Causes of pleural effusions in horses resident in the UK

Pleural effusions (PE) reportedly occur most commonly secondary to bacterial pneumonia with neoplastic effusions contributing a minority of cases. The majority of reports originate from the USA and Australia, where long distance transport of horses, a recognised risk factor, may occur more frequently than in the UK. Anecdotally, a greater proportion of horses with PE are diagnosed with neoplasia in the UK than has been reported. The aim of this retrospective study was to describe the causes of PE in horses in the UK, and to identify markers that can help differentiate between septic and neoplastic causes of PE. Medical records from 4 equine hospitals in the UK were searched for horses diagnosed with PE. Information recorded included case background, admission physical examination and biochemical findings, and characteristics of the effusion ( volume, cell count, total protein [TP] concentration). A total of 69 horses were identified, with 26 (38%) diagnosed with a neoplastic effusion. The remainder were categorised as septic, including 14/43 (32.5%) that had a history of international transport. Horses with septic effusions were significantly younger ( 8 vs. 13 years; P = 0.001) and had significantly smaller volumes of pleural fluid drained at admission (9.8 l vs. 32.2 l; P < 0.001). Horses with septic PE had a significantly higher rectal temperature ( 38.6 degrees C vs. 38.2 degrees C; P = 0.03), fibrinogen concentration ( 7.8 g/l vs. 5.3 g/l; P = 0.01) and serum amyloid A concentration (230 mg/l vs. 59 mg/l; P = 0.02) than those with neoplastic effusions. Significantly higher pleural fluid cell count and TP concentration were identified in horses with septic PE (63.9 x10(9)/l vs. 8.6 x 10(9)/l; P < 0.001; 57.5 g/l vs. 35.9 g/l; P = 0.04). These results suggest that in the UK, neoplastic effusions account for a greater proportion of PE than previously reported. A large volume of PE in an older horse with a low cell count and relatively low TP concentration should increase the index of suspicion of neoplasia.