Positive TW cultures may not signal infection in equine asthma

A new Equine Veterinary Journal study is challenging a common clinical assumption in equine asthma: that a positive tracheal wash bacterial culture points to a meaningful lower-airway bacterial infection. In the retrospective case-control study, researchers found that positive cultures were common not only in horses with severe and mild-to-moderate equine asthma, but also in control horses, and those positive results were not associated with biopsy evidence of infection, more airway remodeling, or more inflammation. The authors concluded that tracheal wash culture results alone should not be a primary driver of treatment decisions in equine asthma. (beva.onlinelibrary.wiley.com)

That conclusion lands in the middle of a long-running diagnostic and treatment debate. Equine asthma is now understood as a heterogeneous inflammatory airway syndrome rather than a single disease, and standard management focuses on environmental control, corticosteroids, and bronchodilators. Tracheal wash remains practical in field settings and offers the advantage of bacterial culture, but its role has limits, particularly when clinicians are trying to distinguish inflammation from infection. Reviews and consensus-oriented discussions have noted ongoing disagreement between tracheal wash and bronchoalveolar lavage findings in some horses, even as tracheal wash remains widely used in practice. (frontiersin.org)

In the new study, investigators evaluated samples from 13 horses with severe equine asthma, nine with mild-to-moderate disease, and nine controls. Positive tracheal wash cultures were found in 8 of 13 horses with severe asthma, 6 of 9 with mild-to-moderate asthma, and 8 of 9 controls. No bacteria were identified on Gram stains of endobronchial biopsy samples, and histologic remodeling scores, bronchoalveolar lavage neutrophil percentages, and tracheal mucus scores did not significantly differ between asthmatic horses with positive and negative cultures. The study’s main limitation was its small sample size, but the direction of the findings was consistent: culture positivity did not track with markers of lower-airway infection or more severe disease. (beva.onlinelibrary.wiley.com)

The paper also speaks directly to prescribing behavior. The authors note that antimicrobial resistance is a growing concern in both veterinary and human health, and that respiratory disease in horses is associated with use of critically important antimicrobials, including fluoroquinolones and later-generation cephalosporins. That concern is echoed by an international survey published earlier this year, which found that 131 respondents said they use antimicrobials at least sometimes in at least one equine asthma scenario, with tracheal wash culture influencing 62% of those prescribing decisions. Another earlier study in horses with severe equine asthma and positive tracheal wash cultures suggested antimicrobials might improve some clinical parameters, but the evidence was limited and did not settle the broader question of whether culture positivity alone identifies a bacterial process worth treating. (beva.onlinelibrary.wiley.com)

Industry and expert-oriented coverage has increasingly framed equine asthma as a management-heavy disease in which air quality, forage exposure, and dust control often matter more than reflexive drug escalation. Educational coverage from The Horse has emphasized that horses can do well when environmental management and anti-inflammatory treatment are prioritized, and that severe cases may require a carefully maintained low-dust breathing zone. That perspective aligns with the new study’s implication that a lab result from the trachea may not tell the full story about what is happening deeper in the lung. (thehorse.com)

Why it matters: For veterinary professionals, this study reinforces a practical stewardship message: in horses with asthma, a positive tracheal wash culture should be interpreted alongside clinical signs, cytology, endoscopy, and the broader history, not as a stand-alone trigger for antibiotics. It also underscores the importance of distinguishing inflammatory airway disease from true infectious pneumonia or other respiratory infections. In day-to-day practice, that could mean more confidence in prioritizing environmental modification and asthma-directed therapy, while reserving antimicrobials for cases with stronger evidence of bacterial involvement, such as systemic illness, compatible cytology, or other diagnostic support. (beva.onlinelibrary.wiley.com)

What to watch: The next step is whether these findings change how clinicians weigh tracheal wash culture in asthma algorithms, and whether future studies with larger populations can identify which positive cultures are clinically meaningful versus incidental, contaminant, or reflective of upper-airway flora. (beva.onlinelibrary.wiley.com)