D: Demodicosis

Demodicosis

BASICS

DEFINITION

An inflammatory parasitic disease of dogs and rarely cats that is characterized by an increased number of mites in the hair follicles and epidermis.
Often leads to furunculosis and secondary bacterial infection.
May be localized or generalized.

PATHOPHYSIOLOGY

Dogs

Three species of mites identified in the dog:
- Demodex canis—follicular mite; part of the normal fauna of the skin; typically present in small numbers; resides in the hair follicles and sebaceous glands of the skin, transmitted from the mother to the neonate in 2–3 days of nursing.
- Demodex injai—large, long-bodied mite found in the pilosebaceous unit, mode of transmission unknown; only associated with adult-onset disease with highest incidence noted in the terrier breeds often along the dorsal midline (West Highland white terrier and wirehaired fox terrier).
- Demodex cornei—lives in the stratum corneum of the epidermis; mode of transmission unknown.
Pathology develops when numbers exceed those tolerated by the immune system.
The initial proliferation of mites may be the result of a genetic or immunologic disorder.

Cats

Poorly understood disorder.
Mites have been identified on the skin and within the external ear canal.
Two species of mites identified in the cat:
- Demodex gatoi is considered to be potentially contagious and associated with pruritic dermatitis.
- Demodex cati infections are often associated with immunosuppressive and metabolic disease.

SYSTEMS AFFECTED

Skin/Exocrine

GENETICS

Initial proliferation of mites may be the result of a genetic disorder

INCIDENCE/PREVALENCE

Dogs—common
Cats—rare

SIGNALMENT

Species

Dogs and cats

Breed Predilections

Potential increased incidence in Siamese and Burmese cat.
West Highland white and wirehaired fox terrier—greasy seborrheic dermatitis associated with D. injai.

Mean Age and Range

Localized—usually in young dogs; median age 3–6 months.
Generalized—both young and old animals.
No data collected for the cat.

SIGNS

Dogs

Localized, Juvenile-Onset

Lesions—usually mild; consist of erythema and a light scale.
Patches—several may be noted; most common site is the face, especially around the perioral and periocular areas and forelegs; may also be seen on the trunk and rear legs.

Generalized, Juvenile-Onset or Adult-Onset

Can be widespread from the onset, with multiple poorly circumscribed patches of erythema, alopecia, and scale.
As hair follicles become distended with large numbers of mites, secondary bacterial infections are common, often with resultant rupturing of the follicle (furunculosis).
With progression, the skin can become severely inflamed, exudative, and granulomatous.
D. injai may be associated with a greasy seborrheic dermatitis of the dorsal trunk, comedones, erythema, alopecia, and hyperpigmentation.

Cats

Often characterized by partial to complete multifocal alopecia of the eyelids, periocular region, head, neck, flank, and ventrum.
Lesions—variably pruritic with erythema, scale, and crust; those caused by D. gatoi are often quite pruritic and may be contagious,
Ceruminous otitis externa has been reported.
D. cati often associated with immunosuppressive disease.

CAUSES

Dog—Demodex canis, Demodex inja, and Demodex cornei.
Cat—Demodex cati and Demodex gatoi.

RISK FACTORS

Dogs

Exact immunopathologic mechanism unknown.
Studies indicate that dogs with generalized demodicosis have a subnormal percentage of IL-2 receptors on their lymphocytes and subnormal IL-2 production.
Genetic factors (especially localized, juvenile-onset), immunosuppression, and/or metabolic diseases may predispose animal.

Cats

Often associated with metabolic diseases (e.g., FIV, systemic lupus erythematosus, diabetes mellitus).
D. gatoi—short and blunted; rarely a marker for metabolic disease; may be transferable from cat to cat within the same household.

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

Dogs

Bacterial folliculitis/furunculosis
Dermatophytosis
Contact dermatitis
Pemphigus complex
Dermatomyositis
Systemic lupus erythematosus

Cats

Allergic dermatitis
Notoedres
Dermatophytosis
Psychogenic dermatitis

CBC/BIOCHEMISTRY/URINALYSIS

Normal unless there is an underlying process

OTHER LABORATORY TESTS

FeLV and FIV serology, toxoplasmosis titers
Fecal samples: rare finding of mites in feces

DIAGNOSTIC PROCEDURES

Skin scrapings—diagnostic for finding mites in the majority of cases.
Trichograms may be an effective technique for mite identification.
Skin biopsy—may be needed when lesions are chronic, granulomatous, and fibrotic (especially on the foot).

TREATMENT

APPROPRIATE HEALTH CARE

Outpatient.
Localized—conservative; most cases (90%) resolve spontaneously with no treatment.
Evaluate the general health status of dogs with either the localized or the generalized form.

CLIENT EDUCATION

Localized—most cases resolve spontaneously.
Generalized—frequent management problem; expense and frustration with the chronicity of the problem are issues; many cases are medically controlled, not cured; juvenile-onset considered an inheritable predisposition therefore breeding of affected animals is not recommended.

MEDICATIONS

DRUG(S) OF CHOICE

A formamidine, which inhibits monoamine oxidase and prostaglandin synthesis; an α2-adrenergic agonist: use weekly to every other week until resolution of clinical signs and no mites are found on skin scrapings; do not rinse off; let air-dry.
Treat for 1 month following negative skin scrape.
Apply a benzoyl peroxide shampoo before application of the dip as a bactericidal therapy and to increase exposure of the mites to the miticide through follicular flushing activity.
Efficacy is proportional to the frequency of administration and the concentration of the rinse.
Rarely used in cats—0.015%–0.025% applied to the entire body every 1–2 weeks (do not use on diabetic cats).
Dogs—0.03–0.05% applied weekly to every other week; total body/additional topical treatments for focal areas (pododermatitis) may be used every 1–3 days with 0.125% solution.
Promeris—topical product applied every 2–4 weeks.
Preventic collar—anecdotal reports of success; change collar every 2–4 weeks; non-FDA-approved usage.
Between 11% and 30% of cases will not be cured; may need to try an alternative therapy or control with maintenance rinse every 2–8 weeks.

Ivermectin

A macrocyclic lactone with GABA agonist activity.
Dog: daily oral administration of 0.3–0.6 mg/kg very effective, even when amitraz fails (recommendation to start at half the dose [or approximately 0.12 mg/kg] for the first week to observe for any signs of toxicity/side effects).
Treat for 30–60 days beyond negative skin scrapings (average 3–8 months).
Reported as a treatment option in the cat; exact dose is unknown (often dosed at 300 μg/kg).

Milbemycin (Interceptor)

A macrocyclic lactone with GABA agonist activity.
Dosage of 1–2 mg/kg PO q24h cures 50% of cases; 2 mg/kg PO q24h cures 85% of cases.
Treat for 30–60 days beyond multiple negative skin scrapings.
Very expensive.

Moxidectin (Avermectin)

Anecdotal reports in the dog when used at 400 μg/kg PO once weekly.
Do not use in ivermectin sensitive breeds.

CATS

Exact protocols are not defined.
Topical lime-sulfur dips every 3–7 days for four to eight treatments often lead to good resolution of clinical signs; recommended therapy in cats.
Studies with milbemycin and ivermectin are lacking, although there are numerous anecdotal reports of efficacy.
Doramectin has also been reported to be effective at 0.6 mg/kg SC once weekly.

CONTRAINDICATIONS

Ivermectin—contraindicated in collies, Shetland sheepdogs, Old English sheepdogs, other herding breeds, and crosses with these breeds; sensitive breeds appear to tolerate the acaricidal dosages of milbemycin (see above).
Sensitive breeds may lack a gene (MDR1/ABCB1 mutation) that codes for a p-glycoprotein (drug-efflux pump) that predisposes to toxicity.

PRECAUTIONS

Amitraz

Side effects—somnolence, lethargy, depression, anorexia seen in 30% of patients for 12–36 hours after treatment.
Rare side effects—vomiting, diarrhea, pruritus, polyuria, mydriasis, bradycardia, hypoventilation, hypotension, hypothermia, ataxia, ileus, bloat, hyperglycemia, convulsions, death.
Incidence and severity of side effects do not appear to be proportional to the dose or frequency of use.
Human beings can develop dermatitis, headaches, and respiratory difficulty after exposure.
Yohimbine at 0.11 mg/kg IV is an antidote.

Ivermectin and Milbemycin

Signs of toxicity—salivation, vomiting, mydriasis, confusion, ataxia, hypersensitivity to sound, weakness, recumbency, coma, and death.
Ivermectin—contraindicated in collies, Shetland sheepdogs, Old English sheepdogs, other herding breeds, and crosses with these breeds; sensitive breeds appear to tolerate the acaricidal dosages of milbemycin (see above).
Sensitive breeds may lack a gene (MDR1/ABCB1 mutation) that codes for a p-glycoprotein (drug-efflux pump) that predisposes to toxicity.

POSSIBLE INTERACTIONS

Amitraz—may interact with heterocyclic antidepressants, xylazine, benzodiazepines, and macrocyclic lactones.
Ivermectin and milbemycin—cause elevated levels of monoamine neurotransmitter metabolites, which could result in adverse drug interactions with amitraz and benzodiazepines.
Spinosad (flea control) usage contraindicated with ivermectin therapy.

FOLLOW-UP

PATIENT MONITORING

Repeat skin scrapings (trichograms) and evidence of clinical resolution are used to monitor progress.

PREVENTION/AVOIDANCE

Do not breed animals with generalized form

POSSIBLE COMPLICATIONS

Secondary bacterial folliculitis and furunculosis

EXPECTED COURSE AND PROGNOSIS

Prognosis (dogs)—depends heavily on genetic, immunologic, and underlying diseases.
Localized—most cases (90%) resolve spontaneously with no treatment; < 10% progress to the generalized form.
Adult-onset (dogs)—often severe and refractory to treatment.
Feline cases with D. cati may have a poor prognosis associated with underlying disease.

MISCELLANEOUS

ASSOCIATED CONDITIONS

Adult-onset—sudden occurrence is often associated with internal disease, malignant neoplasia, and/or immunosuppressive disease; approximately 25% of cases are idiopathic over a follow-up period of 1–2 years.
D. cati associated with FeLV, FIV, toxoplasmosis, and SLE.

AGE-RELATED FACTORS

Young dogs are often predisposed to a localized form

ZOONOTIC POTENTIAL

None

PREGNANCY/FERTILITY/BREEDING

Do not breed animals with the generalized form

SYNONYMS

Mange
Red Mange