Hamartoma affecting ampullary electroreceptors and epitheliotropic lymphoma in a captive electric eel Electrophorus varii

Few reports are available describing lesions in captive electric eels Electrophorus spp. This report describes 2 types of cutaneous proliferative lesions (i.e. hamartoma and neoplasm) in a captive electric eel. Ampullary electroreceptor hamartomas appeared grossly as 2 discrete, smooth, pink, spherical, cutaneous masses measuring 6 and 18 mm in diameter. Histologically, hamartomas were composed of predominately spindle cells that were separated into lobules by a peripheral rim of polygonal cells. Spindle cells were arranged in vague streams and occasionally whorls within a myxomatous matrix. Polygonal cells arranged in variably sized trabeculae and cords within a pre-existing fibrovascular stroma surrounded the streams of spindle cells. Admixed with the polygonal cell population were multiple mucous glands and alarm cells, similar to those seen in normal regions of epidermis. Histochemical stains confirmed similar components in the normal ampullary electroreceptor as in the hamartomas. Lymphoma was also present, appearing grossly as patchy pitting, erythematous, and thickened areas of the skin affecting the entire animal. Lymphoma was diffusely infiltrating and expanding the epidermis, oral mucosa, and branchial mucosa up to 1.5 mm in thickness. It was composed of an unencapsulated, well-demarcated, moderately cellular neoplasm composed of lymphocytes arranged in small dense sheets and clusters that separated and effaced epidermal cells. This is the first report of lymphoma in an electric eel, and the first report of ampullary electroreceptor hamartoma in any animal species.