24
Neoplastic Diseases

James G. Fox, Sureshkumar Muthupalani, Matti Kiupel, and Bruce Williams

Introduction

More than any other factor, the exploding worldwide popularity of ferrets as pets has resulted in a remarkable increase in the amount of information available on neoplasia since the second edition of this book. Unlike the majority of ferrets in laboratory colonies, pet ferrets often reach advanced age, exhibiting a variety and incidence of neoplasms that we had earlier only imagined.

The first reports of neoplasia in this species were published in the 1950s [1–3], and by 1979, only about 20 cases had been documented [4]. In 1988, 161 neoplasms [5] and by 1993, approximately 170 cases were documented in the literature [4], including experimental studies with chemical carcinogens and viral agents [6]. The rarity of ferret neoplasms documented historically in the earlier literature appears to be due to (1) delayed or absent documentation of ferret neoplasms, (2) scarcity of ferrets as laboratory and pet animals in the past, and (3) lack of sufficient time for laboratory maintained ferrets to develop neoplasia in most experimental studies.

The second edition of this book, published in 1998, mined the current literature (1300 cases), and the Veterinary Medical Data Base at Purdue University listed a total of 1945 cases [4,7–11]. In 2004, a retrospective of over 1570 cases of neoplasia in ferrets from the archives of the Armed Forces Institute of Pathology added to the list of tabulated cases [12].

In previous editions [5,8] of this book, the authors of this chapter were able to tabulate the reported cases of spontaneous neoplasia in ferrets. However, the authors of this chapter, in light of the rapidly mounting number of case reports today, have realized the limitations of a similar attempt, and that the absolute number of reported tumors of any particular type is likely of little utility to most practitioners and researchers. Neoplastic cases archived in a given veterinary hospital or pathology service may represent the percentage of neoplastic diseases among the ferrets seen by the hospital or service, but do not reflect the tumor prevalence in the ferret populations, in general. Further, tumor prevalence observed in a given colony of research ferrets may not be accurate because the animals are often subjected to limited life spans and/or experimental manipulations due to specified research programs. Nevertheless, in some closed ferret colonies, tumor prevalence varies from 0.5% to 21% [8].

Even without accurate data on the incidence of neoplasia in today's ferret population, there is a general consensus among authors of large retrospective studies, even from those outside the United States [13], that neoplasia is a significant clinical issue in ferrets [8–14]. A “golden age” for the development of neoplasia in ferrets, between the ages of 4 and 7 years, has been identified in a number of reports [11–13]. No gender predilection has been identified in the development of spontaneous neoplasms in this species [11–14]. A higher incidence of neoplasia has been reported in neutered ferrets [8,11], but likely reflects the relative rarity of intact ferrets in the pet and laboratory populations at large, and the fact that older ferrets at a higher risk for neoplasia have usually been neutered.

Primary neoplasms in ferrets have been reported in every organ system, although in some organs, such as the lung and thyroid gland, primary neoplasia appears to be uncommon [8,12,15]. Most retrospective studies agree that the endocrine, hemolymphatic, and integumentary systems are most commonly affected, with numbers of endocrine neoplasms often outweighing the other two systems combined [8,11–13,16]. Additionally, each of these reviews identified pancreatic islet cell tumors, adrenocortical tumors, and lymphomas as the most common neoplasms, respectively, in this species and the development of multiple neoplasms, especially islet tumors and adrenal tumors, is not uncommon [9,12–14]. The high incidence of several types of neoplasms, especially those listed earlier, makes the simultaneous development of these neoplasms in a single individual likely [12,16]. Twelve to 20% of animals in a number of larger studies were reported as having multiple tumors simultaneously [11–13]. To date, no report has proven a common link in the development of multiple tumor types in a single animal; however, a single report on a ferret with simultaneous development of multiple endocrine tumors (C-cell carcinoma, adrenal adenoma, pheochromocytoma, and insulinoma) demonstrates a strong correlation with the syndrome of multiple endocrine neoplasia (MEN) in humans, although genetic testing is not currently available for ferrets (as it is for human counterparts) [17]. The completion of the ferret genome may make such testing available in the future.

While tremendous thought has been given to elucidating the origin of various neoplasms in ferrets, the answers remain elusive in the vast majority of cases [8,9,12,16]. A few common themes, however, have become apparent. “Husbandry” issues are one of the few theories which have become widely accepted as a significant cause of neoplasia in ferrets. The common practice of early gonadectomy in pet ferrets has been identified as the causative factor in development of proliferative lesions in the adrenal cortex [18,19]. The feeding of high-carbohydrate diets and treats has been suggested as a cause for the high incidence of islet-secreting beta cell tumors in pet ferrets [9,12]. The possibility of genetic predisposition to neoplasia has been advanced by the anecdotal comparison of the higher incidence of neoplasia in American bloodlines versus European bloodlines; however, the lack of genomic studies on ferrets to date has impeded proof of this widely held theory [9,12].

Infectious agents have also been incriminated in the development of neoplasia in ferrets. Cluster outbreaks of lymphoma in ferrets have been linked to retroviral infection [20] in a study that was further strengthened by experimental inoculation and transmission between ferrets using cell-free inoculum [21]. Helicobacter mustelae, a persistent colonizer of the ferret stomach, has been circumstantially incriminated in the development of gastric mucosa-associated lymphoid tissue lymphomas (MALT lymphomas) [22], as in gastric adenocarcinomas in ferrets previously treated with a gastric carcinogen [23] or gastric adenocarcinoma in pet ferrets [24]. A mouse polyomavirus, when injected into neonatal ferrets, resulted in fibrosarcomas in 15% of inoculated animals [25].

Tumor morphology, as judged by histopathology, is still considered the cornerstone for the diagnosis of neoplasms in all animal species, including ferrets. In general, the morphologic appearance and often the clinical behavior of a neoplasm bear a strong resemblance to its tissue of origin. However, a small percentage of neoplasms do not closely enough resemble their tissues of origin to allow identification simply on morphologic grounds; two retrospective studies of neoplasms in ferrets identified the number of these so-called undifferentiated neoplasms as less than 2% [11,16]. In such cases, careful examination of the clinical history, hematologic parameters, and ancillary testing modalities such as immunohistochemistry, ultrastructural analysis, and molecular techniques are essential in order to provide an accurate diagnosis and to predict the prognosis of such neoplasm. The increasingly stringent requirements for publication in today's veterinary literature has necessitated the incorporation of multiple ancillary modalities to establish the diagnosis beyond reproach, lending additional credence to more recent reports in the veterinary literature, as compared with reports from the 1950s, 1960s, and 1970s. However, the use of additional techniques, especially immunohistochemistry, is increasing in many routine diagnostic submissions. Immunophenotyping of lymphomas to establish lineage and clonality is becoming more common, as is the use of immunomarkers for hormone production in tumors of the endocrine system.

The adage that “A ferret is not a dog, nor is it a cat” [12] is especially apropos to the diagnosis and treatment of neoplasia in ferrets. While neoplasms in ferrets are often grossly and histologically similar to those seen in humans and other animal species, their behavior is often significantly different. For example, insulin-secreting pancreatic tumors (also known as insulinomas) are well known for rapid and widespread metastasis in dogs, cats, and humans, but uncommonly metastasize in the ferret. Cutaneous mast cell tumors, which are potentially malignant in dogs and often metastasize to regional lymph nodes, once again, are invariably benign in affected ferrets. For this and many other reasons, clinicians and pathologists unfamiliar with ferret neoplasms should resist the urge to extrapolate knowledge from other, more common domestic species.

For the pathologist, there are some general rules of note regarding the accurate diagnosis of neoplasms in ferrets. Hyperplasia, adenoma, and carcinoma appear to be a continuous spectrum of proliferative lesions in ferrets as in other animals, especially in neoplasms of the endocrine system [9,12]. In the adrenal cortex, the differentiation between hyperplastic lesions, adenoma, and carcinoma is occasionally difficult and often arbitrary in many cases. Luckily, each of these lesions warrants a good prognosis if treated early in its progression [12]. Ferret neoplasms have other morphologic peculiarities, which are not often seen in other species. Focal attenuation or even absence of the capsule with cellular proliferation outside of the capsule may be seen in normal as well as hyperplastic lymph nodes and adrenal glands of ferret [16], as opposed to being evidence of malignancy as it is in many other species. Apocrine sweat gland tumors arising in areas other than the genital or perianal are generally benign, while those arising in the prepuce, vulva, or in the area of the anal sacs generally result in rapid growth as well as metastasis to regional lymph nodes and viscera [16].

Endocrine System

Endocrine tumors are among the most common neoplasms in ferrets [8,11–13]. Although multiple types of endocrine tumors are observed, the majority of them are pancreatic islet cell tumors and adrenocortical neoplasms, are often functional, and secrete excessive amounts of active hormone, resulting in obvious clinical signs. Quantitation of hormones and/or hormonal metabolites in the serum, plasma. and/or urine is often used as corroborative evidence to clinical signs in establishing the presence of one of these neoplasms in the absence of surgical intervention, biopsy, and histologic diagnosis.

Pancreatic Islet Cell Tumors

Neoplasms of the pancreatic islets are among the most common neoplasms in ferrets [8,11–13,26,27]. The pancreatic islets are composed of neuroendocrine cells which secrete a wide range of hormones, including insulin, glucagon, somatostatin, and pancreatic polypeptide, among others. The vast majority of islet cell neoplasms in ferrets arise from beta cells and secrete insulin, hence the common names of “beta cell tumor” or “insulinoma.” The percentage of functional (or insulin-secreting) islet cell neoplasms has been estimated as high as 94% [28]. These neoplasms are also often found as incidental findings at necropsy in geriatric ferrets dying of other conditions.

Islet cell tumors may occur at any age—and have been reported in animals as young as 2 weeks old [11]—however, the peak prevalence is between 4 and 5 years of age [12,26–28]. A slight gender predilection (1.4/1) of males to females has been reported [27].

The clinical behavior of insulinoma in ferrets is quite different than in other domestic species. In ferrets, metastasis of islet cell tumors is an exceedingly rare phenomenon [12,28–30], primarily to liver and lymph nodes [14]. In dogs, metastasis is an early event in disease progression (often occurring prior to clinical diagnosis), and results in a far poorer prognosis (see Chapter 17).

A single pancreatic polypeptide-secreting islet cell neoplasm has been previously reported in the ferret [8].

Clinical Signs

Clinical signs in ferrets with functional islet cell tumors are primarily due to the hypersecretion of insulin by the neoplastic cells. The primary effect of insulin is to regulate glucose levels by causing its uptake into hepatocytes, adipocytes, and muscle cells. The end product of excessive insulin secretion by neoplastic beta cells is profound hypoglycemia.

As the brain has the highest metabolic requirements for glucose, the clinical signs of hypoglycemia are primarily neurologic in origin. These signs include single to recurrent episodes of depression, lethargy, ptyalism, stupor, posterior weakness, ataxia, tremors, and seizures [25–34]. One report identified lethargy (95%), weakness (82%), and difficulty rousing from sleep (64%) as the most common clinical signs of hypoglycemia in ferrets [28]. Pawing at the mouth is also commonly identified as a clinical sign, presumably due to nausea [26–28]. The signs may become severe or more obvious after exercise or fasting, and their frequency increases with more long-standing disease. Seizures, the most severe manifestation of hypoglycemia, are seen less commonly in ferrets than in dogs. Prolonged seizures in affected animals can ultimately result in permanent neurologic damage and visible microscopic lesions within the brain, even if hypoglycemia is subsequently controlled.

Pathology

The gross and microscopic appearance of islet cell tumors in ferrets are similar to those seen in other animal species [8,12,16,29,33], and, upon microscopic examination, closely resemble normal islets in the adjacent pancreas. Multiple neoplasms have been reported as more common than single neoplasms in the pancreas of ferrets undergoing surgical removal [27].

Grossly, islet cell neoplasms are single to multiple red-brown nodules within the pancreatic tissue. Nodules range from almost imperceptible with the naked eye to over 1 cm in diameter and are generally firmer than surrounding tissue. Their gross coloration is different from that of the surrounding pancreas as a result of the large number of blood vessels within this endocrine neoplasm (Fig. 24.1).

Fig. 24.1.  Islet cell tumor (arrows) within the pancreas. Islet cell tumors are usually pink-red as opposed to surrounding tissue. (Photo courtesy of Dr. John Edwards, Texas A&M University.)

Microscopically, neoplasms are well demarcated from the surrounding exocrine tissue; a fibrous capsule may be present. The neoplastic cells are arranged in nests, cords, or packets, and are separated by delicate fibrovascular stroma, or palisade around blood vessels or sinusoids. The cells are cuboidal to columnar, well differentiated, and resemble normal islet cells. The cytoplasm is slightly eosinophilic and finely granular. Cellular features are bland, with minimal cytologic atypia within nodules. Mitotic figures are generally low to absent, and necrosis is uncommon (Fig. 24.2).

Fig. 24.2.  Islet cell neoplasms often resemble normal islet tissue and are often unencapsulated. (HE 80×.)

The well-differentiated nature of the typical ferret insulinoma poses problems for pathologists who require classification as either adenoma or carcinoma. Previous articles have advanced classification schemes based on nodule size, number, infiltration of surrounding tissue, or the presence or absence of a capsule. The classification of single or multiple neoplasms on the basis of histologic features alone is largely arbitrary. In obvious cases of carcinoma (i.e., those in which distant metastasis has occurred), both primary neoplasms and metastatic foci exhibit minimal atypia and still resemble normal islets. Hence, without defined cellular characteristics of malignancy, a histologic diagnosis of beta cell tumor or insulinoma is preferred by many pathologists for those neoplasms in which metastasis is not evident.

Diagnosis

Diagnosis of islet cell tumors is based on clinical signs, laboratory findings, and histopathology [25–37]. In many cases, a provisional diagnosis is based on clinical signs of hypoglycemia as well as determination of blood glucose <60–70 mg/dL [16,26–28]. Fasting for sample collection may be required in animals in early stages wherein insulin release and hypoglycemia may be episodic. A variety of ancillary tests are available for borderline cases or for confirmatory purposes. Serum insulin testing is the most common with hyperinsulinism defined as insulin levels >5–35 μU/mL [26,28]. Insulin : glucose ratios (and their various permutations) are no longer recommended in ferrets due to the high number of false positives [36]. A variety of other testing modalities were evaluated in other domestic species, including fructosamine and glycosylated hemoglobin levels, but these have not been attempted in ferrets [26,28].

A complete blood count and clinical chemistry should be performed on all animals with insulinoma to identify possible underlying and concurrent conditions. Diabetes mellitus with and without ketoacidosis has been diag­­nosed in ferrets with insulinoma [34,35], or ferrets whose hypoglycemia has been managed on a long-term basis with oral glucocorticoids. Many ferrets with insulinoma may also have functional adrenocortical neoplasms, a similarly common neoplasm whose clinical signs overlap or counteract those of insulinoma [9,12–14,32]. Multiple tumors of the endocrine system, including an adrenocortical neoplasm, pheochromocytoma, and C-cell carcinoma of the thyroid gland, were reported in a ferret with a pancreatic islet cell tumor [16].

Diagnostic imaging has been shown to be of little use in the diagnosis or localization of islet cell tumors in ferrets. In a retrospective study, survey radiographs failed to identify islet cell neoplasms in 28 cases; abdominal sonography fared slightly better, identifying 5 neoplasms out of 23 cases [25].

For the pathologist, the diagnosis of islet cell neoplasms is not difficult. Commercially available antisera to insulin, glucagon, somatostatin, pancreatic polypeptide, and neuroendocrine markers chromagranin A and synaptophysin may be useful in identification and further investigation of islet cell neoplasia in ferrets [30,36–38].

Differential diagnoses should include other neurolog­­ical diseases (brain neoplasia, otitis media) or hypoglycemic conditions (sepsis, paraneoplastic syndromes, liver disease), as well as improper handling of blood samples.

Adrenocortical Tumors

Neoplasms of the adrenal cortex are the second most common neoplasms in ferrets [8,11–13,39]. Adrenocortical disease (often referred to as “adrenal-associated endocrinopathy” [12,18]) is a clinical syndrome which is relatively unique to ferrets, although similar syndromes may uncommonly be seen in other species. In ferrets, as opposed to most other species, hyperplastic or neoplastic cells of the zona fasciculata or reticularis elaborate sex steroids rather than cortisol, resulting in a constellation of unique clinical signs.

The pathophysiology of adrenocortical disease in ferrets is widely accepted today to be the result of early neutering, a common practice in ferrets in most areas of the world [12,18,19,28,38,39]. The phenomenon of gonadectomy-induced adrenal neoplasia is not unique to the ferrets, but has been reported in mice, rats, guinea pigs, and hamsters [18,19].

Removal of the gonads in immature ferrets results in disruption of the hypothalamic–pituitary–gonadal axis, culminating in persistently elevated levels of luteinizing hormone (LH) [18,19,40–42]. Upregulation of LH receptors on cells of the zona fasciculata and reticularis results in expression of a gonadal phenotype and secretion of estradiol or its intermediates, 17-hydroxyprogesterone, androstenedione, or dehydroepiandrostenadione sulfate [18,19,28,40–42]. Alternate theories about adrenal tumorigenesis include a genetic predisposition to development [12,28], as well as the exposure of pet ferrets to abnormal light–dark cycles [18,19,28,38,42].

Elevated levels of sex steroids in affected ferrets and the absence of hypercortisolism are characteristic of adrenal disease in ferrets, as opposed to traditional Cushing's disease in other domestic species. Functional neoplasms of the pituitary glands, seen in up to 80% of cases of canine hyperadrenocorticism, are not usually encountered in ferrets [43], although an exemption to this assumption was seen in a report of incidence of nonfunctional pituitary tumors in two ferrets with hyperadrenocorticism [44]. However, in a minority of cases, other adrenocortical steroids may be elevated, such as cortisol [12,39,40,42] and aldosterone [45].

The incidence of adrenal disease in ferrets is actually unknown and estimates in the veterinary literature range widely from 25% to 70% [39,40,45]. There appears to be no gender predilection [35,38,40,45], although one report noted a predominance in females (perhaps as a result of more obvious clinical signs, such as vulvar swelling) [18,46]. Proliferative lesions of the adrenal gland are most common in middle age, from 4 to 6 years of age [11,12,28,39,46], but have been reported in animals as young as 8 months and as old as 9 years [39]. Adrenal disease is reported far more commonly in neutered animals than intact ones [11]; however, this likely is related to the higher prevalence of neutered animals within the general ferret population.

Clinical Signs

Clinical signs include bilateral and symmetric truncal alopecia (due to the catabolic effect of estrogen on hair follicles) (Fig. 24.3) and pruritus [11–14,28,35,38–43,45–48]. Diffuse muscle wasting, coupled with deposition of abdominal fat, often results in a pendulous abdomen, especially in geriatric animals. Up to 70% of spayed females exhibit vulvar swelling, with or without discharge [12,35,39,46]. In males, dysuria is often evident, although urine may be easily expressed from a greatly distended bladder, even though the animal is unable to void on its own [49]. Dysuria may proceed to complete outflow obstruction. A return to reproductive behavior in neutered individuals, including mounting and urine marking, may be seen in affected individuals [12]. Hyperplasia of the mammary gland has been reported in a ferret with a functional adrenocortical tumor [50] (see Chapter 17).

Fig. 24.3.  Severe bilateral truncal alopecia in an animals with an estrogen-secreting adrenocortical neoplasm. (Photo courtesy of Dr. Peter Fisher.)

Pathology

Functional adrenal lesions comprise a histologic continuum ranging from multiple nodules of hyperplasia to benign and malignant neoplasms. The clinical signs listed earlier are common to each of these lesions and specific to none of them, that is, the type and clinical symptoms exhibited by a particular individual have no predictive value as to the type of adrenal tumor present.

The histologic diagnosis of adrenal neoplasms may be confounding to pathologists, especially those unfamiliar with ferrets. This confusion likely makes absolute conclusions of the incidence of each of these lesions somewhat controversial; however, most reviews identify hyperplastic lesions as most common (often over 50%), followed by carcinoma, and adenoma being the least common (usually less that 20%) [11,12,39,45,46]. Up to 20% of individuals will have lesions in both adrenal glands [12,45,46], and, often, different lesions are present in the left versus the right gland [45].

Grossly, affected adrenal glands are usually enlarged (the normal range averages 5–9 mm in length by 2–4 mm in width) with increases in thickness seen most commonly [51,52]. A yellowish discoloration is often noted, with poor delineation or total effacement of the corticomedullary junction, and the gland itself is friable. In some cases of hyperplasia of the right adrenal gland, the cortex and medulla may be effaced by cysts that containing a hard, waxy, translucent material. Adrenal adenomas are nodular lesions often exceeding 1 cm in diameter, and carcinomas may be exceedingly large (often palpable through the skin), with local invasion into surrounding vessels (Fig. 24.4 and Fig. 24.5).

Fig. 24.4.  Ferret adrenal carcinoma: Large adrenocortical carcinoma with areas of necrosis.

Fig. 24.5.  Bilateral adrenal carcinomas with regional caudatum and metastatic spread in relation to kidney mesentery and intestine.

Guidelines for histologic evaluation of proliferative adrenal lesions have not been previously addressed in this species. Early resection of any type of unilateral or bilateral adrenal lesion, including malignant tumors which metastasize late in the course of disease, generally warrants a good prognosis; thus, problems arising from improper classification are rare [8,12,53]. In this regard, a study of 130 cases noted that survival time of ferrets following surgical treatment of hyperadrenocorticism was not affected by the histologic classification of the neoplasm, location of the neoplasms (right, left, or bilateral), or complete excision versus debulking [54].

Nonetheless, in the authors' opinion, a few basic concepts may help to standardize microscopic diagnosis of these common neoplasms.

1. Multinodularity.  Hyperplastic lesions in the adrenal cortex often appear as multiple discrete expansile nodules within the adrenal cortex. Hyperplastic lesions may or may not result in organomegaly; adenomas generally result in gross organomegaly. Hyperplastic nodules vary markedly in size but are always multiple; adenomas are single nodules.
2. Demarcation.  Hyperplastic nodules and adenomas are well demarcated from surrounding tissue, and may be further identifiable by the presence of numerous cytoplasmic vacuoles compared with the surrounding cortex. Carcinomas generally are poorly demarcated from surrounding tissue, making it difficult to precisely identify the interface between neoplasm and surrounding cortex.
3. Cellular appearance.  The cellular appearance of proliferating cortical cells often varies markedly, even in benign lesions. Neoplastic cells range from terminally differentiated adrenocortical cells to actively proliferating cells (dark, angular cells, often resembling lymphocytes) [8,55]. Neoplasms with a predominance of these cells are often identified as pheochromocytoma, even in the presence of irrefutable clinical signs of adrenocortical disease. Large nuclei with prominent nucleoli or evidence of mild anisocytosis or anisokaryosis should not be interpreted as cytologic evidence of malignancy.
4. Mitotic rate.  Mitotic figures are rarely (<1/10 400× fields) seen in hyperplastic lesions and adenomas. Mitotic rates exceeding this should be considered evidence of malignancy.
5. Necrosis.  Significant areas of necrosis are generally considered features of malignancy in adrenocortical lesions.
6. Metastasis.  Metastasis to distant organs is prima facie evidence of malignancy in ferret adrenocortical tumors. Ectopic adrenocortical tissue is a common finding in ferrets and should not be interpreted as metastasis.

One particular phenotype of adrenocortical neoplasm has been identified as highly malignant with a high incidence of metastasis. In a review of 15 cases, adrenal carcinomas composed of poorly differentiated adrenocortical cells (having the small polygonal phenotype described earlier), which formed lumen-like spaces filled with acidic mucopolysaccharides, exhibited a 40% chance of developing invasive or metastatic disease [56]. Contralateral adrenal atrophy is infrequent in ferrets with unilateral tumors, as is seen in other species with traditional hypercortisolism [8,12,33].

Pathologists should also be familiar with the many other pathologic conditions which accompany functional proliferative lesions of the adrenal cortex. An associated lesion in a minority of cases of nodular hyperplasia of the right adrenal gland is biliary cysts. In these cases, large areas of the adrenal gland may be effaced by large cysts lined by cuboidal epithelium, which contain a bright pink, homogenous secretory material. In biopsies of debulked adrenal glands, this waxy secretory material may be the only material submitted, as the remainder of the friable gland is lost during surgical removal. The epithelium lining of these cysts is immunoreactive for CK7, suggesting that they are of biliary origin, and that the lesion may be developmental in nature.

Evidence of hyperestrogenism is often seen in other organ systems in affected animals. Examination of haired skin will demonstrate global telogenization and atrophy of hair follicles, thinning of the epidermis, and evidence of trauma in areas of pruritus. In male ferrets, the prostate gland is enlarged with development of multiple cysts, often filled with pus. These cells develop following squamous metaplasia of the normal prostatic glandular epithelium under the effects of estrogen, and a subsequent suppurative reaction by the body [49]. In female ferrets, in addition to vulvar enlargement and discharge, endometrial hyperplasia and pyometra may be seen in the uterine stump. Bone marrow suppression and variable pancytopenia are occasionally seen [39], but with far less frequency than in the hyperestrogenism seen in persistent estrus in the intact jill (see Chapter 15).

Diagnosis

In the vast majority of cases, diagnosis of proliferative adrenocortical lesions is accomplished based on clinical signs. A wide range of other testing modalities may be utilized to confirm early cases. Ultrasonography has repeatedly proved superior to radiography in several studies [39,51,52,57], with success rates of up to 97% in one report [51].

Specific testing for sex steroids and their progenitors is the most reliable test available for diagnosis of adrenocortical disease. Testing panels for estradiol, 17-hydroxyprogesterone, and androstenedione are commercially available through the Clinical Endocrinology Laboratory at the University of Tennessee College of Veterinary Medicine [39]. Increased urinary cortisol : creatinine ratios have been demonstrated to have some utility in the diagnosis of adrenocortical disease in the ferret [58,59] as a result of increased secretion of cortisol in the urine in animals with hyperadrenocorticism [59], although use remains controversial [39]. Blood tests which are used in the diagnosis of traditional Cushing's disease in other species, such as high- and low-dose dexamethasone suppression, cortisol, adrenocorticotropic hormone (ACTH), and alpha-melanocyte stimulating hormone (α-MSH), have not been shown to be of any benefit in the diagnosis of adrenocortical disease in ferrets [12,39,60] One report demonstrated a higher percentage of cornified preputial epithelial cells in the preputial lavage of castrated males with adrenocortical disease, which correlated with elevated levels of 17-hydroxyprogesterone [61].

Routine hematologic and clinical chemistry data, outside of the relatively rare cases of long-standing adrenal disease with bone marrow suppression, are rarely useful in the diagnosis of adrenocortical disease in ferrets but may identify concurrent disease, such as insulinoma or gastric ulceration.

In attempting to differentiate benign from malignant lesions, several immunomarkers may be of utility. Adrenocortical malignancies may variably express cytokeratin, vimentin, and inhibin. GATA-4, a transcription factor, has been identified as a marker of anaplasia in ferret adrenocortical neoplasms [62]. In one study, nuclear immunoreactivity was identified in 19/22 (86%) of adrenocortical carcinomas in ferrets, and in 7/14 adenomas [62]. Additionally, increased expression of cytochrome b, a regulator of cortisol and adrenocortical androgens, was demonstrated in 24/25 adrenal adenomas and carcinomas via the use of specific immunomarkers [63].

Adrenocortical Spindle Cell Neoplasms

Spindle cell neoplasms have been described in the adrenal cortex of ferrets, both with and without concurrent proliferative lesions of adrenocortical cells [8,12,57,58]. Spindle cell proliferation is most prominent in the cortex, but the number of spindle cells varies widely. Histologically, the spindle cells resemble smooth muscle cells, with oval to elongated nuclei and a small amount of amphophilic cytoplasm. Ultrastructurally, they contain thin contractile filament. Neoplastic spindle cells exhibit immunopositivity for smooth muscle actin and desmin, and are negative for cytokeratin and S-100 protein (a neural marker) [64]. A study of 24 such neoplasms [57] identified positive for estrogen receptors in 9/24 neoplasms. Classification of these neoplasms incorporated both the diagnosis associated with the epithelial component and a modifier of “mixed” to identify the presence of spindle cells as part of the neoplasm (hence the diagnoses of mixed adenomas and mixed adenocarcinomas). In this series, the disease-free interval was markedly reduced in neoplasms with abundant smooth muscle, a more malignant histologic grade for the epithelial component, or the expression of estrogen receptors. Infrequently, smooth muscle proliferation may efface the adrenal cortex; adrenal leiomyoma or leiomyosarcoma may be a more appropriate diag­­nosis for cases in which an epithelial component is not visualized.

Pheochromocytomas

Pheochromocytomas have been rarely diagnosed in ferrets [11,12,39]. As mentioned earlier, the morphologic appearance of poorly differentiated adrenocortical cells in the far more common adrenocortical neoplasms can resemble the neuroendocrine cells of the adrenal medulla, from which pheochromocytomas arise. In order to definitively confirm a diagnosis of pheochromocytoma, several clinical, histologic, and/or ultrastructural criteria must be identified.

Clinical signs of pheochromocytoma are lethargy, tachycardia, dyspnea, and cardiovascular collapse, and have been identified in one anecdotal report without further histologic confirmation [39]. Various histochemical stains can be used to pinpoint these rare neoplasms, including silver stains (which highlight the presence of argyrophilic granules), or the immunomarker synaptophysin, a glycoprotein found in the synaptic vesicles of neuroendocrine cells. Ultrastructural examination of cells in pheochromocytoma will also demonstrate the characteristic dense core granules of cells of neuroendocrine lineage. In today's literature, only one case report [17] has fulfilled these criteria, identifying argyrophilic granules within neoplastic cells. This particular report also identified other neoplasms of endocrine origin in the affected individual, including a medullary carcinoma of the thyroid, an islet cell neoplasm, and tumors of the adrenal cortex. Pathologists reviewing adrenal neoplasms in ferrets are cautioned to carefully examine the clinical history on each submission to note clinical signs associated with neoplasms of the adrenal cortex prior to making a diagnosis of this tumor type [65]. A single case of neuroblastoma of the adrenal medulla has been reported. While the neoplastic cells morphologically resemble those of the adrenal medulla, the criteria, evidence of characteristic clinical signs was not demonstrated, nor was synaptophysin immunoreactivity or ultrastructural examination investigated [66].

Adrenal Gland Teratomas

Teratomas are suspected to be developed from the parthenogenetic germ cells and consist of multiple tissues from all three germ layers. In the majority of species, teratomas arise in the gonads; however, the adrenal glands are the most common site of origin in ferrets [65,66]. Unilateral and bilateral neoplasms of this type have been documented in ferrets [67].

As teratomas do not elaborate hormones as primary neoplasms of the adrenal cortex and medulla do, clinical signs, when present, are limited to lethargy and perhaps a palpable unilateral or ovarian mass. These masses are usually identified upon routine physical examination or examinations or procedures for other reasons (e.g., insulinoma removal). They may also be identified as an unexpected finding on routine radiologic examination due to the common presence, or bone or tooth-like structures.

Histologically, adrenal teratomas are composed of various combinations and concentrations of well-differentiated epithelial, mesenchymal, and endodermal tissues which are not native to the adrenal gland. Bone, cartilage, skin (replete with hair follicles and apocrine and sebaceous glands), brain, fat, smooth muscle, pancreatic acinar tissue and glandular epithelium resembling that of the gastrointestinal or respiratory tract, and even a rudimentary tooth, in one case, have been described. In each case, the adrenal cortex and medulla are markedly compressed [65,66].

Thyroid Gland Neoplasms

Diseases of the thyroid gland appear to be rare in ferrets, but several cases of thyroid adenomas and adenocarcinomas have been reported [8,15,66].

The reported cases of thyroid follicular neoplasm did not appear to result in clinical signs referable to hyperthyroidism, and thyroid profiles were not performed. The adenomas were small and nonpalpable; the thyroid adenocarcinomas in each case were firm and nonmovable, one resulting in dysphagia [8].

The follicular adenomas were 2–3 mm in diameter, unilateral, and well demarcated, while the adrenocarcinomas were larger (up to 2 cm in one report) and infiltrated adjacent tissue. In each report, the neoplasms retained a follicular structure which allowed identification on morphologic characteristics. Neoplastic cells were cuboidal to columnar with abundant eosinophilic granular cytoplasm and basilar nuclei. Colloid-containing follicles were present in all tumor types. In thyroid follicular adenocarcinomas, karyomegaly and a high mitotic rate (up to 7 per 10 400× fields) were present. Immunomarkers for thyroid hormones (thyroglobulin and calcitonin) were positive when attempted on the thyroid adenocarcinomas. Today, definitive diagnosis of thyroid follicular neoplasms should include these markers, as well as thyroid transcription factor-1 (TTF-1), a transcription regulator found in thyroid follicular and parafollicular cells.

As mentioned previously, a single case of a C-cell carcinoma, a tumor of parafollicular cells, occasionally referred to as a thyroid medullary carcinoma, has been identified in a ferret [17]. This 3-cm nonmovable mass effaced the thyroid gland and infiltrated an adjacent lymph node. The neoplasm was composed of irregular cords and packets of columnar cells which, in less differentiated areas, assumed a spindled appearance. Mitotic figures ranged up to 6 per 400× fields. Neoplastic cells were positive for argyrophilic stains for neuroendocrine granules. At present, the use of immunomarkers for calcitonin, thyroglobulin, and TTF-1 would be required for the definitive diagnosis of C-cell neoplasms in ferrets.

Hemolymphatic System

After the endocrine system, the hematolymphatic system is the site of origin for the largest number of neoplasms, as well as the most common number of malignancies [8,12]. While an excellent discussion of hematologic neoplasms is present in Chapter 14, a brief review of these important neoplasms is presented here.

Numerous reports have identified malignant lymphoma as the third most common neoplasm in ferrets [8,11–13,15,67–69]. Malignant lymphoma has been identified in every organ system in ferrets, although the majority of cases arise in the hematolymphatic organs, that is, the lymph nodes and spleen [12]. The term lymphoma denotes the presence of soft tissue tumors in any organ, while leukemia denotes the presence of neoplastic lymphocytes in the blood or bone marrow. Other terms in common use for this disease include malignant lymphoma (a redundant term as all lymphomas are malignant), as well as lymphosarcoma (an older term which is rarely used today). The terms acute and chronic may be applied to leukemia in ferrets to describe the level of differentiation of lymphocytes from immature forms (acute) to well differentiated (chronic).

One problem in the classification of lymphoma in ferrets is the lack of standardization of protocols in use today. There are a number of schemes utilized for the classification of lymphomas in ferrets; some are derived from other species, such as the National Cancer Institute Working Formulation [69], a classification scheme for lymphoma in humans. Some classifications are based solely on morphologic criteria (lending to tremendous inter- and intraobserver variation), while some utilize immunophenotyping data (which determine the origin of neoplastic lymphocytes as either T- or B-cell type) In most studies, the use of a particular classification is primarily the result of local preference.

Most ferret lymphomas appear to arise spontaneously in affected animals, but unlike endocrine neoplasms, there does not appear to be any particular age susceptibility [12]. Ferrets of all ages may develop lymphoma, and several age-related syndromes have been historically identified in this species. However, as more sophisticated tools are developed to investigate lymphoma, and more and larger studies begin to appear in the literature, it is becoming apparent that some previous assumptions may not be entirely true [12]. Cluster outbreaks of lymphoma have also been identified, suggesting a potential infectious cause [20,21]; transmission of lymphoma between individuals following inoculation of cell-free media has suggested a viral origin [21].

Clinical Signs

The clinical presentation of lymphoma is nonspecific in most cases, and is generally referable to the primary organ(s) affected. In younger ferrets, a syndrome of visceral lymphoma (often referred to as lymphoblastic lymphoma) is characterized by infiltration of the thymus (Fig. 24.6), liver, and spleen by large blastic lymphocytes; clinical signs are usually that of dyspnea as the thymic mass fills the thoracic cavity and compresses the lungs. In older ferrets, more differentiated lymphocytes expand peripheral lymph nodes, often resulting in marked lymphadenomegaly (Fig. 24.7) (often referred to as lymphocytic leukemia), but few other clinical signs. One particular form of lymphoma in older animals, cutaneous lymphoma, differs from other forms of lymphoma in that visceral metastasis is uncommon, and surgical excision may be curative [70].

Fig. 24.6.  In younger ferrets with lymphoblastic lymphoma, large thymic masses are common.

Fig. 24.7.  Marked lymphadenomegaly of cervical lymph nodes in an older ferret with lymphocytic lymphoma.

Hematologic abnormalities also reflect the primary organs affected; anemia is a common accompanying sign [12,67]. The presence of abnormal lymphocytes in the peripheral blood is uncommon, with leukemia occurring in less than 10% of cases [12].

Paraneoplastic syndromes, such as hypercalcemia, are uncommon in ferrets. In one study, 2/28 ferrets with lymphoma exhibited evidence of hypercalcemia [15], and this finding has been reported one other time in a ferret with multiple myeloma (a neoplasm of terminally differentiated B-cells) of the vertebrae [71].

Pathology

Histology or cytopathology of affected tissues is required for the diagnosis of lymphoma in ferrets. Histopathology is considered the diagnostic gold standard, although cytologic diagnosis may be sufficient in animals with advanced disease and typical clinical signs, such as generalized lymphadenopathy.

Cytologic diagnosis via fine needle aspiration is a rapid and inexpensive way to obtain a preliminary diagnosis of lymphoma. The cytologic hallmarks of lymphoma are a monotonous population of lymphocytes with a paucity of other hematogenous elements. Marked variation in lymphocyte size, as well as the presence of plasma cells, macrophages, and neutrophils, is suggestive of reactive hyperplasia. Lack of generalized lymphadenopathy in some cases, as well as poor sample preparation, may lead to false negative results.

Many cases will require histologic evaluation of excised lymph nodes or organ biopsies to confirm a diagnosis of malignant lymphoma. As a general rule, the diagnosis of lymphoma in nonhematolymphatic organs on the basis of solid nodules of variably differentiated lymphocytes is not difficult; however, the identification of lymphoma, especially in early stages, in lymph nodes and spleen may pose problems even for experienced pathologists. This problem is further compounded by the variability in morphology of neoplastic lymphocytes which may resemble the normal, well-differentiated lymphocytes typically populating these organs. Lymph node sampling may also complicate diagnosis, for example, markedly hyperplastic lymph nodes, such as seen in animals with chronic inflammatory disease of the gastrointestinal tract, may pose special problem for the identification of malignant lymphoma. In general, biopsy of peripheral nodes, such as the scapular or popliteal, less likely to be affected by local inflammation and complications of their excision are rare [12].

The increasing use of immunophenotyping holds great promise in identifying and ultimately deriving prognostic information in cases of lymphoma. Numerous case reports and several larger retrospective studies have validated the use of commercially available antibodies for the identification of T- and B-cells in ferrets [15,67–75].

The identification of T- or B-cell lymphomas [15,69–72], as well as its more differentiated forms [73–76], has become widespread in the veterinary literature, both in individual case reports and retrospective studies. One retrospective study has validated the use of tissue microarray for high-throughput phenotyping of ferret lymphomas [71]. Existing retrospective studies largely have tried to correlate the immunophenotype of the neoplastic cells with existing morphologic classification [15,69–71]; however, variability in experimental design with regard to the location of the neoplasm and the age of affected animals, as well as the correlation with differing morphologic classification schemes, has adversely impacted the ability to produce useful clinical data. One study, however, which investigated the immunophenotype of mediastinal lymphoma in young ferrets, identified a T-cell lineage for this particular syndrome [72]. This focused approach to immunophenotyping lymphomas should be used as a model for future investigations and scaffolding upon which appropriate prognostic information can be identified.

For a more in depth discussion of malignant lymphoma, to include treatment options, the reader is directed to Chapter 14, Hematopoietic Diseases.

Myelolipoma

Myelolipomas are rare: benign incidental neoplasms are composed of mature adipose tissue and hematopoietic tissues in variable proportions. Most myelolipomas do not cause signs of disease unless they attain considerable size and/or acquire certain secondary changes.

A case of splenic myelolipoma has been described in a 5-year-old male pet ferret [77] with many 1–4 mm diameter, white-to-yellow fatty nodules within the spleen. Histologically, the splenic nodules were unencapsulated, well demarcated, expansile, and composed of a mixed population of mature adipose cells (20–60%) with hematopoietic cells at the periphery. The hematopoietic population was composed predominantly of immature myeloid and erythroid cells admixed with megakaryocytes and primitive blast cells.

Diagnosis of myelolipoma is based currently on gross and histologic findings, although immunohistochemical staining for leukocyte and/or megakaryocyte markers may be useful. The main differential diagnosis of myelolipoma is splenic extramedullary hematopoiesis; how­­ever, this common splenic finding is not associated with adipocytes.

Integumentary System

The skin and subcutaneous tissues are the third most common location for neoplasia in ferrets [8,11–13,78]. In this system, the vast majority of neoplasms are primary skin tumors, and the vast majority pursue a benign course [12,78]. Benign neoplasms of basal cell origin, including sebaceous epitheliomas and sebaceous adenomas, are the most common skin neoplasms, followed by mast cell tumors and apocrine sweat gland neoplasm.

Basal Cell Tumors

Basal cell tumors are the most common neoplasm in the skin of ferret, accounting for up to 60% of skin tumors [8,12,78]. Basal cell tumors arise from the primitive pluripotential basal cells of the epidermis, which have the ability to differentiate into both squamous and sebaceous epithelium [8,79]. These neoplasms generally contain both types of cells in varying proportions, and, as such, may be diagnosed as basal cell tumors (with a predominance of squamous cells), sebaceous epitheliomas (approximately equal amounts of squamous and sebaceous), and sebaceous adenomas (with a prominence of sebaceous cells). Older reports referred to these neoplasms as “baso-squamo-sebaceous tumors,” a term which has fallen out of favor today.

There is no gender, age, or site predilection for the development of basal cell neoplasms. Grossly, these neoplasms present as raised, white-to-pink exophytic warty masses, are often hyperpigmented or partially covered by a serocellular crust as a result of self-trauma (Fig. 24.8). Occasionally, these neoplasms may be plaque-like with a necrotic center [12]. While they may attain a large size, these neoplasms are usually well demarcated and do not invade underlying structures [12,78].

Fig. 24.8.  Sebaceous epithelioma in ferrets is often raised, wartlike, and ulcerated, and may exhibit central areas of necrosis. (Photo courtesy of Dr. John Edwards, Texas A&M University.)

Microscopically, these neoplasms are composed of well-differentiated basaloid epithelial cells arranged in sheets or lobules, with a variable amount of fibrous stroma. In each neoplasm, a percentage of basal epithelial cells exhibit sebaceous and/or squamous differentiation. In most tumors, sebaceous differentiation is prominent, with sebaceous cells forming islands and nests (sebaceous epithelioma) (Fig. 24.9) or well-defined adenomeres (sebaceous adenomas). Squamous differentiation is most often limited to the formation of duct-like structures throughout the neoplasm. Undifferentiated basal cells have round to oval densely basophilic nuclei, indistinct nucleoli, and little cytoplasm, without intercellular bridges. Mitotic figures may be numerous.

Fig. 24.9.  Histologically, sebaceous epitheliomas are composed of variable concentrations of basophilic reserve cells, as well as groups of larger vacuolated epithelial cells undergoing sebaceous differentiation. (H&E, 240×).

While long-standing basal cell neoplasms have rarely given rise to sebaceous adenocarcinomas, squamous cell carcinomas, or a combination of both [12], these neoplasms are overwhelmingly benign and surgical excision is considered curative (Fig. 24.10, Fig. 24.11, and Fig. 24.12). Traumatized or inflamed neoplasms of this type may pose a special diagnostic problem, as they may appear more aggressive both grossly and histologically, with increased mitoses, cellular atypia, and apparent infiltration at inflamed margins. In such cases, diagnostic focus should be given to irrefutable evidence of invasion at the margins rather than cytologic abnormalities when considering the possibility of malignancy.

Fig. 24.10.  Skin (H&E, 40×): A biopsy specimen of a ferret with multiple chronic skin tumors showing in this section a large dermal mass diagnosed as sebaceous adenocarcinoma (thick arrows) merging with squamous cell carcinoma arising from the superficial epidermis (thin arrows).

Fig. 24.11.  Skin (H&E, 100×): Higher magnification of Figure 24.10 showing both malignant sebaceous glands (disorganized darker staining cells with some foamy sebaceous differentiation) and inward growing squamous cell component with keratinization in a background of dermal inflammation.

Fig. 24.12.  Skin (H&E, 100×): Higher magnification of Figure 24.10 showing dermally invasive islands of neoplastic squamous epithelial cells with keratin pearls arising from an eroded superficial epidermis.

Mast Cell Tumors

Mast cell tumors in ferrets are also common and invariably benign neoplasms. There is no age or gender predilection for development of these neoplasms, and they may arise in any area of the body [8,80–82]. While usually single, multiple concurrent neoplasms have been occasionally identified.

Mast cell neoplasms are generally flat to slightly raised red macules which often have an overlying crust, often as a result of pruritis and self-trauma. Microscopically, cutaneous mast cell neoplasms are usually less than 5 mm in diameter and composed of sheets of well-differentiated mast cells with centrally placed nuclei and few metachromatic granules. They may be infiltrated by low numbers of eosinophils; however, collagen degradation (“flame figures”) has not been identified in ferret mast cell tumors. Granules are best demonstrated by the use of toluidine blue staining [9,12].

In contrast to those in dogs, mast cell neoplasms of ferrets are invariably benign; neither cutaneous malignancy nor metastasis has been reported in ferrets. One visceral mast cell tumor affecting multiple abdominal organs has been reported in ferrets [81]; however, no cutaneous neoplasm was identified.

Apocrine Gland Neoplasms

Tumors of the apocrine sweat glands are the third most common neoplasm of ferret skin [8,11–13], and one of the more common malignancies in this organ. The apocrine sweat glands are scent glands of the ferret's haired skin. While distributed throughout the haired skin, these glands are present in highest numbers on the head, neck, and around the prepuce and vulva [12]. Apocrine glands of the prepuce have historically been referred to as “preputial gland tumors,” but these glands are not modified in any way and are considered to be similar to apocrine glands in other areas of haired skin. The mammary gland and ceruminous glands of the external ear canal are modified apocrine glands which are covered separately in this chapter. Apocrine glands of the anal sacs are modified glands which give rise to malignancies in dogs and in minks [83,84], but have not been yet identified in ferrets. Proliferative lesions of the apocrine glands form a continuum from benign non-neoplastic apocrine cysts, through adenoma to carcinoma. Interestingly, apocrine neoplasms of the perineal area, specifically the prepuce and perivulvar skin, have a profoundly increased incidence of malignancy.

While previous reports suggest that most apocrine lesions occur around the perineum [8,9,11], the increased number of aggressive malignancies requiring medical attention at this location has likely skewed these data, and actual numbers reflect a more even distribution among areas with high concentrations of apocrine sweat glands. There is no apparent age predilection; however, these neoplasms appear to be more common in the male, especially affecting the prepuce.

Grossly, these lesions run from small intradermal cysts (often resembling blue to black beads under the skin) to benign cystadenomas which are small, fluctuant, freely moving cysts or multiloculated neoplasms. Often, a small amount of grayish-brown fluid may be aspirated from these lesions. During aspiration, these lesions may collapse, but will generally regain their previous size over a course of days. Apocrine adenocarcinomas are large nodules which are initially freely moving, but become anchored to underlying tissues over time. They appear as hard, solid masses without any material to aspirate. The presence of a large mass on the prepuce or adjacent to the vulva of a ferret is very likely one of these neoplasms.

Histologically, benign neoplasms are uni- or multilocular cysts, which are lined by cuboidal epithelium that often exhibits apocrine secretion (blebbing of secretory product into the central lumen). The lining is usually a single cell layer thick, although rarely, multiple cell layers may be seen. Some of these lesions may contain neutrophils and/or macrophages, especially at their periphery as a result of trauma and release of secretory material into the surrounding dermis.

Apocrine carcinomas are usually neoplasms composed of glandular acini, but occasionally are solid lobules of neoplastic cells. The cells are usually cuboidal, with vesicular nuclei and prominent nucleoli which may pile up several layers thick. Mitotic figures are common, as opposed to cysts and benign cystadenomas. Glands are often separated by abundant fibrous connective tissue, which is part of a characteristic desmoplastic response initiated by the malignant epithelial cells. Occasionally, necrosis may be seen within these tumors [8].

As previously stated, malignancy is a significant concern with neoplasms in the vicinity of the prepuce and vulva. These tumors tend to recur at sites of excision [8,9,12,85,86], and readily and quickly metastasize to internal lymph nodes and viscera, such as the lung (Fig. 24.13), even prior to presentation [12,78,85,86]. For this reason, any possible neoplasms in the area of the prepuce or perineum should be removed quickly following identification and submitted for pathologic examination. Adenocarcinomas at these sites warrant a guarded prognosis at best, while benign cysts or adenomas warrant a good prognosis.

Fig. 24.13.  Large nodule of metastatic apocrine gland carcinoma within the lung of a ferret. (H&E 60×.)

Ceruminous Gland Adenocarcinoma

Ceruminous glands are specialized apocrine glands of the ear canal, and neoplasms of these glands are quite similar to other apocrine neoplasms of the head and neck. A complex ceruminous gland adenocarcinoma was described in a 7-year-old, spayed female ferret [87]. The affected ferret had a right-sided head tilt, circled to the right, and was ataxic. A pedunculated mass was present within the ear canal and penetrated the tympanic membrane.

The mass was 2 cm in diameter, red, firm, hairless, and occluded the left external ear canal [87]. Microscopically, the mass consisted of poorly defined lobules of acini, supported by a fibrous stroma. The acini were lined by single to double layers of cuboidal to polygonal epithelial cells, surrounded by an intact basement membrane, and filled with eosinophilic secretions. Mitotic figures were rare. Bundles of proliferating myoepithelial cells surrounded and separated acini, and areas of cartilage were present within the fibrous stroma [87]. The presence of myoepithelial cells resulted in the diagnosis of complex ceruminous gland adenocarcinoma; however, the presence of cartilage within the neoplasm and the lack of metastasis or mitotic figures suggest that a diagnosis of mixed ceruminous gland adenoma might have been more appropriate.

Cutaneous Hemangioma and Hemangiosarcoma

Vascular neoplasms are occasionally seen in the skin of ferrets, and reports appear to be evenly divided between hemangioma and hemangiosarcoma [12,16,78,88]. As opposed to dogs and cats, in which vascular tumors are often seen in lightly haired or pigmented skin exposed to UV light, ferret vascular tumors usually have no history of UV exposure. Moreover, actinic damage, which often surrounds sun-induced vascular neoplasms in other domestic species, has not been reported in ferrets.

Grossly, vascular neoplasms appear as well-demarcated blue to black raised lesions within the skin. There is no sex or age predilection. Microscopically, they are composed of narrow blood-filled spaces lined by well-differentiated endothelial cells with various amounts of intervening collagen. Criteria for the diagnosis of benign versus malignant lesions are not well defined, and vascular hamartomas and telangiectatic vessels [89] can further confound the issue. Criteria associated with malignancy in vascular neoplasms include the presence of mitotic figures, cytologic atypia, piling up of endothelial cells, and protrusion into vascular lumina. The presence of well-defined collagenous or smooth muscle wall around vessels suggests the possibility of hemangioma or vascular hamartoma. Telangiectasia of normal vessels in the superficial dermis has been identified in association with adrenal disease in ferrets [89].

Following complete excision, the prognosis of both benign and malignant vascular neoplasms is good. Only one case of metastatic cutaneous neoplasia has been recorded [88], and no cases of primary visceral hemangiosarcoma with metastasis to the skin has been documented to date.

Leiomyoma/Leiomyosarcoma

Leiomyomas and leiomyosarcomas are neoplasms arising from smooth muscle. Neoplasms of smooth muscle may arise in any organ, but are most commonly seen in the reproductive, gastrointestinal, endocrine, and integumentary systems [9,11,12]. Leiomyomas and leiomyosarcomas compose a significant portion of spindle cell neoplasms of the adrenal gland as previously discussed, and will subsequently be also discussed under tumors of the digestive and reproductive systems.

While early reports suggested that smooth muscle neoplasms of the skin arise from the smooth muscle of cutaneous vessels, recent reports have identified these neoplasms primarily as piloleiomyosarcomas, arising from the arrector pili muscles of the hair follicle [90–92]. These neoplasms are reported to occur more often in males, and more often along the head, neck, or dorsal midline (sites where arrector pili muscles are most prominent). Neoplasia in adult ferrets between the ages of 2 and 6 years have been reported to be affected.

In ferrets, reported smooth muscle neoplasms have all been classified as piloleiomyosarcomas, low-grade malignancies which are locally invasive, but rarely metastasize. The neoplasms are found associated with hair follicles, although multiple sections may be required to demon­­strate their proximity. Grossly, the neoplasms are well-demarcated dermal nodules which rarely ulcerate and occasionally infiltrate subcutaneous tissues. Microscopically, the neoplasms are composed of unencapsulated interweaving bundles of smooth muscle cells with moderate atypia and a mitotic rate greater than 2 mitoses per 400× field (Fig. 24.14). Neoplastic cells exhibit cytoplasmic immunoreactivity to smooth muscle actin and desmin. Metastasis to regional nodes or viscera has not been reported [12,90–92].

Fig. 24.14.  Piloleiomyoma-hair follicles are surrounded by spindled-shaped smooth muscle cells with large nuclei and abundant eosinophilic cytoplasm. (H&E, 160×.)

A syndrome of multiple piloleiomyomas has recently been described in a 2-year-old female ferret [92]. In this case, multiple smooth muscle neoplasms arose over the tail, and over the next 3 years, additional lesions appeared in the vicinity of the scar and then further rostrally over the trunk and thorax. Grossly, lesions appeared as raised cords in the dermis, histologically composed of bundles of proliferating smooth muscle which replaced continuous piloadnexal unit. The neoplasms exhibited minimal anisokaryosis and a low mitotic rate, consistent with benign smooth muscle tumors.

Piloleiomyosarcomas are low-grade, poorly infiltrative neoplasms with minimal metastatic potential. Complete surgical excision of these neoplasms is considered curative.

Fibroma and Fibrosarcoma

Fibroblastic neoplasms have been occasionally reported in the skin of the ferret [9,11,12,93,94]. In recent years, there have been two reports of fibrosarcomas arising at the site of a previous vaccination [93,94].

Previous reports of fibrosarcoma not associated with vaccination have shown a predominance of males without any age predilection [11,78]. The affected sites included the face, neck, axilla, and thorax. Fibromas in ferrets were mostly well circumscribed in the dermis or subcutis. These neoplasms are composed of unencapsulated nodules of spindle cells with variable degrees of atypia and variable amounts of collagen. Fibroblasts in fibrosarcomas are less well differentiated, with prominent nuclei and some mitotic activity. Descriptions of these early reports, in the absence of immunohistochemical investigation, suggest a possibility that some, if not all, of these neoplasms may actually be of smooth muscle origin.

While most fibrosarcomas have a dense collagenous stroma, a few myxosarcomas (a variant of fibrosarcoma with abundant mucinous ground substance) have also been described [11]. A single case of acrochordon (also known as a skin tag or fibrous polyp) has been reported in a large collection of ferret skin tumors [78].

A series of seven fibrosarcomas arising at sites of previous vaccination has been reported in ferrets, as well as three additional fibrosarcomas unassociated with previous vaccination [93]. Vaccination-site fibrosarcomas were identified by the presence of macrophages with intracytoplasmic vaccine material and/or the presence of prominent lymphoplasmacytic infiltrates. As compared with non-vaccine-related fibrosarcomas, vaccination-derived neoplasms had a higher level of cellular pleomorphism, multinucleated neoplastic cells, and, occasionally, expression of smooth muscle actin and desmin. These findings suggest that, like in cats, postvaccinal inflammation may result in tumorigenesis of local pluripotent mesenchymal cells.

Peripheral Nerve Sheath Tumor

Peripheral nerve sheath tumors are tumors of the skin and soft tissues in the ferret. These neoplasms are seen primarily in the skin of the face, particularly in the areas around the eyes, likely as a result of the numerous small nerves in this area [12]. These neoplasms are low-grade malignancies with low metastatic potential; however, like other soft tissue sarcomas, repeated surgical excisions often result in more aggressive behavior by remaining neoplastic cells. Designation of benign versus malignant peripheral nerve sheath tumors is generally unnecessary, as all of these neoplasms are of low-grade malignant potential.

Grossly, these neoplasms appear as variably sized, firm, nonulcerated dermal nodules which may be anchored to underling tissues. Microscopically, the neoplasms are composed of interwoven streams and bundles of spindle cells with moderate amounts of finely vacuolated, eosinophilic cytoplasm. These neoplasms occasionally have alternating areas of poor cellularity and increased cellularity (Antoni A and B areas, respectively), as well as areas in which nuclei of adjacent cells palisade in one plane, known as “Verocay bodies.” Nuclei are irregular with finely stippled chromatin; anisokaryosis and mitotic rate increase markedly following unsuccessful surgical interventions.

Due to their locally infiltrative nature, many of these neoplasms recur following initial surgery, and often become highly aggressive and unresectable after several attempts.

Squamous Cell Carcinoma

Squamous cell carcinomas are malignant neoplasms of squamous epithelium. In ferrets, it is a relatively rare neoplasm, arising both in the skin and in the oral cavity (to be discussed under Gastrointestinal Tumors) [7,11,12,78,95–97]. Squamous cell carcinoma has also been reported as arising from the squamous epithelium lining the anal sacs [12,95], as well as within long-standing basal cell neoplasms [12].

Reported cases of squamous cell carcinoma in ferrets show no apparent sex or age predilection, with reports in animals ranging from 2 to 10 years. Affected cutaneous sites included the footpads, face, thigh, trunk, and face.

Squamous cell carcinomas in ferrets present as single or multiple, gray-to-white, hard nodules or plaques in the skin. Alopecia and ulceration of the overlying skin is common, as is infiltration to underlying structures in long-standing neoplasms. Grossly, the neoplasms may resemble basal cell neoplasms. These neoplasms are usually solitary but occasionally multicentric [96,97]. Histologically, neoplastic squamous cells are arranged in sheets, nests, and irregular rete peg-like structures growing downward into the subjacent dermis. Characteristic features of squamous cell carcinoma include marked dysplasia, infiltration of keratinized neoplastic cells through the basement membrane, a high mitotic index, formation of horn cysts, and marked neutrophilic inflammation (Fig. 24.10 and Fig. 24.12). Neoplasms are composed of large, polygonal, often keratinized epithelial cells with prominent nuclei, with nucleoli and intercellular bridges (desmosomes) often visible between neoplastic cells. Mitotic index is often high. Squamous cell carcinomas often have a prominent neutrophilic inflammatory component as a result of exposure of keratin within the dermis.

Squamous cell carcinomas are generally locally invasive and may metastasize to local lymph nodes and distant organs [96]. If treated early, wide surgical excision may be curative; chemotherapy and radiation of squamous cell carcinomas in the ferret have not proven effective.

While the majority of squamous cell carcinomas in ferrets are infiltrative, a recent report describes the first squamous cell carcinoma in situ, as well as a possible viral etiology. In situ squamous cell carcinomas (also known as Bowenoid in situ carcinoma (BISC), or Bowen's disease, is a full-thickness dysplastic lesion with loss of normal epithelial stratification. These lesions may ultimately progress to infiltrative squamous cell carcinomas. Most commonly seen in cats and humans, these plaques are often associated with papillomavirus infection in cats. In one report [97], neoplastic cells exhibited morphologic changes consistent with “koilocyte” formation, a cytopathic effect associated with papillomavirus infection. Affected cells are swollen with abundant clear cytoplasm and 5–7 μm intranuclear viral inclusions. A novel papillomavirus in crystalline array was identified on ultrastructural examination and nuclei exhibited immunoreactivity for papillomavirus antigens.

Adrenocortical-Like Cutaneous Neoplasms

There is a report of neoplasia [98] in two unrelated adult female ferrets of ventral abdominal subcutaneous masses resembling tumors of the adrenal cortex. Neoplasms are composed of islands of polygonal cells separated by interlacing streams of spindle cells, which are reminiscent of ferret adrenocortical tumors with smooth muscle proliferation. Neoplastic cells demonstrate strong cytoplasmic reactivity for inhibin and weak cytoplasmic reactivity for pancytokeratin and S-100 protein; spindle cells exhibit cytoplasmic reactivity for alpha smooth muscle actin, muscle-specific actin, desmin, and glial fibrillary acidic protein. Ultrastructurally, vesicular tubular mitochondria were found in polygonal cells in one neoplasm. The given differential diagnosis for this unusual cutaneous neoplasm was an ovarian gonadal stromal tumor. Neither ferret had a clinically detected primary adrenal gland tumor nor clinical signs of adrenal-associated endocrinopathy.

Lipoma

Lipomas, tumors of well-differentiated adipocytes, have occasionally been identified in the skin of ferrets [11,12]. Grossly, peripheral nerve sheath tumors may resemble lipomas in their fluctuant nature, bland appearance on cut section, and rare infiltration of adjacent structures. Surgical excision, even if not totally complete, is usually curative. Liposarcoma, neoplasms of malignant adipocytes, have not been identified in the skin of ferrets, but one neoplasm has been previously reported in the marrow cavity of the jaw of ferrets (see Chapter 14).

Melanocytoma

There is a single report [99] of a solitary melanocytoma in the dorsal lumbar skin of a 4-year-old, spayed female ferret. The neoplasm was composed of densely packed polygonal to spindle cells with varying amounts of intracytoplasmic melanin pigment, as demonstrated by Warthin-Starry and Fontana-Masson histochemical stains. Neoplastic stains were immunoreactive for S-100 protein and vimentin. A low mitotic rate (1 per 10 400× fields), as well as a lack of recurrence following surgery, or metastatic disease, suggests a benign nature for this neoplasm.

Digestive System

Neoplasms have been described in every segment of the digestive tract, from the oral cavity to the anus, as well as in the digestive glands, including liver and pancreas. Lymphoma is the most common neoplasm of the digestive system, as well as the most common malignancy [12]. Primary neoplasms are less common and will be described herein.

Neoplasms of the Oral Cavity

Squamous cell carcinomas are the most common neoplasm of the ferret oral cavity [11,12,100–102]. As in cats (in which infiltrative squamous cell carcinoma is also the most common neoplasm of the oral cavity), these malignancies originate in the gingiva and infiltrate the alveolar and, ultimately, lamellar bone of the jaws, resulting in pain, difficulty in prehension and mastication, and pathologic fracture. Squamous cell carcinomas of the maxilla have been reported to invade and destroy the adjacent tissues, including the cranial bones and nasal turbinates [102].

Grossly, these tumors appear as firm, invasive tumors of the gingiva which result in tooth loss, halitosis, and, eventually, distortion of the maxilla or mandible. Microscopically, these tumors resemble their counterparts in the skin, with sheets of keratinized squamous epithelium with large nuclei and multiple prominent nucleoli, and often a profound neutrophilic response, engendered by the presence of keratin fragments within tissue [100–102].

While these neoplasms have low metastatic potential, their aggressive infiltration within the oral cavity requires equally aggressive and invasive therapeutic measures. Various techniques of surgical excision, including rostral maxillectomy, chemotherapy, and radiation therapy, have not proven successful [101,102], as local recurrence has proven formidable. For this reason, a guarded to poor prognosis is warranted in these cases.

Various forms of sarcomas have also been documented in the oral cavity [9,12] but, due to their infiltrative nature, have not responded to treatment.

Gastric Neoplasms

Neoplasms of the stomach may arise from a variety of cell types common to the stomach. Lymphoma is the overall most common neoplasm of the stomach in ferrets, both arising as a primary neoplasm [22,103,104] (to be described later) or as part of a neoplasm with widespread metastasis [12]. Neoplasms of the mucosal epithelium are occasionally seen in ferrets [12,24,105–107], and rarely, neoplasm of stromal tissues of the stomach may be seen [108].

Gastric adenocarcinomas are neoplasms of the epithelium of the mucosal lining and have been occasionally reported [9,11,12,105–107]. These aggressive malignancies often invade transmurally with local extension into surrounding tissues as well as distant metastasis. They often incite a prominent schirrous response, which easily differentiates them from gastric lymphoma on gross inspection.

Gastric adenocarcinomas have been epidemiologically associated with gastric Helicobacter spp. infection in humans and in selected species of animals [23,24]. This association has been corroborated by the fact that reported cases of gastrointestinal carcinoma have originated in the pyloric stomach, where helicobacter colonizes most heavily [11]. A study in ferrets to investigate the carcinogenic potential of Helicobacter pylori in the development of human gastric adenocarcinoma demonstrated that 90% of ferrets infected with H. mustelae and dosed with a known gastric carcinogen developed gastric carcinomas [23].

Reports indicate that gastric adenocarcinoma is a disease of animals over 2 years of age. Clinical signs include lethargy, anorexia, vomiting, weight loss, dehydration, and a bloated abdomen [24,105–107], and rarely, anemia. Physical examination often reveals a mass in the cranial abdomen; survey and contrast radiographs demonstrate a distended stomach due to stenosis in the pyloric region. Gross examination will confirm radiographic interpretation, with a mildly to markedly distended stomach, and a 2–3 cm white-yellow plaque-like constriction in the areas of the pylorus. The constriction is fibrous and firm, and may be gritty in areas due to ossification [107]. The neoplasm may result in fibrous adhesions to adjacent tissues, including mesentery, pancreas, intestine, and liver.

Microscopically, there is often transmural infiltration of well-differentiated epithelial cells, often forming glands and acini within a dense fibrous stroma. A prominent lymphoid infiltrate, which may form lymphoid follicles, is occasionally seen as a result of absorption of antigens across the devitalized gastric mucosa. In some cases, neoplastic epithelium may contain large mucin vacuoles (“signet-ring cells”), or the neoplasm may contain large lakes of mucin in the stroma, in so-the called “mucinous” adenocarcinomas. Subclassification of gastric adenocarcinomas as “tubular,” “signet ring,” and “mucinous” are common but yield no prognostic implication. Some carcinomas contain large areas of osseous metaplasia within the stroma. Invasion of and adhesion to other tissues is common; metastasis to local lymph nodes and portal vein has also been noted [105]. While generally unnecessary, neoplastic epithelial cells will exhibit strong cytokeratin immunoreactivity, which may help to demonstrate individual cell infiltration in areas of dense desmoplasia or in regional lymph nodes.

Due to the desmoplastic response and confinement of neoplastic cells in a well-demarcated area, surgical resection may be considered in the absence of obvious metastasis or local invasion. The presence of adhesions to adjacent organs should warrant a poorer surgical prognosis.

Lymphoma is the most common neoplasm of the gastrointestinal tract [11,12], and may have a slightly different pathogenesis in the stomach than in the rest of the gastrointestinal tract [9,23]. In one report [22], similarities were noted between mucosal-associated lymphoid tissue (MALT) lymphomas in the human stomach and those noted in H. mustelae-infected ferrets. In the four animals in this report, the lymphomas were of a monoclonal B-cell phenotype (based on a κ-clonal shift) which contrasted with the 1 : 1, κ : λ ratio seen in ferrets with simple helicobacter gastritis. The authors postulate that these B-cell lymphomas develop from a T-cell-mediated mechanism during chronic helicobacter infection, similar to the mechanism proposed for H. pylori-associated gastric MALT lymphomas in humans [11,22,24,104].

Grossly, lymphomas of the stomach appear as a focally extensive to multifocal thickening of the gastric wall which blend imperceptibly into surrounding tissue. Gastric rugae may be thickened, and affected tissue is white and often bulges on cut section. Microscopically, the neoplastic lymphocytes range from well-differentiated to large immature lymphocytes. T-cell (CD3) and B-cell (CD 79A, PAX-5) may be used in ferrets to determine immunophenotype.

Treatment and prognosis of gastric lymphoma is based on the immunophenotype and proposed etiology. Multicentric lymphomas, in which the neoplastic cells are present in other organs, generally carry a poor prognosis. As a general rule, chemotherapy is more successful in animals whose neoplastic lymphocytes are of a more differentiated phenotype. In general, however, lymphomas of the gastrointestinal tract generally have shorter survival times than those of most other organ systems as a result of the inevitable consequence of devitalization of the mucosal border and absorption of bacteria and toxins directly from the lumen into the bloodstream [12]. Solitary MALT lymphomas in humans often respond to antibiotic therapy directed at eradicating the presence of the inciting helicobacter agent; similar attempts in ferrets have not been well documented but hold logical promise.

Small Intestinal Neoplasms

Intestinal neoplasms are much similar to those seen in the stomach, with lymphomas being the most common, and adenocarcinomas of the intestinal epithelium occasionally seen. The gross and microscopic aspects of these neoplasms are similar, with desmoplasia and stenosis a major differentiating factor.

A single case of gastrointestinal stromal tumor (GIST) has been reported in ferrets [108]. These neoplasms have been reported in the human, nonhuman primates, horses, and dogs, and a variety of exotic species. GISTs are tumors of the stroma which are derived from the interstitial cells of Cajal, which serve as pacemakers for gastrointestinal motility. There is interspecies variation for location, with most neoplasms in the human being present in the stomach, and most canine GISTs being present in the intestine or cecum.

In this particular case, the neoplasm was present in the ileum. As is typical in other species, this neoplasm originated within the tunica muscularis and extended outward. A characteristic gross feature of most GISTs is outward growth, the neoplasms often extend outward from the wall of the gastrointestinal tract; marked encroachment into the lumen is uncommon.

Microscopically, the neoplasm is composed of broad bands of spindle cells with prominent cigar-shaped nuclei and a variable mitotic rate. The major differential is leiomyoma or leiomyosarcoma; it is possible that neoplasms of smooth muscle origin reported prior to the widespread use of immunohistochemistry [11] may have been misdiagnosed as being of smooth muscle origin.

The most specific way to make this diagnosis is via the use of immunohistochemistry. These neoplasms are differentiated from tumors of smooth muscle or peripheral nerve sheath origin by their strong immunoreactivity for c-kit (CD 117), a protein kinase receptor encoded by the proto-oncogene c-kit, and strongly expressed by neoplastic cells in GISTs but not other mesenchymal neoplasms of the gastrointestinal tract [108]. While these neoplasms are considered to be low-grade malignancies, the animal in the single report was considered disease free 4 years following surgical resection [108].

Colonic Neoplasms

Neoplasms of the colon and rectum are similar to those in more proximal regions of the gastrointestinal tract, with lymphoma seen most commonly, and adenocarcinoma seen occasionally.

There is a single case report of an adenomatous polyp of the colon in a ferret [109]. Polyps are non-neoplastic proliferation of intestinal or colonic glands on a fibrous stroma. Polyps may result in gastrointestinal disease as a result of luminal encroachment, local stasis, or ulceration with pain and absorption of luminal bacteria and toxins.

In this particular case, clinical signs were limited to intermittent diarrhea, staining, and a mild blood loss anemia. The polyp, measuring 1.5 × 2 cm was palpable within the abdomen. At surgery, the polyp was revealed to be present in the descending colon and to have caused an intussusception. The intussusception and polyp were removed en bloc and anastomosis was performed. The animal recovered uneventfully and no further gastrointestinal issues were observed for the remainder of its life [109].

Microscopically, the polyp was composed of tortuous but normal colonic glands and abundant fibrovascular stroma. One unusual finding was the presence of smooth muscle, which differs from other species. With such a limited sampling, few conclusions should be drawn regarding gastrointestinal polyps in ferrets until other cases are published.

Mesenchymal Neoplasms of the Abdominal Cavity

Mesenchymal neoplasms are occasionally encountered as free-floating masses within the abdominal cavity of the ferret [10–12,110,111]. The majority of these neoplasms are of smooth muscle origin, although the organ from which they developed cannot be identified. Neoplasms are most often composed of long streams and bundles of spindle cells with an elongate nucleus, finely stippled chromatin, and a variable mitotic rate. Necrosis is often seen within the neoplasm, which may have severed connection to original tissue of origin. An origin from the smooth muscle of mesenteric vessels has been proposed [12]. While these neoplasms exhibit cellular features of low-grade malignancy, including a brisk mitotic rate, metastasis has not been documented.

There has been one report of a malignant intra-abdominal mesenchymoma surrounding nonabsorbable sutures placed in the abdomen during ovariectomy 60 months previously [111]. The term “mesenchymoma” in this report was used to describe a poorly differentiated sarcoma which had areas of bone formation, but lacked immunoreactivity for smooth and striated muscle markers (actin, desmin, and myoglobin). The diagnosis of mesenchymoma in this case was based on the presence of two distinct cell lines of differentiation within the same mesenchymal neoplasm, in this case “osteosarcomatous” and “fibrosarcomatous.” Evidence of metastasis was not seen in this case.

Hepatic Neoplasms

Hepatic neoplasms are seen with some frequency in ferrets. In a large retrospective study, neoplasms of the liver comprised 4.5% of total neoplasms in ferrets [12]. Both primary and metastatic neoplasms are well documented, with malignant lymphoma being the overall most common neoplasm in the liver [11,12]. The liver is also a common site of metastasis for malignancies of other organs, with metastatic adrenal carcinoma [12,56], exocrine pancreatic carcinoma, hemangiosarcoma [112,113], insulinoma [12], and a variety of poorly differentiated carcinomas having been reported in the liver.

Primary hepatocellular carcinomas are those derived from liver-specific tissues, to include hepatocytes, and biliary epithelium.

Hepatocellular Neoplasms

Both benign and malignant neoplasms of hepatocytes have been reported in ferrets, with reports of hepatocellular carcinomas [5,7,11,12,114,115] significantly outnumbering those of hepatocellular adenoma [7,12]. Hepatocellular carcinoma appears to be a disease of adult ferrets without gender predilection [5,7,12,114,115]. Clinical signs consisted of lethargy, anorexia, and weight loss, and often a cranial to midabdominal mass may be palpable. While elevations in liver-specific enzymes, such as alanine aminotransferase or alkaline phosphatase may be elevated, in most cases, clinicopathologic findings are variable and nonspecific [12]. Survey radiographs may show a mottled abdominal density or enlarged liver outline; the presence of the neoplasm is usually readily apparent on ultrasound examination.

Grossly, hepatocellular carcinoma results in marked hepatomegaly (up to four to five times its normal weight in one case [68]), with white to gray nodules of variable number, size, and shape in the parenchyma. Ascitic fluid may be present. Histologically, carcinomas were composed of large cuboidal to pleomorphic cells that were arranged in acini, sheets, or cords. Metastasis to the spleen and the mesenteric lymph node were observed [68]. Hepatocellular adenoma was diagnosed as an incidental in a 5-year-old, male ferret [68]. The tumor mass was well circumscribed and consisted of pleomorphic epithelial cells resembling normal hepatocytes.

Biliary Neoplasms

In one review [12], neoplasms of the biliary system outnumber those of hepatocellular origin. These lesions occupy a continuum between common benign biliary cysts, benign cystadenomas, and frankly infiltrative cholangiocellular carcinoma.

Biliary cysts are common focal or multiple subclinical and incidental lesions in ferrets. Biliary cystadenomas are the most common neoplasms of the ferret biliary tract, but pose special problems, as they commonly exhibit infiltrative growth, replacing one or more lobes of the liver. Grossly, the two lesions are easily differentiated, as cystadenomas often arise at the edge of liver lobes and are composed of interconnected oval to spherical cysts with minimal intervening fibrous stroma. Cysts often contain clear intraluminal fluid, which is rarely opaque. On microscopic examination, the two lesions may look similar, as both are composed of large empty cystic spaces lined by attenuated epithelium [3,12,68,116].

Pathologists attempting to differentiate biliary cysts, biliary cystadenoma, and biliary cystadenocarcinoma on limited surgical biopsies should closely scrutinize the clinical history for the presence of hepatic-specific clinical symptoms, clinicopathologic abnormalities, or expansive growth over time documented by progressive growth via abdominal palpation or ultrasound [16]. A diagnosis of cystadenocarcinoma should be considered in cases in which there is infiltrative growth and replacement of extensive amounts of hepatic tissue even in the absence of cellular features of malignancy.

True cholangiocellular carcinoma is far easier to diagnose—these neoplasms are composed of acini lined by cuboidal epithelial cells with large nuclei and a variable mitotic rate. Unlike cystadenomas or those of hepatocellular origin, these carcinomas are usually associated with a prominent desmoplastic response and, at diagnosis, are often in advanced stages resulting in elevations of alanine aminotransferase and other liver-specific enzymes [16].

One report describes the presence of biliary malignancies (two cholangiocellular carcinomas and two biliary cystadenomas) in a series of eight ferrets with chronic cholangiohepatitis arising from chronic infection with a novel Helicobacter species [117]. The connection between chronic infectious cholangiohepatitis is an interesting one and has been identified in other species with hepatic infections with helicobacter (i.e., specific inbred strains of mice infected with Helicobacter hepaticus) [118].

Pretreatment with vitamin K should precede hepatic biopsy whenever possible in animals with clinical liver disease [16]. Surgical removal of any cystic lesion of the liver should be accomplished with wide margins due to the aggressive nature of biliary cystadenoma in this species. The long-term prognosis of any hepatic neoplasm involving multiple lobes is poor [12].

Pancreatic Neoplasms

A variety of proliferative lesions may be seen in the acinar or exocrine pancreas (the cells which manufacture, store, and ultimately secrete digestive enzymes). Hyperplasia of the exocrine pancreas is a nodular aging change which is seen in other species and is considered to be an incidental finding. Of the true neoplasms, malignant neoplasms (pancreatic adenocarcinoma) far outnumber benign neoplasms in this organ.

Pancreatic exocrine adenocarcinomas are aggressive neoplasms in ferrets which arise in adult animals without apparent gender predilection [4,5,7,9–12,114,119–121]. These neoplasms tend to spread widely, often by a process known as carcinomatosis, which is the seeding of the abdominal cavity with hundreds or thousands of small nests of neoplastic cells, often in the absence of visceral metastasis [121]. In other cases, visceral metastasis may be widespread [120]. The neoplasm is often accompanied by significant ascites.

Clinical signs included lethargy, anorexia, and distended abdomen; clinicopathologic abnormalities were nonspecific. Radiography and ultrasonography often revealed widespread abdominal masses in the pancreas and other abdominal viscera, with variable degrees of peritoneal effusion.

Grossly, nodular masses of variable number and size are firm to friable, white to tan, and up to 3 cm in diameter in the pancreas. The masses may also be observed in the abdominal wall, diaphragm, mesentery, omentum, and gastrointestinal wall. The pancreas is often effaced by the neoplasm and may be difficult to identify (Fig. 24.15).

Fig. 24.15.  Exocrine pancreas: Neoplasms of the exocrine pancreas often are multinodular, efface pancreatic tissue, and may explant throughout the abdomen.

Microscopically, neoplastic cells are mildly pleomorphic but often retain an acinar structure in parts of the neoplasm, although in some regions, the neoplastic cells may be arranged in sheets. The loss of acinar architecture may be an important diagnostic feature in distinguishing well-differentiated neoplasms from areas of pancreatitis. In most tumors, neoplastic cells contain variable amounts of zymogen granules within their cytoplasm, although they may stain more lightly than in adjacent normal pancreas. These zymogen granules are demonstrable on ultrastructural examination, along with apical microvilli [120]. Nuclear size, size and number of nucleoli, and the mitotic rate are often variable. Focal necrosis, cyst formation, and intralesional osseous metaplasia have been described [120,121]. Regional lymphatic and distant pulmonary metastases have also been observed.

Pancreatic adenomas are rare incidental findings in ferrets, and are differentiated from foci of nodular hyperplasia largely by size and the presence of a fibrous capsule [5,11,68]. Grossly, they are single to multiple, white to gray, tan, round to oval nodules. Neoplastic cells resembled normal pancreatic acinar cells and maintain a typical acinar structure. Differentiating these from foci of exocrine hyperplasia is often arbitrary.

Salivary Gland Adenocarcinoma

Salivary gland carcinomas have been diagnosed in two ferrets [8,68]. The neoplastic nodule observed by the authors was poorly demarcated and locally infiltrative to the adjacent salivary parenchyma and other soft tissues. The neoplastic cells were arranged in acini, nests, or trabeculae. The cytoplasm was eosinophilic and contained eosinophilic granules in many cells. The nuclei were round to oval, with a high mitotic index.

Reproductive System

Tumors of the reproductive system are generally underreported in the veterinary literature due to the practice of early neutering in this species, both in the pet and laboratory trade. Furthermore, in breeding operations, the number of intact females far exceeds that of males. For this reason, data concerning the overall incidence of neoplasia in the reproductive system of ferrets are likely skewed, both as far as overall incidence, as well as in the male : female ratio.

In the female reproductive tract, neoplasms of smooth muscle are overrepresented, both in the ovary and in the uterine tube [7,11,12,68] Nonmuscular neoplasms of the uterus are extremely rare in ferrets [9]. Most smooth muscle neoplasms in the female reproductive tract have been reported as low-grade malignancies based on cytologic characteristics; however, metastasis has not been reported [12]. Other than reproductive failure, most neoplasms of the reproductive tract are clinically silent and are often considered incidental findings.

Testicular neoplasms are the most common neoplasms of the male reproductive tract [7,11,12,68] with no clear determination of which is most common. Neoplasms of the accessory sex glands (i.e., prostate) have not yet been reported in ferrets.

Neoplasms of the Female Reproductive Tract

Ovarian Neoplasms

Ovarian Neoplasms of Smooth Muscle

A wide variety of mesenchymal neoplasms have been identified in the ovary of the ferret [3,7,11,12,68,122–124]. These early reports may be confusing, as a range of terminology is present based on the presence or absence of a fibrous stroma, leading to the diagnosis of “fibromyoma” [123] and “fibroleiomyoma” [124], as well as “fibromatoid tumor” [3]. Other spindle cell neoplasms of the ovary, such as thecoma and ovarian stromal tumors, have been described [125]. The overlapping morphology of the spindle cell tumors of the ovary, coupled with the lack of immunohistochemical proof of origin, lends additional confusion to previous reports.

Leiomyomas and leiomyosarcomas (as well as their fibrous variants ) are spontaneous neoplasms of the ovaries of adult female ferrets [4,8,10–12,16,123–125], although they have been identified in 20 individual ferrets from a single colony [125]. Ovarian smooth muscle neoplasms are single or multiple, firm, solid, up to 2 cm in diameter, and white on cut section. Microscopically, neoplasms appear to be more common at the ovarian hilus extending into the ovarian stroma. Neoplastic cells are present in long streams and bundles, and have elongate nuclei, often with blunt ends. Mitotic figures are typically rare. In one study [12], low-grade malignant tumors predominated, while in others, benign tumors were more common [7,11,68], suggesting a variation in interpretation of cellular features of malignancy between observers. Fibroblasts and a dense fibrous stroma between neoplastic cells may warrant a diagnosis of fibroleiomyoma based on histologic grounds.

Definitive diagnosis of ovarian smooth muscle tumors requires demonstration of immunoreactivity for smooth muscle markers, including smooth muscle actin and desmin. Calponin, a marker for myofibroblasts, may also be useful in identifying a true fibroblastic component.

Cystic endometrial hyperplasia was noted in a ferret with ovarian fibroleiomyoma [125], but no cause-and-effect relationship was proven.

Ovarian Sex-Cord Stromal Tumors

Sex-cord stromal tumors of the ovary [7,11,12,16,68,124,126] are neoplasms of ovarian cells which surround the developing oocytes. These neoplasms are often functional, secreting estrogen, androgens, or their intermediates. As in other steroid producing cells, these neoplasms are often yellow to orange in color and, microscopically, have prominent lipid vacuoles.

In one report [125], abdominal neoplasms were identified in two ferrets which exhibited evidence of bilateral truncal alopecia. In one of these ferrets, high levels of androstenadione and 17-hydroxyprogesterone resolved following surgical removal, and a normal haircoat was restored.

Microscopic evaluation of the two neoplasms revealed biphasic neoplasms composed of prominent spindle cells admixed with polygonal epithelial cells with foamy cytoplasm, characteristic of a typical sex-cord stromal tumor [125] (Fig. 24.16, Fig. 24.17, and Fig. 24.18). No further differentiation was made beyond obvious morphologic characteristics.

Fig. 24.16.  Ovary (H&E, 100×): An ovarian sex-cord stromal cell tumor with complete effacement and replacement of normal ovarian tissue by a predominant spindle cell component with abundant stroma.

Fig. 24.17.  Ovary (H&E, 400×): A higher magnification of Figure 24.16 showing high cellularity with a rich stroma that is composed of mostly intersecting fascicles of spindle cells with some embedded pale to foamy cells.

Fig. 24.18.  Ovary (H&E, 400×): Another segment of the ovarian sex-cord stromal cell tumor showing a predominant sex-cord component comprising of sheets of polygonal cells with abundant cytoplasm that is frequently pale staining to foamy in appearance.

Thecomas are benign ovarian sex-cord stromal neoplasms which resemble cells of the theca interna of the ovarian follicle. One affected ferret also had endometrial hyperplasia [122]. Three thecomas have been diagnosed in ferrets [8,66,124]. These neoplasms were firm, smooth, and lobulated, with a homogenous yellow cut surface; one of these was bilateral and associated with cystic endometrial hyperplasia [124].

Sex-cord stromal tumors and thecomas may resemble tumors of smooth muscle on cursory microscopic examination, but unlike smooth muscle tumors, the former are immunoreactive for inhibin and negative for desmin.

Granulosa cell tumors are a third form of sex-cord stromal neoplasm which has occasionally been documented in ferrets [5,7,9,68]; however, clinical signs have not been reported in association with these neoplasms. Grossly, these neoplasms are often cystic and hemorrhagic. Microscopically, the neoplasms are composed of small cells arranged in sheets, which occasionally form rosettes (Call–Exner bodies). One case of arrhenoblastoma, a virilizing form of granulosa cell tumor, has been described in ferrets [5].

Neoplasms of the Ovarian Epithelium

Ovarian adenocarcinomas arise from the surface epithelium covering the ovary or epithelial nests within the ovarian cortex. One case of adenocarcinoma has been reported composed of cystic acini on a dense fibrous stroma [66]. The neoplastic epithelial cells lining the acini were cuboidal, with numerous papillary infoldings. While epithelial neoplasms of the ovary are considered to be aggressive, especially in women, metastasis was not reported in the affected ferret.

To date, metastasis has not been reported in cases of ovarian neoplasia treated surgically; surgical excision therefore should be considered curative.

Uterine Neoplasms

Uterine Smooth Muscle Neoplasms

Leiomyomas and leiomyosarcomas are the most common neoplasm of the ferret uterus [7,11,12,68,126]. In one study [12], low-grade leiomyosarcoma was the most common uterine tumor. These neoplasms are nodular, may be unilateral or bilateral, and are up to 3 cm in diameter [66]. As in the ovary, neoplastic cells are arranged in long streams and bundles on a fine fibrovascular stroma. Nuclei are elliptical with blunt ends. The mitotic rate is generally low. Tumors in which the neoplastic cells are interspersed with abundant collagen and fibroblasts, that is, fibroleiomyoma, have also been reported [66].

Smooth muscle neoplasms of the uterine horn are rarely associated with clinical signs, although large neoplasms may be felt on abdominal palpation. There is a single report of hydrometra in association with a uterine leiomyoma [126]. Surgical excision is considered curative.

Other Uterine Neoplasms

A uterine teratoma has been described in a primiparous 1-year-old jill [4]. An abdominal mass noted after the weaning of her first litter was palpated in the affected ferret. The mass was located on the right uterine horn. The cut surface was gritty and cystic, with thick brownish to green fluid in the cysts. Histologically, the mass consisted of various tissues derived from all three germ layers, for example, fibrous tissues, lymphoid, adipose, sebaceous, cartilage, bone, tooth, ciliated and mucous-secreting epithelia, and dermoid tissues. The right ovary was not identifiable.

Implantation sites in the ferret uterus are unique findings which may be seen following parturition or pseudo­pregnancy in the jill. Microscopically, the marked pleomorphism of the decidual epithelium (also termed presymplasma) strongly resembles a malignant neoplasm to pathologists unfamiliar with this normal finding [12].

Mammary Gland Neoplasms

While mammary neoplasia is a relatively common finding in black-footed ferrets (Mustela nigripes), comprising approximately 7% of neoplasia in a retrospective study of a large captive population [127], it is relatively uncommon in domestic ferrets. Both benign and malignant mammary neoplasms have been reported in ferrets [9,16,128]; diagnostic schema used in diagnosis of mammary neoplasia in dogs and cats have been applied to ferrets in reported cases.

Simple and complex mammary adenomas [12,68] have been reported in ferrets. Simple mammary adenomas are expansile nodules composed of cuboidal glandular epithelium which forms tubules and acini. Within dilated glands, the epithelium may pile up or form papillary structures, but infiltration of the surrounding tissue or invasion of lymphatics or blood vessels is not seen. Complex mammary adenomas have the characteristics of simple mammary adenomas; however, a population of neoplastic myoepithelial cells is arranged around and often within neoplastic glands.

Mammary adenocarcinoma has been reported once in an adult female domestic ferret [7]. Mammary carcinomas are also composed of epithelial cells forming tubules and acini; however, these cells are more pleomorphic, exhibit marked loss of polarity, and often have a high mitotic rate. Neoplastic cells infiltrate adjacent tissues, and may metastasize both locally and to distant sites, primarily via lymphatics.

Testicular Neoplasms

Testicular neoplasms in domestic ferrets include interstitial cell (Leydig) tumors, seminomas, and Sertoli cell tumors, with interstitial cell tumors being the most numerous in two large retrospectives [10,12]. In one study, neoplasms were most numerous in cryptorchid testes [12], which is consistent with other domestic species. The presence of multiple, different neoplasms in the same testis has been reported several times [12], and one report includes a total of four testicular neoplasms (including a rare carcinoma of the rete testis) [12]. A case of testicular mixed germ cell tumor (sex-cord and germ cell type) has also been recently diagnosed by the authors by histomorphological features, although additional immunohistorchemical stains would be needed for definitive diagnosis (Fig. 24.19, Fig. 24.20, and Fig. 24.21).

Fig. 24.19.  Testis (H&E, 20×): A large high cellular and mostly well-demarcated testicular mass showing varying tinctorial appearance with a presumptive histological diagnosis of testicular mixed germ cell tumor.

Fig. 24.20.  Testis (H&E, 200×): A segment of the testicular tumor and adjacent non-neoplastic seminiferous tubules from Figure 24.19, showing a complex morphology with interspersed disorganized sheets/cords of less differentiated cells.

Fig. 24.21.  Testis (H&E, 400×): A higher magnification of a section of the testicular tumor from Figure 24.21, showing superficial packed cords with dark nuclear staining of polygonal cells blending into cords of large polygonal paler cells separated by distinct stroma.

Interstitial (Leydig) Cell Tumors

The interstitial (or Leydig) cells of the testis are responsible for the manufacture and secretion of androgens in male ferrets. These cells are normally located between the seminiferous tubules of the developed testis.

These neoplasms are seen in adult males. Testicular enlargement is the only clonical described in association with these neoplasms [5,10,12,68,128,129]. Grossly, they are firm, nodular or multinodular, yellow to orange (as a result of their manufacture of sterols), and often exhibit hemorrhage on cut section. Microscopically, neoplastic cells are arranged in irregular lobules, solid nests or sheets, and are separated by a fine fibrovascular stroma. The neoplastic cells are polygonal with moderate mounts of a foamy eosinophilic cytoplasm. Nuclei exhibit minimal variation and mitoses are rare. Neoplastic cells strongly resemble normal interstitial cells in areas of normal testis. There are often large areas of necrosis and hemorrhage with these neoplasms. Metastasis of interstitial cell tumors has not been reported, so castration is considered curative.

Seminoma

Seminomas are neoplasms of the germ cells of the seminiferous tubules, which ultimately become spermatozoa after a series of mitotic and meiotic divisions. These neoplasms have been occasionally reported in ferrets [12,129]. They may also result in testicular enlargement, and, upon cut section, tend to bulge from the surface and are soft, white, fleshy masses. Multiple or bilateral seminomas may be seen. Neoplastic cells are large and round, and usually contained within seminiferous tubules, although they may occasionally be present in sheets, effacing tubular architecture. Nuclei are round and nucleoli are prominent; mitotic figures are rare. Metastasis has not been reported in seminoma in ferrets, so castration is considered curative.

Sertoli Cell Tumors

Sertoli cell tumors are neoplasms of the sustentacular cells of the seminiferous tubules which support the developing spermatocytes. These are also neoplasms of adult male ferrets and may be unilateral or bilateral, resulting in testicular enlargement [4,5,12,128]. Grossly, these neoplasms are firm (due to a prominent fibrous stroma not present in other types of testicular neoplasia) and white. Microscopically, these neoplasms are composed of oval to spindle cells arranged in irregular tubules and/or sheets separated by dense fibrous septa. In some areas, neoplastic cells will palisade along fibrous stroma or basement membranes, reminiscent of their arrangement in mature seminiferous tubules. Atrophy of adjacent tubules may be seen, likely as a result of increased intratesticular pressure [8,128,129]. Visceral metastasis to the liver has been reported in one case of Sertoli cell tumor in a ferret [12].

Musculoskeletal System

While not a common site for neoplasia in ferrets, tumors of the musculoskeletal system of the ferret generally result in obvious clinical signs which are apparent to owners and veterinarians. Tumors of the skeletal system are relatively common in ferrets, while tumors of the muscles are relatively rare.

Chordomas

Chordomas are the most commonly reported musculoskeletal neoplasm in ferrets [5,11,12,68,130,131]. Early reports of this neoplasm in the veterinary literature misdiagnosed them as chondrosarcomas or chondromas.

Chordomas are neoplasms of the axial skeleton which arise from rests of primitive notochord, the pluripotent mesenchyme upon which the fetal skeletal is developed. These neoplasms are low-grade malignancies of adult ferrets which are primarily seen in the caudal (tail) vertebrae [5,11,12,68,130,132,133], but have been reported in other sites, including the cervical [134,135] (Fig. 24.22) and thoracic [136] spinal cord. These low-grade malignancies are slow growing, but eventually destroy the compact bone of the vertebral body, and infiltrate adjacent tissues. In the cervical and thoracic spine, chordomas result in paresis or paralysis due to spinal cord compression and/or pathologic vertebral fracture. In tail chordomas, a large mass at the tail tip is generally the only clinical sign; however, some animals may manifest pain resulting from bony destruction at the tumor site. Cutaneous, but not visceral, metastasis has been reported as a rare complication of vertebral chordoma in ferrets [134,137].

Fig. 24.22.  Cervical chordoma in a ferret. (Photo courtesy of Dr. Taryn Donovan, Animal Medical Center.)

Radiography is often helpful in the diagnosis of chordomas arising in the cervical or thoracic skeleton, but rarely necessary in those arising in the tail. In such cases, survey radiographs will show lysis of affected vertebrae, and multiple small gravel-like mineralized densities (representing areas of bone formation within the infiltrating neoplasm) in the surrounding soft tissues (Fig. 24.23).

Fig. 24.23.  Radiograph of a cervical chordoma in a ferret. Note the gravel-like radiodensities corresponding to areas of bone within the neoplasm. (Photo courtesy of Dr. Mark Finkler, Armed Forces Institute of Pathology.)

At the tail tip (the most common location), chordomas are typically hard, round, or club-like, and up to 5 cm in diameter. In other areas of the axial skeleton, they are hard masses anchored to underlying muscle and bone. Alopecia or ulceration of the overlying skin at the tail tip may be observed in larger tumors. Upon sectioning, the neoplasm is often multilobulated, with gritty areas of bone, opalescent areas of chondroid matrix, and, often, scattered areas of hemorrhage. Histologically, chordomas are multilobular neoplasms composed of three zonal components: trabecular bone in the center, surrounded centrifugally by areas of cartilaginous matrix, and bounded by an area of fibrous connective tissue. Trabecular bone is often well differentiated and bone marrow elements may be present. Neoplastic cells (known by the term “physaliferous cells”) are round to polygonal, with vacuolated foamy cytoplasm and centrally located, round to angular, hyperchromatic nuclei (Fig. 24.24). Mitotic figures are rare. Cytologic diagnosis is rarely needed in these neoplasms, but impression smears of fresh tissue may yield a rapid diagnosis at necropsy. The cytologic appearance of physaliferous cells is very similar to their histologic appearance [135,138].

Fig. 24.24.  Foamy “physaliferous cells” separated by bluish chondroid matrix border area of bone (bottom center). (H&E, 140×.)

While immunophenotyping is rarely used in their diagnosis, chordomas have an immunophenotype with epithelial and mesenchymal characteristics [137,139]. Physaliferous cells express both cytokeratin and vimentin intermediate filaments, and are variably positive for S-100 protein and neuron specific enolase.

Chondromas

Chondromas have rarely been reported in ferrets, arising in the axial skeleton as well. Chondromas are well-differentiated neoplasms of cartilage and populated by slightly pleomorphic chondrocytes in a basophilic fibrous matrix [8]. Careful attention should be paid to differentiating these very uncommon lesions from the far more common chordoma, which arises in a similar location.

Osteomas

Osteomas are slow growing, benign neoplasms of bone which arise in adult ferrets. These neoplasms generally arise on flat bones (especially of the bones of the skull) (Fig. 24.25 and Fig. 24.26), but occasionally on the thoracic vertebrae [138]. They are usually slow growing, well circumscribed, and exophytic [5,7,11,12,68,138,140–142]. While slow growing, unresected neoplasms result in compression and/or atrophy of soft tissues, including the respiratory passages or muscles of mastication.

Fig. 24.25.  Ferret mandibular osteoma: Osteomas most often present as smooth dense nodular bony neoplasms from the flat bones, often of the head.

Fig. 24.26.  Osteoma. Dorsoventral (A) and lateral (B) skull radiographs. There is a mineralized mass extending rostrally from the left tympanic bulla. (Reprinted with permission, Voe et al. (2002) Vet Radiology and Ultrasound 43(2): 346–348.)

Radiographically, these neoplasms are diagnosed as luminous bony densities attached to the surface of flat bones. They are usually more radiodense as compared with the surrounding normal bone, as they are composed of densely packed compact bone (Fig. 24.26).

Grossly, tumor masses are hard, smooth protuberances from the surfaces of flat bones. Histologically, tumor masses consist of well-differentiated, compact lamellar bone, and normal-appearing osteocytes, with scattered adipose tissue and fibrovascular stroma. The capsule of the tumors resembles the active or inactive periosteum.

Osteosarcoma

Malignant bone tumors are far less common than their benign counterparts, and documented cases are split evenly between long bones and flat bones [12,143]. These malignancies are invasive neoplasms in ferrets, and may result in invasive growth and, ultimately, pathologic fracture.

Clinical signs include pain as well as marked deformation of affected bones. Radiography generally discloses a combination of bony lysis and proliferative growth, usually in the area of the adjacent cortex, often as a result of periosteal bone growth [12].

Grossly, these neoplasms are firm with an irregular contour, often effacing the cortex of affected bones and extending into adjacent soft tissues (Fig. 24.27). Affected bones break easily, and sectioning often has a gritty feel proportional to the amount of osteoid within the neoplasm.

Fig. 24.27.  Osteosarcoma arising from a vertebral body. The extent of the preexisting vertebra is outlined in blue. (H&E, 4×.)

Microscopically, the neoplasm is composed of haphazardly arranged pleomorphic spindle cells which are multifocally aligned along or within a pink, eosinophilic matrix (osteoid). Neoplastic cells often have large nuclei, prominent nucleoli, and frequent mitotic figures. Adjacent lamellar or compact bone is often fragmented and in the process of resorption.

If neoplasms arise in the appendicular skeleton, amputation is the most appropriate form of therapy [16]. Metastasis has not been reported in ferrets; however, the number of neoplasms of this type is small.

Rhabdomyosarcomas

Rhabdomyosarcomas are uncommon neoplasms in ferrets, which arise from striated muscle and have been reported in five male ferrets, ranging in age from 1.6 to 7 years [7,122,144].

These neoplasms are poorly demarcated, low-grade malignancies, which infiltrate and replace skeletal muscles from which they arise. Clinical signs include difficulty in locomotion and paresis, although in one case, a subcutaneous mass originating from the thoracic wall was the only clinical sign [144]. Histologically, these tumors are composed of spindle cells arranged in bundles. Occasionally, multinucleated “strap” cells are seen, but should not be confused with atrophic preexistent skeletal muscle cells. While histochemical stains such as phosphotungstic acid-hematoxylin (PTAH) and Masson's trichrome may demonstrate cross-striations of neoplastic cells, this diagnosis is most commonly confirmed via immunohistochemical investigation. Neoplastic skeletal muscle cells will be positive for muscle-specific actin, desmin, myoglobin, and myogenin, a marker of early skeletal phenotype. In one case, neoplastic cells were immunoreactive for skeletal muscle-type creatine phosphokinase [144]. Ultrastructurally, neoplastic cells will show varying amounts of dis­organized myofibrils and z-band like structures, often proportional to their level of differentiation.

Synovial Cell Sarcoma

Synovial cell sarcoma arises from pluripotent fibrocytes of the synovial membrane and/or subsynovial tissues. It has been described in a 2-year-old male ferret [145] with an abnormal gait. Radiography revealed a multinodular soft tissue mass encompassing the femorotibial joint. At necropsy, a 4 × 6 cm long, firm, immobile, and nonpainful mass surrounded the joint, crossing the joint space. Histologically, the nodules consisted of broad sheets, and nests of epithelial cells, which were often interrupted by prominent clefts. The cells had vacuolated cytoplasm, pleomorphic nuclei, and a high mitotic rate. Nuclei were pleomorphic, mitotic figures were numerous, and occasional multinucleated neoplastic cells were present. The anaplastic cells infiltrated adjacent periosteum, bone, and subcutis.

Synovial cell sarcomas are in a small subset of neoplasms which exhibit both cytokeratin and vimentin immunoreactivity. Today, immunohistochemistry is required for definitive diagnosis of these rare neoplasms.

Liposarcoma

Lipomas and liposarcomas are benign and malignant neoplasms of adipocytes, respectively. An intraosseous liposarcoma was described in the marrow cavity of the mandible of 2-year-old male ferret [146]. Radiography revealed an infiltrative osteolytic mass within the mandible. Grossly, the mass was a 2-cm white to tan multilobulated mass. Histologically, the mass was unencapsulated and composed of large pleomorphic cells with finely vacuolated cytoplasm, round to oval nuclei, single nucleoli, and a low mitotic index. Ultrastructurally, the vacuoles were variably sized and nonmembrane bound. The cytoplasmic vacuoles were oil-red-O-positive histologically, and osmium tetroxide-positive ultrastructurally, indicating that the vacuoles contained lipids [146].

Fibrosarcoma

Fibrosarcomas are malignant neoplasms of fibroblasts and are most commonly described in the skin of the ferrets, and rarely in the oral cavity. Fibrosarcomas have also been rarely reported involving the musculoskeletal system of the ferrets. These neoplasms are characterized by mildly pleomorphic spindle cells in long streams and bundles separated by abundant fibrous to mucinous stroma [147]. Fibrosarcomas with abundant mucinous stroma are referred to as myxosarcomas [83].

In one report, a 4 × 2 cm firm mass was identified between the 15th thoracic and 2nd lumbar vertebrae of a ferret with hind limb paralysis [148]. The neoplasm infiltrated the vertebral body and in areas, it compressed the spinal cord. Mildly pleomorphic cells were present in long interweaving streams and produced variable amounts of collagen as demonstrated by a Masson's trichrome stain. The neoplasms infiltrated through the vertebral body, meninges, and into the spinal cord, resulting in malacia.

In another report, a neoplasm of spindle to stellate cells was identified in the carpal joint of a ferret [149]. Neoplastic spindle cells were embedded in a variably mucinous and collagenous stroma. The mucinous stroma was confirmed with the use of Alcian blue and mucicarmine stains. Neoplastic cells were positive for vimentin intermediate filaments and negative for cytokeratin, confirming a mesenchymal origin. The ferret made a routine recovery following amputation of the affected limb.

Cardiovascular System

Neoplasms of the cardiovascular system are primarily derived from the vascular endothelium, for example, hemangioma and hemangiosarcomas. A single report of lymphangioma was recorded in a year-old male ferret [7]. Because of the wide distribution of blood and lymphatic vessels, the endothelial cell neoplasms may occur anywhere in the body. Interestingly, with the exception of one undetermined tumor, primary neoplasms of the heart have not been documented [7]. Metastatic lymphoma is occasionally seen in the heart, making it the most common neoplasm in this organ.

Hemangiomas and Hemangiosarcoma

Hemangiomas and their malignant counterparts, hemangiosarcomas, have been reported most commonly in the skin and liver [5,7,9,12,78,113], and occasionally in the spleen and rectum [8,12]. In a colony of 60 ferrets, five hemangiomas and eight hemangiosarcomas were observed in the liver of 10 adult male and 3 adult female ferrets, with a prevalence of 22% [113]. Affected ferrets often died unexpectedly and 9/13 ferrets had hemoperitoneum. In hemangiomas, single to multiple, dark-red nodules of 2–20 mm in diameter were in the liver parenchyma and/or raised above the liver surface. In this report, hemangiomas were single to multiple nodules ranging up to 2 cm, consisting of cavernous blood-filled spaces, lined by a single layer of well-differentiated endothelial cells and separated by fibrous septa. Hemangiosarcomas consisted of small blood-filled clefts lined by pleomorphic endothelial cells, which projected into the vascular lumina. Metastasis to the omentum and lung were observed in two ferrets with hemangiosarcoma [113].

Vascular neoplasms are also commonly seen in the skin and subcutis of the ferret [12,16,78,88], and have been previously described at length. Benign and malignant tumors appear in the skin of adult ferrets with a relatively even distribution. Extracutaneous metastasis of hemangiosarcoma has not been reported, so surgical excision should be considered curative.

Nervous System

Primary neoplasms of the nervous system are uncommon in ferrets, accounting for less than 0.5% of neoplasms in one large retrospective [12]. Overall, metastatic lymphoma is the most common neoplasm of the nervous system, both in the central (brain) and the peripheral (spinal cord and peripheral nerves) nervous systems.

A range of primary neoplasms of the central nervous system have been reported in ferrets, including astrocytoma [12], meningioma [7,12], two cases of granular cell tumors [12,150], and a choroid plexus papilloma [151]. One primitive neuroepithelial tumor has also been reported [12].

In the peripheral nervous system, after lymphoma, peripheral nerve sheath tumors (also known as schwannomas, neurofibromas, neurilemmomas, and neurofibrosarcomas) are the most common neoplasm [4,5,7,12,68,114], and ganglioneuromas [12,68] have also been reported.

The organs of special senses are also considered to be part of the central nervous system. Once again, lymphoma [7,12,68] is the most common neoplasm of the eye and orbit; other neoplasms are considered rare.

Neurologic disease in ferrets may also result from damage to the brain or spinal cord from neoplasms in proximity, or at a distance. Anyone working with ferrets should be constantly aware of the fact that, by far, the most common cause of neurologic dysfunction in ferrets is hypoglycemia, resulting from sporadic or sustained release of insulin by functional pancreatic islet cell tumors. The incidence of cases in which neurologic dysfunction results from primary or metastatic neoplasia within nervous tissue is extremely low on the list of differential diagnosis.

Neurologic dysfunction may also result from compression of the brain or spinal cord by neoplasms, such as lymphoma [74,152] or chordoma [132–134], infiltrating adjacent structures.

Tumors of the Central Nervous System

Meningioma

Meningiomas are neoplasms derived from the meningothelial cells of the arachnoid membrane and pia mater of the nervous system [153]. These benign neoplasms grow outward from the meninges until they reach the skull bones, and continued growth results in the compression of underlying tissue within the cerebrum, cerebellum, and brainstem. Neurologic deficits may reflect the affected area of the brain. These neoplasms have been rarely reported [7,12] and histologic features were not described. Due to their benign progression, meningiomas are likely the only primary tumors of the cranial vault which might be successfully treated, provided an early diagnosis is made.

Granular Cell Tumor

Granular cell tumors are of presumed meningeal origin and are rare neoplasms of the central nervous system [154]. While they are also seen in the skin and upper respiratory system of other domestic species, the only two reported granular cell tumors in the ferret occurred within the cerebrum [12,150]. Regardless of tissue of origin, granular cell neoplasms receive their name from the large numbers of lysosomes within their cytoplasm, which give them a characteristic, bright pink granular appearance.

In one report [150], a 4-year-old, castrated male ferret presented with profound neurologic signs, including intermittent right head tilt which progressed to ipsilateral circling, ataxia, torticollis, and seizures. At necropsy, a well-demarcated oval mass was present in the edematous middle right forebrain. The neoplasm was composed of sheets of large polygonal cells with abundant eosinophilic granular cytoplasm, small eccentric round to oval dark nuclei, and occasional small single nucleoli. The intracytoplasmic granules were positive for periodic acid-Schiff reaction and diastase resistant.

While histochemical stains and the morphologic appearance of granular cell tumors is characteristic, cerebral granular cell tumors also are immunoreactive for vimentin intermediate filaments, ubiquitin, and variably positive for alpha-1-antitrypsin and alpha-1-antichymotrypsin [154]. Ultrastructurally, the presence of polygonal cells with numerous electron-dense angular lysosomes is also a characteristic feature.

Choroid Plexus Papilloma

Choroid plexus neoplasms are uncommon neoplasms in domestic species, which are most frequently diagnosed in dogs [153]. These tumors, which develop from the choroid plexus in the lateral, 3rd, and 4th ventricles, are subdivided into slow-growing papillomas and more aggressive choroid plexus carcinomas. Metastasis of choroid plexus carcinomas often occurs by translocation via the cerebrospinal fluid (CSF) to other locations within the ventricular system.

A focal choroid plexus papilloma was diagnosed in a 6-year-old ferret with progressive neurologic signs including head tilt, circling to the right, ataxia, and quadraparesis [151]. At necropsy, a 1 × 1 × 2 cm grayish granular mass was identified, expanding the 4th ventricle and impinging on the right ventrocranial area of the cerebellum. The mass was accompanied by hydrocephalus of the lateral and 3rd ventricles as a result of impedance of CSF flow. Histologically, the mass was composed of a single layer of cuboidal epithelium on an arborizing fibrovascular stroma. A choroid plexus papilloma was diagnosed based on the lack of infiltration of the surrounding neuropil, low mitotic rate, and absence of any metastatic foci.

Tumors of the Eye and Orbit

Neoplasia of the eye and orbit is also uncommon in ferrets. Malignant lymphoma is the most common neoplasm in both the eye and the orbit [12,68]. Within the eye, clinical signs result in the areas of the eye affected; visual deficits are common. When present within the orbit, exophthalmos is the most common presenting sign [155]. Retrobulbar lymphoma should be a primary ruleout for exposure keratitis in this species; a systemic workup may yield other signs of multiorgan lymphoma in affected ferrets.

Melanocytoma [12], eyelid squamous cell carcinoma [155], and a retrobulbar adenocarcinoma [156] have been reported in the ferret. The retrobulbar adenocarcinoma was proposed to be of lacrimal gland origin, but this could not be effectively proven due to the prolonged period before diagnosis as well as the lack of normal structures at enucleation. The neoplasm responded well to radiation therapy.

Tumors of the Peripheral Nervous System

Peripheral Nerve Sheath Tumors

As previously described, peripheral nerve sheath tumors are most commonly described in the skin of the ferrets, especially in the area of the face and eyelids, which have a high number of small nerve bundles. A multitude of names for this particular neoplasm populate the literature, including schwannoma, neurofibroma, neurilemmoma, and neurofibrosarcoma, based on the presumption of a particular cell of origin, whose identity has yet to be proven [4,5,7,9,12,68,114]. This neoplasm, regardless of nomenclature, is a soft tissue sarcoma with an infiltrative nature and low metastatic potential. Moreover, with each unsuccessful attempt to remove these neoplasms, the remaining neoplastic cells exhibit increasingly malignant behavior, including more rapid growth and infiltration of adjacent tissue, rendering complete excision even more difficult [12].

Peripheral nerve sheath tumors are primarily observed in the skin of ferrets, but have also been reported in adrenal gland [8]. While tumors of the cranial nerves or spinal plexuses (common sites for nerve sheath tumors in other domestic species) have not been reported in ferrets, a schwannoma of the vagus nerve was noted in the subcutaneous tissue of the neck of a ferret, interrupting vagal innervation [114].

As previously discussed, tumors of the nerve sheath are composed of interwoven streams and bundles of spindle cells with moderate amounts of finely vacuolated eosinophilic cytoplasm. These neoplasms occasionally have alternating areas of poor cellularity as well as increased cellularity (Antoni A and B areas, respectively), as well as areas in which nuclei of adjacent cells palisade in one plane, known as “Verocay bodies.” Nuclei are irregular with finely stippled chromatin; anisokaryosis and mitotic rate increases markedly following unsuccessful surgical interventions. Necrosis is often seen in larger neoplasms. Rarely, peripheral nerve sheath tumors may be pigmented as a consequence of their origin from the neural crest.

As peripheral nerve sheath tumors often resemble other spindle cell neoplasms of soft tissues such as fibroma, fibrosarcoma, leiomyoma, and leiomyosarcoma, definitive diagnosis requires immunohistochemical investigation. Neoplastic cells are immunoreactive for glial fibrillar acidic protein and S-100 protein, but should be negative for muscle markers such as smooth muscle actin and desmin. Ultrastructurally, Schwann cells have a definitive basement membrane, unlike other spindle cell neoplasms.

Ganglioneuroma

Ganglioneuromas are rare benign neoplasms derived presumably from surviving primitive neuroepithelial cells, with differentiation toward postmitotic neurons [150]. Ganglioneuromas have been reported twice in ferrets [12,68]. These nodules may arise in close proximity to the adrenal gland [12] or other ganglia throughout the abdomen or thorax [68].

Grossly, these tumors may be mistaken for adrenal gland or lymph nodes on initial inspection; one report identified a 1.5 cm nodule in the area of the right adrenal gland. Histologically, these tumors are composed of well-differentiated neurons and glia within a neural stroma. Neurons are oval to pyramidal in shape with abundant Nissl substance; nuclei were large and often eccentric. In one case, the neuron also contained abundant lipo­­fuscin [68].

Urinary System

Primary neoplasms of the urinary system are rare in ferrets. In a large retrospective study [12], urinary tract neoplasms accounted for less than 1% of recorded tumors, and once again, lymphoma was the most common neoplasm. In a study of stage IV lymphomas in ferrets, 8/18 neoplasms were present in the kidney at necropsy [157].

Primary neoplasms in the urinary system are divided into those of the kidneys and those of the lower urinary tract (including the ureters, bladder, and urethra). In the kidney, transitional cell carcinoma appears to be the most common primary neoplasm, arising in the renal pelvis [12,158]. Benign and malignant neoplasms of tubular epithelium (renal adenomas and carcinomas, respectively) have also been reported [7–9,159]. While uncommon in domestic ferrets, neoplasms of tubular epithelium are commonly seen in black-footed ferrets (M. nigripes), being identified in over 20% of adult animals at necropsy [160].

Hematuria, dysuria, or urinary incontinence may be seen in ferrets with urinary neoplasms [161]; however, the majority of these rare neoplasms are clinically silent.

Tumors of the Kidney

Transitional Cell Carcinoma

Transitional cell carcinomas are neoplasms of the epithelial (urothelial) lining of the lower urinary tract, which runs from the renal pelvis down the ureters into the urinary bladder. While they may arise in any of the location, the most commonly reported site in ferrets is in the renal pelvis.

In one report, a renal transitional cell carcinoma was found via palpation in a ferret exhibiting clinical signs of anorexia, weight loss, and lethargy. The affected kidney was replaced by a lobulated neoplasm measuring 10 cm in diameter. Microscopically, neoplastic epithelial cells were arranged in variably sized tubules, papillary projections, and solid sheets of cells. A transitional cell carcinoma has also been reported in the urinary bladder [8].

Transitional cell carcinomas are often diagnosed by location as well as cellular morphology. A major ruleout, prostatic carcinoma, has not yet been reported in ferrets. Neoplastic urothelium is strongly immunoreactive for cytokeratin and uroplakin, a transmembrane protein associated with urothelial differentiation.

Tumors of renal tubular epithelium

Renal adenomas and carcinomas are uncommon neoplasms in domestic species which arise from tubular epithelial cells [162]. Benign neoplasms (adenomas) are usually silent; malignant neoplasms often metastasize widely. A benign renal adenoma was found incidentally in a mature female ferret at necropsy [3]. The tumor was cystic and measured 3 cm in diameter. The tumor cells were arranged in large cystic tubules which contained papillary projections of a single layer of neoplastic cells on a fine fibrovascular stroma. The lack of pleomorphism, low mitotic rate, and lack of metastasis coupled with the morphologic features of the neoplasm resulted in a diagnosis of papillary tubular cystadenoma.

In another report, a 3 × 2 × 3 cm mass was identified at necropsy of an emaciated 7-year-old female ferret with a history of ambulatory difficulty [159]. The neoplasm was firm, grayish white, and replaced the kidney and adrenal gland. Neoplastic cells were arranged in tubules and occasionally solid sheets. Tubules had prominent periodic acid-Schiff (PAS) stain positive basement membranes and were lined by mildly pleomorphic cuboidal epithelium. In solid areas, neoplastic cells showed marked karyomegaly and pleomorphism, with a high mitotic rate. Metastatic foci with similar cellular morphology were seen in the lungs, omentum, liver, contralateral renal pelvis, and multiple lymph nodes. Neoplastic cells were immunoreactive for cytokeratin and CD10, a cell surface marker expressed on the brush border of renal tubular epithelium. Based on the morphologic appearance and immunophenotype, the neoplasm was diagnosed as a pleomorphic renal adenocarcinoma.

Tumors of the Respiratory System, Mediastinum, and Serosal Membranes

While the small capillaries of the lung commonly entrap circulating neoplastic cells, resulting in a high incidence of metastatic neoplasm, primary pulmonary neoplasms of ferrets are not yet documented in the veterinary literature. As in most other systems, lymphoma is the most common neoplasm in the respiratory system, as well as the most common malignancy [12]. Carcinomas, especially of apocrine glands, may also be found as metastatic foci in the lungs.

Tissues of the mediastinum and serosal membranes give rise to two neoplasms in ferrets. Thymomas, neoplasms arising from the epithelial stroma (not the resident lymphocyte population) of the thymus gland, have been rarely reported in this species [163]. Mesotheliomas, malignant neoplasms of the serosal membrane of the thoracic and abdominal cavities, have been occasionally reported in ferrets [164].

Thymoma

Thymomas are rare neoplasms of domestic species which originate from the epithelium of the thymus gland. The native population of lymphocytes within the thymus is not neoplastic, but may obscure the proliferating epithelium. Thymomas are generally slow-growing neoplasms with low metastatic potential.

Clinical signs in affected ferrets are most often the result of a space-occupying mass within the chest [163]—vomiting, coughing, respiratory distress, and esophageal dilatation. In the two documented cases, a tan-brown, multilobulated mass occupied the cranial mediastinum, compressing the lungs and resulting in marked deviation of the esophagus. Metastasis was not noted in either case. Myasthenia gravis, which may accompany thymoma in domestic species, has not been documented in association with this neoplasm in ferrets.

Mesothelioma

Mesotheliomas are malignant neoplasms which are seen in a wide range of species and are always associated with a poor prognosis. These neoplasms have been occasionally reported in ferrets [5,7,164]. Mesotheliomas spread widely within the abdomen or thorax via explantation and are often significantly advanced before clinical signs are noticed.

The predominant clinical sign in affected ferrets is profound abdominal distention due to the presence of abundant abdominal fluid (occasionally referred to as “malignant ascites”).

At necropsy, pink to white, pearlescent nodules of variable size are present within the omentum, mesentery, and serosal surfaces of abdominal and visceral organs, and abundant viscous exudate is present within the abdomen. Cytologic evaluation of ascitic fluid often demonstrates clusters or sheets of pleomorphic polyhedral cells with a prominent pink brush border.

Microscopically, mesotheliomas may demonstrate a predominantly epithelial, mesenchymal, or biphasic pattern, often on a moderately dense fibrous stroma. Within the epithelial parts of the neoplasm, cells may form prominent exophytic papillary projections into the abdominal or thoracic cavity. The mesenchymal component is often located beneath an epithelial component and is composed of spindle cells arranged in long streams [164]. Neoplastic cells demonstrate multifocal immunoreactivity to both cytokeratin and vimentin intermediate filaments. Ultrastructurally, surface microvilli, interdigitating cytoplasmic membranes, desmosomes, and tonofilaments are characteristic [164].

Experimental Neoplasms

A possible role of viral etiology in ferret lymphomas was demonstrated by Erdman et al. by transmitting a spontaneous lymphoma in a ferret to six laboratory ferrets using the lymphoma cell or cell-free inocula [21]. All six recipient ferrets developed lymphocytosis in peripheral blood in 6 weeks and 3/4 of the ferrets developed polymorphous lymphomas in 24–36 months post inoculation. Reverse transcriptase activity and retrovirus-like particles were observed in the cultivated cells from the recipient ferrets [21].

Association of H. mustelae with gastric carcinomas and lymphomas has been demonstrated in ferrets [22–24]. Fox and coauthors induced gastric adenocarcinomas by intragastric inoculation of a single dose of 100 mg/kg N-methyl-N-nitro-N′-nitrosoguanidine (MNNG), a gastric carcinogen in several animal species, into H. mustelae infected ferrets [22]. Nine of 10 H. mustelae-infected ferrets given the MNNG developed gastric adenocarcinoma 4–5 years after inoculation. None of the five H. mustelae-infected control ferrets developed gastric tumors.

Newborn ferrets were also shown to be susceptible to Mill Hill polyoma virus infection, a polyoma virus originally isolated from the spleen of a leukemic mouse [165]. The virus induces multiple tumors in newborn or young rodents, for example, mice, rats, and hamsters. Harris et al. induced fibrosarcomas in 4 of 32 neonatal ferrets with the virus. When injected subcutaneously into three newborn ferrets, phenol-treated virus induced subcutaneous fibrosarcomas in two ferrets. When untreated virus was injected subcutaneously into five ferrets, no tumors were induced. Intraperitoneal injection of untreated virus induced fibrosarcomas in the abdominal wall in 2 of the 24 newborn ferrets [165].

Rous sarcoma virus is a retrovirus of chicken origin. Chicken sarcoma cells of Rous sarcoma virus origin induced tumors or cystic lesions in some rodents, but did not induce any lesions in ferrets [7]. Likewise, well-known chemical carcinogens 1,2,5,6-dibenzanthracene and 3,4-benzpyrene also failed to induce tumors when injected into six ferrets [6].

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