Abstract
A Quarter Horse, a gelding aged 22, was subjected to a clinical examination for colic syndrome. During admission to the clinic, blood counts and ultrasound examination were performed. Ultrasound revealed abdominal masses and abundant accumulation of pleural (50 L) and abdominal fluid (100 L). Cytology was performed on the aspirated fluid. The patient was euthanized. The autopsy examination revealed abundant effusion and nodular masses on the peritoneum, omentum, lungs, heart, and mediastinum. A diagnosis of epithelioid mesothelioma was made via histopathology and confirmed with immunohistochemistry; it showed positive antibodies against cytokeratin (CK) and vimentin. Mesothelioma is a rare cancer in older horses. It is important to employ the correct differential diagnostics using the available methods, providing valid ante-mortem support to the clinical veterinarian and monitoring the territory using this species as a valid biological indicator.