Melanoma of unknown primary origin metastasizing to visceral organs in a sheep

Abstract

Malignant melanomas are quite uncommon in sheep and goats. A three-year-old native female sheep with a history of sudden death was referred for postmortem examination. Gross evaluations revealed multiple brown to black neoplastic masses with compact nodular appearance, involving lungs, mediastinal lymph nodes, liver and kidneys. Comprehensive physical examination of skin, all mucous membranes and mucocutaneous junctions revealed no evidence of concomitant pigmented lesions or dark discoloration. Microscopically, the masses were predominantly composed of round and polyhedral-shaped neoplastic cells of various sizes with abundant pale eosinophilic cytoplasm containing variable amounts of dark brown melanin pigment and peripherally located, oval to round, vesicular nuclei with prominent nucleoli. Immunohistochemically, the neoplastic cells displayed diffuse and positive expression for S100 protein, vimentin and melan-A. These histopathological and immunohistochemical findings were consistent with a final diagnosis of metastatic malignant melanoma, “epithelioid” type. To the author’ knowledge, metastatic melanoma without an identified primary origin has not yet been reported in the domestic animals.