Disseminated lipid-rich peritoneal mesothelioma in a horse

A 9-year-old Haflinger mare presented to the Liphook Equine Hospital with a history of weight loss, azotemia, and repeated episodes of ascites over a period of 10 days. The horse was euthanized after exploratory laparotomy revealed large numbers of variably sized masses distributed throughout the peritoneal cavity. Macroscopically, some masses were papillary, while others were nodular. Histologically, the masses were comprised of large to giant, variably shaped, and occasionally multinucleated neoplastic cells with marked anisokaryosis and anisocytosis and a high mitotic rate. Small to moderate numbers of neoplastic cells were swollen by 1 to several, moderately sized to large, clear, circular or ovoid vacuoles, which stained positive with oil red O. Immunohistochemically, the neoplastic cells co-expressed vimentin and cytokeratin. Electron microscopy demonstrated tumor cells with tight junctions, microvilli, and numerous intracytoplasmic lipid droplets. These findings are consistent with a lipid-rich form of mesothelioma, which should be considered as a differential diagnosis if lipid vacuoles are present in potentially neoplastic cells in equine abdominocentesis samples.