Zygomycosis is used to describe infection with fungi in the class Zygomycetes and 2 genera in the order Entomophthorales, Basidiobolus and Conidiobolus. True zygomycete infections are rare, but conidiobolomycosis and basidiobolomycosis are more common and cause pyogranulomatous lesions that are grossly and histologically similar to those caused by pythiosis and lagenidiosis. This is primarily an infection of the nasal mucosa and subcutaneous tissue of horses and rarely other animals (llamas, sheep) by C coronatus, C incongruus, C lamprauges, or B ranarum. These ubiquitous fungi are present in soil and decaying vegetation and, in the case of basidioboli, the GI tracts of amphibians, reptiles, and macropods. C coronatus affects almost exclusively the mucosa of the nose and mouth. Basidiobolus infects the lateral aspects of the head, neck, and body. C coronatus is also an important insect pathogen.

Clinical Findings

Ulcerative pyogranulomas of the mucous membrane of the nostril or mouth, or nodular growths of the nasal mucosa and the lips caused by C coronatus may cause mechanical blockage, resulting in dyspnea and nasal discharge. Lesions caused by B ranarum are large, usually single, circular, ulcerative, pruritic nodules of the skin of the upper body. Fistulous tracts discharge a serosanguineous fluid from the lesions, which frequently are traumatized. Extension to regional lymph nodes results in swelling of the nodes and development of yellow necrotic foci. Lesions may contain a creamy, yellow central core of necrotic tissue. Disseminated basidiobolomycosis is rare but has been described in dogs and a mandrill.

Lesions

In excised tissues or necropsy specimens, a thickened fibrotic dermis has scattered, red or creamy white areas. The lesions, which contain hyphal forms, a heavy infiltrate of eosinophils, and sequestered areas of necrosis, have histologic features of infectious granulomas.

Diagnosis

Clinically, zygomycosis may be confused with cutaneous habronemiasis (see Helminths of the Skin: Cutaneous Habronemiasis) and oomycosis (see Fungal Infections: Oomycosis) but can be differentiated by microscopic examination of tissues. In H&E sections, the fungus appears as holes and elongated channels, and many hyphae have an eosinophilic cuff; in sections stained for fungi, the organism consists of large, branching, sometimes septate, 4–20 μm hyphae. Cultural examination is required to identify the causative fungus.

Treatment

Surgical excision or immunotherapy, or both, have been successful. The immunotherapy consists of intradermal injections of 0.02–0.1 mL particulate fungal material. Localized mycotic disease has been treated with amphotericin B given systemically or locally, or both. Ideally, treatment includes early surgical removal of the lesion, followed by administration of amphotericin B.

Last full review/revision March 2012 by Joseph Taboada, DVM, DACVIM