Causes of Pleural Effusion in Horses in the UK

Reasons for performing study Pleural effusion ( PE ) is reported to occur most commonly secondary to bacterial pneumonia or lung abscesses, with neoplastic effusions contributing the minority of cases. The majority of these reports originate from America and A ustralia, where long distance transport of horses, a recognised risk factor, appears to occur more frequently. Anecdotally, neoplastic PE is more commonly diagnosed in the UK . Objectives To describe the causes of PE in horses resident in the UK , and to identify potential markers that can help differentiate between infectious and neoplastic causes of PE . Study design Retrospective clinical study. Methods Medical records from 4 referral hospitals in southern England were searched for horses diagnosed with PE . Information gathered from medical records included signalment, diagnosis (infectious vs. neoplastic), admission physical examination and biochemical findings, and characteristics of the effusion (volume, cell count, total protein [ TP ] concentration). Statistical comparisons were made between the neoplastic and infectious group using appropriate testing. Results Seventy horses were identified, of which 28 (40%) were neoplastic and 42 were infectious. Horses with infectious effusions were significantly younger (median 7 vs. 13 years; P = 0.002) and had significantly smaller volumes of pleural fluid drained at admission (9.8 vs. 32.3 l; P<0.001). Horses with infectious PE had a significantly higher rectal temperature (38.6 vs. 38.2° C ; P = 0.03), fibrinogen concentration (7.8 vs. 5.7 g/l; P = 0.02) and serum amyloid A concentration (223 vs. 104 mg/l; P = 0.02). Pleural fluid characteristics identified a significantly greater cell count and TP concentration in horses with infectious PE (47 x 10 9 /l vs. 3.4 x 10 9 /l; P<0.001; 54 vs. 31 g/l; P = 0.001). Conclusions 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 TP concentration should increase the index of suspicion of neoplasia. Ethical animal research:  This was a retrospective study of clinical cases. Explicit owner informed consent for inclusion of animals in this study was not stated. Source of funding:  None. Competing interests:  None declared.