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

Bacterial pneumonia in horses often arises due to factors that weaken their natural defenses. This study analyzed 81 cases over ten years at a European veterinary clinic to understand causes, bacterial profiles, treatment challenges, and outcomes in affected horses.

Study Overview and Population

• The study was a retrospective analysis conducted at the equine clinic of Freie Universität Berlin from January 2015 to December 2024.
• A total of 81 horses diagnosed with bacterial pneumonia were included.
• Horses infected with primary pathogens such as Streptococcus equi or related ssp. were excluded to focus on secondary bacterial pneumonia cases.

Predisposing Factors and Demographics

• The leading predisposing factors that impair equine respiratory defenses and led to pneumonia were:
  • Aspiration, mostly following oesophageal obstruction, occurring in 37.0% of cases.
  • Post-anaesthetic complications (27.2%).
  • Primary respiratory signs as an initial trigger (24.7%).
  • Other miscellaneous conditions (11.1%).
• Horses experiencing aspiration were typically older than horses in other categories.

Clinical Signs and Diagnosis

• Common clinical signs observed included:
  • Tachypnoea (rapid breathing)
  • Tachycardia (rapid heart rate)
  • Fever
• These signs, however, were not present in every horse, indicating variability in clinical manifestation.
• The primary diagnoses were:
  • Bronchopneumonia overall, especially predominant post-aspiration.
  • Pleuropneumonia, more often observed following general anaesthesia or with primary respiratory signs.

Bacteriological Findings

• Bacteriological examinations were performed in 51 of the 81 horses.
• Mixed bacterial infections were common rather than a single isolate causing infection.
• Among identified pathogens, certain subspecies (designated as “ssp.” in the study) were frequently isolated, indicating their role in equine pneumonia.
• Antimicrobial resistance was a significant concern:
  • Resistant bacterial isolates were detected in most tested cases.
  • Multidrug resistance was particularly prevalent in horses developing pneumonia after anaesthesia.

Treatment Outcomes and Prognosis

• Overall survival to hospital discharge was 77.8%, indicating a favorable but not guaranteed outcome.
• The survival rate did not significantly differ between different predisposing causes.
• Several clinical and diagnostic indicators were associated with poorer survival:
  • Initial lymphopenia (reduced lymphocyte count).
  • Presence of pleural effusion (fluid accumulation around lungs).
  • Need for thoracic drainage procedures, suggesting more severe thoracic involvement.
• Other factors such as anaerobic bacterial infection and pulmonary abscesses were linked to increased risk of readmission due to recurrent respiratory problems.

Long-Term Follow-Up

• Most horses that survived to discharge had acceptable long-term outcomes regarding their respiratory health.
• Despite survival, recurrent respiratory signs remained common, indicating a need for ongoing monitoring and management in some cases.

Implications for Veterinary Practice

• This study emphasizes the importance of recognizing predisposing factors like aspiration and post-anaesthetic complications as key contributors to bacterial pneumonia in horses.
• The high rates of antimicrobial resistance highlight the need for targeted bacteriological cultures to guide antibiotic therapy rather than relying solely on empirical treatment.
• Monitoring clinical signs and diagnostic parameters early can help identify horses at higher risk of mortality and tailor more aggressive interventions.
• Awareness of potential for recurrent respiratory issues suggests that owners and vets should plan for long-term follow-up care.