Tracheal wash culture is not associated with bronchial infection, remodelling or inflammation in horses with asthma.
Authors: Leduc Laurence, St-Jean Guillaume, Lavoie Jean-Pierre
Journal: Equine veterinary journal
Summary
# Tracheal Wash Culture Results Should Not Drive Antimicrobial Decisions in Equine Asthma Antimicrobials are routinely prescribed for asthmatic horses based on positive tracheal wash (TW) bacterial cultures, yet evidence supporting this practice remains sparse. Leduc and colleagues conducted a retrospective, blinded case-control study comparing TW aerobic culture results against tissue-level evidence of infection and inflammation in 13 horses with severe equine asthma (SEA), 9 with mild-to-moderate asthma (MEA) and 9 controls, using endobronchial biopsies, bronchoalveolar lavage fluid (BALF) cytology and tracheal mucus scoring to validate their findings. Positive TW cultures were common across all groups (62% of SEA, 67% of MEA and 89% of controls), yet no bacteria appeared on Gram stain in any endobronchial biopsy; moreover, histological remodelling scores, neutrophil percentages in BALF and tracheal mucus accumulation did not differ significantly between asthmatic horses with positive versus negative TW cultures (p = 0.4, p = 0.5 and p = 0.7 respectively). This work challenges a widespread clinical assumption: positive TW cultures do not confirm secondary bacterial infection, bronchial colonisation or increased inflammation, suggesting practitioners should reconsider antimicrobial prescription decisions based solely on culture results and instead integrate additional diagnostic markers—such as direct tissue evidence and inflammatory cell profiles—when managing asthmatic horses.
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Practical Takeaways
- •Do not rely solely on positive tracheal wash culture results to justify antimicrobial therapy in asthmatic horses, as these results do not confirm bacterial infection or predict disease severity
- •A positive tracheal wash culture in an asthmatic horse may represent contamination or colonisation rather than pathogenic infection, so clinical judgment should integrate other diagnostic findings
- •Consider withholding or reconsidering antimicrobial treatment in asthmatic horses with positive tracheal wash cultures unless other evidence of bacterial infection is present
Key Findings
- •Positive tracheal wash bacterial cultures were found in 62% of asthmatic horses (14/22) and 89% of control horses (8/9), with no significant difference in prevalence by disease severity
- •No bacteria were identified on Gram stain in any endobronchial biopsy samples across all groups, indicating absence of true bronchial infection
- •Bronchial remodelling histologic scores and neutrophil percentages in BALF were not significantly different between asthmatic horses with positive versus negative tracheal wash cultures (p=0.4 and p=0.5 respectively)
- •Positive tracheal wash cultures are not associated with bronchial infection, colonisation, increased remodelling, or increased airway inflammation in equine asthma