THE MERCK VETERINARY MANUAL
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Mycetomas

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Mycetomas are granulomatous nodules of the subcutaneous tissues that contain tissue grains or granules. Within the grains are dense colonies of the organism. When such lesions are caused by fungi, they are known as eumycotic mycetomas. The causal agents of eumycotic mycetomas include a variety of saprophytic geophilic fungi. Eumycotic mycetomas caused by pigmented fungi such as Curvularia spp and Madurella spp are called black- or dark-grain mycetomas. White-grained mycetomas are caused by unpigmented fungi such as Acremonium spp and Scedosporium apiospermum (the asexual state of Pseudallescheria boydii).

Most eumycotic mycetomas are confined to the subcutaneous tissue, but white-grain mycetomas may be extensions of abdominal cavity disease. Peritonitis or abdominal masses are typically seen in white-grain mycetomas. Black-grain mycetomas are usually characterized by relatively poorly circumscribed cutaneous nodules on the extremities or face. The lesions may ulcerate or form fistulas. When the feet or limbs are involved, the infection may extend to the underlying bone.

The fungal mycelia proliferate in the lesions and organize into aggregates known as granules or grains. In these granules, the mycelium is compact and frequently bizarre and distorted in form. Chlamydospores are frequent, especially at the periphery, and the mycelium may or may not be embedded in an amorphous, cement-like substance. Histologically, the granules are frequently surrounded by eosinophilic deposits. Granules may be of various colors and sizes, depending on the species of fungus involved.

A presumptive diagnosis can be made if there are grains within the exudate of draining tracts. For cytology, the grains should be examined for the presence of fungal elements. If no tissue grains are found in the exudate, a biopsy of the lesion should be taken for histopathologic examination. Cultures should be performed to confirm cytologic findings and to identify the causative agent. Either tissue grains or biopsy specimens should be cultured.

The prognosis for abdominal mycetomas is guarded, because tissue involvement is usually extensive. Cutaneous mycetomas, while not life-threatening, are often difficult to resolve. Radical surgical excision, including limb amputation, may be effective for some cases of cutaneous mycetomas. Effectiveness of antifungal chemotherapy has been reported in only a few cases. In one report, fluconazole, 50 mg/day for 6 wk, was used to successfully treat a dog with intra-abdominal maduromycosis. In another report, longterm treatment with itraconazole, 5–10 mg/kg/day, failed to resolve a disseminated Acremonium infection in a dog.

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

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