Malignant Mesothelioma of the Vaginal Tunic and Peritoneum in a Young Stallion.

Mesothelioma is a rare malignancy in horses. This report describes a case in which marked hemoperitoneum, moderate anemia, and moderate dependent edema were the presenting clinical signs in a 2-year-old Standardbred stallion with mesothelioma of the vaginal tunic. On necropsy, approximately 40 L of dark red fluid distended the abdomen. A dark red mass infiltrated and effaced the right pampiniform plexus, and red nodular masses were present multifocally on the parietal and visceral peritoneal surfaces. Histopathologically, the masses were composed of malignant spindle cells with a sarcomatoid appearance. Using immunohistochemistry, tumor cells were cytokeratin and vimentin positive and factor VIII-related antigen negative: compatible with a diagnosis of mesothelioma. This is the first report of mesothelioma arising from the vaginal tunic of a horse with subsequent spread to the peritoneum. This case adds additional information regarding the clinical presentation and behavior of mesothelioma originating from the vaginal tunic in horses.