[Monorchism in a tomcat].

In a 9-month-old tomcat presented for routine castration, only the right testis was palpable in the scrotum. This tomcat was part of an inbred litter. Unilateral cryptorchism was suspected, but further diagnostic procedures (ultrasound, diagnostic laparotomy) were unsuccessful in identifying a cryptorchid left testicle. Histopathologic evaluation of the resected right testis revealed a normal organ with spermatogenesis. In tissue resected from the subcutaneous tissue of the left inguinal region side during laparotomy, only a small left epididymis with surrounding fatty tissue was identified at the proximal end of the vas deferens. Morphologic controls of the phenotype and the remaining primary sexual characteristics as well as the results of a hCG stimulation test 7 months after the surgery further established the diagnosis of monorchism. Possible causes for monorchism are unilateral testicular agenesis and testicular atrophy because of vascular insults. This case shows that even during routine procedures, like tomcat castration, the general practitioner can encounter rare anomalies like monorchism, which must be differentiated from cryptorchidism by appropriate diagnostic procedures.