Cystic, peritoneal mesothelioma in a dog.

A 10-year-old, spayed female beagle was presented with a 2.5-year history of ascites. The ascitic fluid was a modified transudate with no abnormal cells present. Cardiac and hepatic functions were normal. Abdominal ultrasonography revealed cystic abnormalities of the peritoneal serosal surfaces. On exploratory laparotomy, the parietal and visceral peritonea were covered with variably sized, thin-walled cysts containing serous fluid. Histopathological, immunohistochemical, and ultrastructural analyses of the lesions were consistent with a diagnosis of cystic, peritoneal mesothelioma.