Retrospective study of fatal pneumonia in racehorses.

Respiratory diseases have a major impact on racehorses in training and are often cited as the second most common reason of horses failing to perform. Cases were submitted by the California Horse Racing Board to the California Animal Health and Food Safety laboratory for postmortem examination between January 1, 2005 and December 31, 2014. We determined the demographics of racehorses with fatal pneumonia, characterized the pathologic findings in animals with a postmortem diagnosis of respiratory infection, and determined the most significant pathogens associated with lower respiratory tract disease. We analyzed autopsy reports from 83 horses with a diagnosis of pneumonia, bronchopneumonia, and/or pleuropneumonia. The most common presentation was pleuropneumonia (71% of cases), with extensive areas of lytic necrosis and abscesses of the pulmonary parenchyma. Streptococcus equi ssp. zooepidemicus, a normal mucosal commensal of the upper respiratory tract of healthy horses, was the most commonly isolated organism (72% of cases), either in pure culture or accompanied by other aerobic or anaerobic bacteria. Its presence in the pulmonary parenchyma is associated with severe and extensive damage to the lung. Furthermore, this agent has zoonotic potential, which stresses the importance of early detection and proper management of cases of pneumonia in racehorses.