Bacterial Pneumonia in Horses: Insights from a 10-Year European Single-Centre Retrospective Study

Bacterial pneumonia in horses typically occurs secondary to predisposing factors that compromise defence mechanisms. This 10-year retrospective study evaluated bacterial pneumonia in 81 horses treated at the equine clinic of Freie Universität Berlin from January 2015 to December 2024. Horses infected with primary pathogens such as Rhodococcus equi or Streptococcus equi ssp. equi were not included in the study. The most common predisposing events were aspiration, mainly following oesophageal obstruction (37.0%), post-anaesthetic disease (27.2%), primary respiratory signs (24.7%), and other conditions (11.1%). Horses with aspiration were significantly older than the "other" groups. Tachypnoea, tachycardia, and fever were common but not universal. Bronchopneumonia was the most frequent diagnosis overall and predominated after aspiration, whereas pleuropneumonia was more common after general anaesthesia and in horses with primary respiratory signs. Bacteriological examinations were performed in 51 horses. Mixed infections were common, and Streptococcus equi ssp. zooepidemicus was frequently isolated. Resistant isolates were detected in most tested cases, and multidrug resistance was especially common in post-anaesthetic horses. Overall survival to discharge was 77.8% and did not differ significantly between predisposing events. However, initial lymphopenia, pleural effusion, and the need for thoracic drainage were associated with reduced survival. Anaerobic bacterial isolation, pulmonary abscessation, and diagnosis were associated with readmission because of respiratory signs. Most survivors had acceptable long-term outcomes, although recurrent respiratory signs were common.