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Phaeohyphomycosis refers to chronic cutaneous, subcutaneous, mucosal, cerebral, or systemic infection caused by one of several genera and species of pigmented fungi of the family Dematiaceae. Several fungal genera have been reported to affect people and other animals, including Alternaria, Bipolaris, Cladophialophora (Xylohypha, Cladosporium), Curvularia, Exophiala, Fonsecaea, Moniliella, Phialophora, Ramichloridium, and Scolecobasidium. Fungi in this category are saprophytic, widely distributed organisms found in soil, water, and decaying vegetable matter. Infection may result from fungal implantation into tissue at the site of an injury.

Phaeohyphomycosis has been described in cows, cats, horses, and dogs. The most common clinical presentations include ulcerated cutaneous nodules of the digits, pinnae, nasal planum, and nasal/paranasal tissues in cats. The nodules may ulcerate and have draining fistulous tracts. These pyogranulomas contain pigmented, septate hyphae with irregular enlargements and thin-walled, budding, yeast-like forms. Granulomatous meningoencephalitis caused by pigmented fungi has been reported in dogs and cats. Dogs treated with multiple immunosuppressive agents, especially cyclosporine, appear to be predisposed to developing multifocal cutaneous lesions. Systemic dissemination is most likely in animals treated with immunosuppressive drugs.

Phaeohyphomycosis can be diagnosed by microscopic examination of exudate and biopsy specimens, which reveals pigmented, dark-walled, irregularly septate filamentous hyphae (2–6 μm in diameter) or yeast-like cells. Infected tissues may be grossly pigmented, giving an appearance of melanoma. The several causative fungi cannot be identified by their histologic features in tissues; cultural isolation and/or PCR are required. The differential diagnosis should include neoplasia, other granulomas, and epidermoid cysts.

Phaeohyphomycosis is generally poorly responsive to treatment. Wide excision of cutaneous or subcutaneous lesions is recommended, followed by 6–12 mo of treatment with itraconazole (10 mg/kg/day). Nonresectable disease should be treated with itraconazole. Voriconazole or posaconazole may be more effective, but voriconazole is not recommended in cats. In dogs being treated with immunosuppressive therapy, the prognosis may be better if the immunosuppressive drugs (especially cyclosporine) can be discontinued.

Last full review/revision May 2014 by Joseph Taboada, DVM, DACVIM

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