Retrospective study of pneumonia in non-racing horses in California.

Pneumonia is a significant disease of horses. Although pneumonia has traditionally been studied in racehorses, little information is available for non-racing horses. Non-racing horses that died with pulmonary lesions (n = 156) were available from cases submitted for autopsy from January 2015 to June 2020. Bronchopneumonia (35%), interstitial pneumonia (29%), embolic pneumonia (21%), granulomatous pneumonia (13%), and pleuritis (2%) were observed in the examined horses. Seventy-four horses died or were euthanized because of pulmonary diseases, and 82 horses died or were euthanized because of non-pulmonary causes but had lung lesions. Of the horses that died from pulmonary causes, the most common finding was bronchopneumonia, with abscesses and/or necrosis in the cranioventral aspect of the lung. Bacteria isolated from cases of bronchopneumonia were Streptococcus equi subsp. zooepidemicus (48.5%), Klebsiella pneumoniae (12.1%), and Actinobacillus equuli subsp. haemolyticus (9.1%). The most common extrapulmonary lesions responsible for death in horses that also had lesions in the lung were mainly in the gastrointestinal system (30%), multiple systems (septicemia and/or toxemia; 27%), and musculoskeletal system (12%). The main postmortem findings in cases of bronchopneumonia of non-racing horses were similar to those reported previously in racehorses. However, some non-racing horses also had interstitial and granulomatous pneumonia, patterns not described previously in racehorses in California, likely as a result of the inclusion of extended age categories for non-racing horses. We also found that the equine lung was frequently affected in cases of sepsis and gastrointestinal problems of infectious origin.