43 Aspergillus species
Although the genus Aspergillus contains more than 190 species, only a limited number have been implicated in opportunistic infections in animals and humans. Aspergillus species are saprobes which are widely distributed in the environment. Aspergillus fumigatus is the species most often involved in tissue invasion; other potentially invasive species include A. niger, A. flavus, A. nidulans, A. flavipes, A. deflectus and A. terreus. Although the teleomorph or sexual stage is not known for many Aspergillus species, the sexual reproductive cycle of Aspergillus fumigatus has been demonstrated and its teleomorph Neosartorya fumigata has been described.
Aspergilli are aerobic and grow rapidly, forming distinct colonies after incubation for two to three days. The colour of the obverse side of colonies, which may be bluish-green, black, brown or yellow, varies with individual species and cultural conditions. Aspergillus fumigatus, a thermotolerant species, grows at temperatures ranging from 20 to 50°C.
The hyphae are septate, hyaline and up to 8 μm in diameter. Unbranched conidiophores develop at right angles from specialized hyphal foot cells. The tip of the conidiophore enlarges forming a vesicle which becomes partially or completely covered with flask-shaped phialides. The phialides produce chains of round pigmented conidia which are up to 5 μm in diameter. Respiratory infection may occur following inhalation of spores. Occasionally, infection can result from ingestion of spores or following the introduction of spores into tissues by trauma. Systemic infection is invariably associated with immunosuppression. Species such as A. flavus, which elaborate potent toxins when growing on cereals and other crops, cause mycotoxicoses.
Aspergillus species grow on standard laboratory media such as Sabouraud dextrose agar. Because the genus contains a large number of species, differentiation is difficult. Colonies can be up to 5 cm in diameter after incubation for five days. The colonies of A. fumigatus become velvety or granular and bluish-green with narrow white peripheries. Colonies of A. niger are black and granular, features imparted by the large pigmented sporing heads. Aspergillus flavus colonies are yellowish-green with a fluffy texture.
Infection with Aspergillus species, mainly A. fumigatus, has been recorded in many species of animals. Aspergillosis, which is primarily a respiratory infection, follows spore inhalation. Immune competence of the host largely determines the outcome of infection. Factors which modify immune competence such as corticosteroid therapy, cytotoxic drugs and immunosuppressive viral infections may predispose to tissue invasion. Hyphal invasion of blood vessels leads to vasculitis and thrombus formation. Mycotic granulomas may develop in the lungs and occasionally in other internal organs.
Clinical cases of aspergillosis are comparatively uncommon and usually sporadic. The clinical conditions caused by Aspergillus species in domestic animals are summarized in Table 43.1. Brooder pneumonia affects newly hatched chickens which are exposed to high numbers of A. fumigatus spores. Affected chickens develop somnolence and inappetence and many may die. Yellowish nodules are present in the lungs, air sacs and, occasionally, in other organs. Histopathological evidence of tissue invasion by fungi and culture of A. fumigatus from lesions are required for confirmation. Strict hygiene and routine fumigation of incubators are effective control measures. Aspergillosis in mature birds frequently follows inhalation of spore-laden dust derived from contaminated litter or feed. Poultry, captive penguins, raptors and psittacine birds may be affected. Clinical signs, which are variable, include dyspnoea and emaciation. Yellowish nodules resembling lesions of avian tuberculosis can be observed in lungs and air sacs. Diagnosis is confirmed by histopathology and culture.
Table 43.1 Clinical conditions caused by Aspergillus species in domestic animals.
| Hosts | Condition | Comments |
| Birds | Brooder pneumonia | Occurs in newly hatched chickens in incubators |
| Pneumonia and airsacculitis | Chickens and poults up to 6 weeks of age are most susceptible; older birds sometimes affected | |
| Generalized aspergillosis | Dissemination of infection usually from the respiratory tract | |
| Horses | Guttural pouch mycosis | Confined to guttural pouch, often unilateral |
| Nasal granuloma | Produces a nasal discharge and interferes with breathing. Fungi other than Aspergillus species may initiate this condition | |
| Keratitis | Localized infection following ocular trauma | |
| Cattle | Mycotic abortion | Occurs sporadically; produces thickened placenta and plaques on skin of aborted foetus |
| Mycotic pneumonia | Uncommon condition of housed calves | |
| Mycotic mastitis | May result from the use of contaminated intramammary antibiotic tubes | |
| Intestinal aspergillosis | May cause acute or chronic diarrhoea in calves | |
| Dogs | Nasal aspergillosis | Invasion of nasal mucosa and turbinate bones; occurs periodically |
| Otitis externa | Aspergillus species may constitute part of a mixed infection | |
| Disseminated aspergillosis | Uncommon; may result in osteomyelitis or discospondylitis | |
| Cats | Systemic aspergillosis | Rarely encountered; immunosuppressed animals are at risk |
In horses, guttural pouch mycosis, which is frequently associated with A. fumigatus infection, is usually unilateral. Plaque-like lesions develop in the mucosa of the pouch wall. When fungal hyphae penetrate to deeper tissues they cause tissue necrosis, thrombosis, erosion of blood vessel walls and neural damage. Clinical signs include epistaxis, dysphagia and laryngeal hemiplegia. Diagnosis is based on clinical signs, radiographic evidence of fluid accumulation in the pouch and demonstration of characteristic lesions by endoscopy. Confirmation is based on the demonstration of fungal hyphae in biopsy specimens and isolation of A. fumigatus from lesions. Therapeutic options include infusion of antifungal agents such as itraconazole into the pouch and surgical intervention to deal with serious haemorrhage. The spores of A. fumigatus are among the allergens reported to be capable of inducing the allergic condition recurrent airway obstruction in horses, also known as chronic obstructive pulmonary disease and ‘heaves’.
Nasal aspergillosis in dogs is encountered predominantly in young to middle-aged dolichocephalic breeds. Clinical signs, which are often unilateral, include persistent profuse sanguino-purulent nasal discharge with sneezing and bouts of epistaxis. Radiography may reveal an increased radiolucency of turbinate bones and computerized tomography scans are of high diagnostic value. Rhinoscopy is also useful for clinical examination. Culture and histopathological examination of biopsy material are essential for confirmation. Flushing of the frontal sinuses and nasal chambers with clotrimazole may be used together with systemic treatment with itraconazole, fluconazole or voriconazole. Treatment for six to eight weeks may be required.
Mycotic abortion in cows occurs sporadically and its prevalence may be influenced by poor-quality contaminated fodder harvested in wet seasons. Aspergillus fumigatus can proliferate in damp hay, in poor-quality silage and in brewer's grains. Infection, which reaches the uterus haematogenously, causes placentitis, leading to abortion late in gestation. Affected cows usually show no signs of systemic illness. Intercotyledonary areas of the placenta are thickened and leathery and the cotyledons are necrotic. Aborted foetuses may have raised cutaneous plaques, resembling ringworm lesions. Diagnosis is based on culture of A. fumigatus from foetal abomasal contents and histopathological evidence of mycotic placentitis.