Diagnostic dichotomy: A question of thoracic mesothelioma

Summary A 12-year-old Irish Sport Horse gelding was presented with a two-day history of acute onset dyspnoea, pyrexia, tachycardia and ventral sternal oedema. A pleural effusion was identified, and thoracocentesis yielded a haemorrhagic modified transudate. Cytological examination of the fluid showed numerous red blood cells with smaller numbers of large nucleated cells that exhibited mild anisocytosis and anisokaryosis. Thoracoscopy identified adhesions between the visceral and dorsal parietal pleura, haemorrhagic effusion ventrally and haemorrhagic soft tissue plaques scattered over the diaphragm and mediastinum; biopsies of tissue on the caudal mediastinum were obtained. Histopathological examination of the biopsies identified inflammation, with a suspicion of underlying neoplasia; adenocarcinoma, haemangiosarcoma or mesothelioma were considered most likely, but could not be confirmed on the basis of these examinations. The horse was euthanased, and mesothelioma was diagnosed following post mortem examination. This case demonstrates some of the difficulties associated with the accurate premortem diagnosis of mesothelioma.