Scrotal enlargement is a problem in stallions that often leads to poor semen quality. Although there are many causes for scrotal enlargement, testicular neoplasia is often high on the differential list, especially in an aged stallion.
This case report describes a stallion with scrotal enlargement caused by a rare neoplasia located in the tunica vaginalis, and not in the testicular parenchyma. Diagnosis of mesothelioma was made following hemicastration. Prognosis in other species is variable, but this stallion returned to his successful breeding career following treatment and has remained healthy one year following the diagnosis.