Zygomycosis is used to describe infection with fungi in the class Zygomycetes and two genera in the order Entomophthorales, Basidiobolus and Conidiobolus. True zygomycete infections are rare, but conidiobolomycosis and basidiobolomycosis are more common and cause pyogranulomatous lesions grossly and histologically similar to those caused by pythiosis and lagenidiosis. This is primarily an infection of the nasopharyngeal 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.
Ulcerative pyogranulomas of the mucous membrane of the nasopharyngeal tissue, mouth, or nodular growths of the nasal mucosa and the lips may be seen with conidiobolomycosis, resulting in mechanical obstruction of the nasal cavity, dyspnea, and nasal discharge. Local dissemination into the retropharyngeal, retrobulbar, or other tissues of the face may be noted. Lesions in basidiobolomycosis are 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.
Clinically, zygomycosis may be confused with cutaneous habronemiasis (see Cutaneous Habronemiasis in Animals) and oomycosis (see 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 a wide eosinophilic cuff; in sections stained for fungi, the organism consists of large, branching, sometimes septate, 2.5–25 μm hyphae. Cultural examination is required to identify the causative fungus.