Malignant mesothelioma of the tunica vaginalis in a dog

A 12-year-old Scottish terrier had a scrotal mass which was diagnosed as a malignant mesothelioma of the tunica vaginalis. The tumour resulted in a rapidly fatal clinical course with direct extension into the peritoneal cavity via the inguinal canal; there were no haematogenous or lymphatic metastases. This is the first case report of such a tumour in the dog. The histology and differential diagnoses of these tumours and a review of similar tumours in other species and possible causative agents are given.