46 Zygomycetes of veterinary importance
Fungi in the phylum Zygomycota usually have broad (up to 15 μm in diameter), coenocytic (relatively aseptate) hyphae and replicate asexually by producing sporangiospores within a sporangium. Sexual reproduction involves fusion of gametangia from two different strains, resulting in the production of a thick-walled zygospore.
Three orders in the class Zygomycetes – Mucorales, Mortierellales and Entomophthorales – are of veterinary significance. Genera in these orders contain potentially pathogenic species. These rapidly growing fungi, which are widely distributed saprobes, can cause sporadic opportunistic infections in animals.
Infection with these fungi is uncommon in healthy immunocompetent animals. Factors which may predispose to infection are listed in Box 46.1. Following ingestion or inhalation of spores from a contaminated environment, hyphae invade the mucosa, submucosa and local vessel walls, producing an acute necrotizing thrombotic vasculitis. Chronic lesions are usually localized and granulomatous.
Clinical infections
The zygomycoses of domestic animals are presented in Table 46.1. Apart from Mortierella wolfii, which may produce abortion followed by acute pneumonia, members of the Mucorales rarely cause recognizable disease syndromes in animals. Mycotic lesions caused by members of the Mucorales are less commonly encountered than those caused by Aspergillus species. Laboratory procedures, including isolation of the fungus and demonstration of hyphae in affected tissues, are essential for the diagnosis of zygomycoses.
Table 46.1 Zygomycoses of domestic animals.
| Fungal disease | Hosts | Clinical conditions |
| Mucormycosis (fungi belonging to the sub-phylum Mucoromycotina, orders Mucorales and Mortierellales) | Cattle | Mesenteric and mediastinal lymphadenitis Abortion Pneumonia, following abortion caused by Mortierella wolfii Oesophagitis and enteritis in calves Rumenitis, abomasal ulceration Cerebral mucormycosis |
| Pigs | Enteritis in piglets Mesenteric and mandibular lymphadenitis Gastrointestinal ulcers | |
| Cats | Focal necrotizing pneumonia Necrotic enteritis | |
| Dogs | Enteritis | |
| Entomophthomycosis (fungi belonging to the order Entomophthorales) | Horses | Cutaneous granulomas caused by Basidiobolus species Nasal granulomas caused by Conidiobolus species |
| Dogs | Subcutaneous, gastrointestinal and pulmonary granulomas caused by Basidiobolus species Subcutaneous granulomas caused by Conidiobolus species | |
| Sheep | Nasal granulomas caused by Conidiobolus species |
Although Aspergillus species account for the majority of mycotic abortion cases in cattle, M. wolfii, Lichtheimia (Absidia) species, Mucor species and Rhizopus species have been implicated. Mycotic abortion, which usually occurs late in gestation, is often linked to the feeding of mouldy hay or silage. The location of lesions on cotyledons suggests haematogenous infection of the uterus. The cotyledons are enlarged and necrotic and the intercotyledonary placental tissue is thickened and leathery. Occasionally, lesions may be observed grossly on the skin of aborted foetuses.
Mycotic rumenitis in cattle may follow mucosal damage associated with ruminal lactic acidosis. The microscopic appearance of the causal fungi in ruminal lesions suggests that, in most cases, zygomycetes are involved. Infarction due to thrombotic arteritis, necrosis and haemorrhage are major features of the mycotic lesions. Extension of the inflammatory process through the ruminal wall results in fibrinous peritonitis. Zygomycotic abomasitis in calves, which may follow neonatal infection, can also produce perforation and peritonitis.
Two genera in the Entomophthorales, Basidiobolus and Conidiobolus, are sometimes associated with opportunistic infections in animals. Basidiobolus species and Conidiobolus species are saprobes in soil and in decaying vegetation. The route of entry of these fungi is probably through minor abrasions in the skin or nasal mucous membranes, giving rise to granulomatous lesions. Basidiobolus species cause cutaneous lesions in the horse, while Conidiobolus species cause nasal granulomas in horses, sheep and llamas.
Specimens for laboratory examination should include biopsy or postmortem tissues for histopathology and culture. Staining of tissue sections by the PAS or methenamine silver techniques facilitates detection of hyphae. Isolation is carried out on Sabouraud dextrose agar without cycloheximide. Cultures are incubated aerobically at 34°C for up to five days. Growth of Lichtheimia (Absidia), Mucor, Rhizomucor and Rhizopus species is rapid, filling the Petri dish with greyish or brownish-grey fluffy colonies within a few days. Mortierella wolfii has characteristic white velvety colonies with lobulated outlines after incubation for four days. Differentiation to species level is carried out in mycological reference laboratories.