Tumours of the prostate and penis.

Tumours of the male genital tract, excluding the testes, are relatively rare in the six major domestic animals. The most important tumours are prostate carcinoma and transmissible venereal tumour of the penis in dogs, fibropapilloma of the penis in bulls, squamous papilloma and squamous cell carcinoma in horses, and squamous papilloma in pigs. Four histological types of canine prostate carcinoma exist: alveolar papillary, acinar, organoid, and poorly differentiated. The biological behaviour of prostate carcinomas is similar to that in man, with frequent metastasis to the regional pelvic nodes, bones, and lungs. There appears to be no relationship between the common diffuse glandular hyperplasia and carcinoma in the prostate of dogs. A unique lesion of dogs is squamous metaplasia of the prostate related to estrogen-producing Sertoli cell tumours of the testis. Three different transmissible tumours of the penis occur in domestic animals. The canine venereal tumours can be transmitted only by intact tumour cells during licking and coital contact, whereas bovine fibropapillomas and porcine squamous papillomas can be transmitted by cell-free material. In cattle, the fibropapillomas are caused by the same virus that produces cutaneous papillomatosis. All three tumours are benign and usually regress spontaneously.