A PENILE SQUAMOUS CELL CARCINOMA WITH SYSTEMIC METASTASIS IN A THOROUGHBRED HORSE

A 22-year-old gelding Thoroughbred horse had been in poor physical condition with a prolapsed penis. On necropsy, numerous white nodules/plaques were found on the lungs, omentum, and multiple lymph nodes, as well as several ulcerations and cauliflower-like growths throughout the glans penis to the internal fold. Histologically, the white nodules/plaques were composed of similar neoplastic squamous cells, which were immunostained positive for cytokeratin (CK), CK 5/6, and P63. The cauliflower-like growths were diagnosed as squamous papilloma, inverted papilloma, and penile squamous cell carcinoma (PSCC). It is speculated that the lung metastasis and peritoneal carcinomatosis in this case are derived from the PSCC. The lesion tissues from the penis and omentum were characterized negative, by polymerase chain reaction, for Equus caballus papillomavirus 2 (EcPV2). This finding suggests that the penile lesions may have been irrelevant to the EcPV2, further implying that multiple pathogeneses exist for the development of an equine PSCC.