Study questions antibiotic signals in equine asthma cultures

A positive tracheal wash culture has long complicated equine asthma cases, especially when clinicians are deciding whether a horse needs antimicrobials. A new study in Equine Veterinary Journal argues that culture positivity, by itself, may be a poor proxy for true lower-airway bacterial disease. Researchers reported that positive tracheal wash cultures were not associated with bronchial infection, colonization, increased airway inflammation, or increased bronchial remodeling in horses with asthma. (beva.onlinelibrary.wiley.com)

The question matters because equine asthma is common, chronic, and often managed at the intersection of environmental control, anti-inflammatory therapy, and performance expectations. Standard treatment centers on antigen avoidance, corticosteroids, and bronchodilators, but antibiotics are still used in a meaningful share of cases. In the study's introduction, the authors note that an international survey found more than 30% of equine veterinarians prescribe antimicrobials for case descriptions consistent with equine asthma, and that tracheal wash culture results are among the most commonly cited reasons for doing so. (beva.onlinelibrary.wiley.com)

To test whether those positive cultures actually reflect clinically meaningful infection, the investigators reviewed tracheal wash aerobic bacterial cultures, endobronchial biopsies, BALF cytology, and tracheal mucus scores from 31 horses in an equine respiratory tissue biobank and referral population. The cohort included 13 horses with severe equine asthma, nine with mild-to-moderate equine asthma, and nine control horses without respiratory disease. 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, a finding that undercuts the assumption that bacterial growth on culture necessarily signals asthma-related infection. (beva.onlinelibrary.wiley.com)

The rest of the dataset pointed in the same direction. No bacteria were identified on Gram stain in any endobronchial biopsy samples. Among asthmatic horses, positive versus negative tracheal wash cultures were not associated with significantly different histologic remodeling scores, BALF neutrophil percentages, or tracheal mucus scores. The paper also reported no meaningful correlation between colony counts and BALF neutrophilia, and no significant link between colony counts and mucus scores. In other words, bacterial growth in the tracheal sample did not track with the markers clinicians would expect if infection were contributing materially to disease severity. (beva.onlinelibrary.wiley.com)

The broader clinical context supports that interpretation. AAEP's infectious respiratory disease field diagnostic guidelines recommend tracheal wash when bacterial or fungal pneumonia is suspected, but bronchoalveolar lavage when asthma is the leading differential. Other AAEP educational material similarly notes that positive bacterial culture in a sample showing neutrophilic inflammation but few or no visible bacteria often points to noninfectious airway inflammation, including equine asthma, particularly when growth is scant. Together, those sources suggest the new paper is less a radical departure than a sharper evidence base for a more disciplined diagnostic split between infectious and noninfectious lower-airway disease. (aaep.org)

There is some nuance. A small 2018 clinical study found that adding antimicrobials to conventional therapy improved some outcomes in horses with severe equine asthma and positive tracheal wash cultures, so the question of when bacteria are incidental versus clinically relevant isn't fully closed. But the new study differs in what it asks: not whether some horses may improve with antibiotics, but whether culture positivity itself is a reliable marker of infection, colonization, or worse airway pathology. Based on these data, the answer appears to be no. (pubmed.ncbi.nlm.nih.gov)

Why it matters: For veterinary professionals, this is an antimicrobial stewardship story as much as a respiratory medicine story. If positive tracheal wash cultures are common even in controls, and don't align with biopsy, cytology, or mucus burden, then treating the lab result instead of the horse could expose patients to unnecessary antimicrobials while muddying case management. That has implications for referral workups, racehorse medicine, client communication, and practice protocols, especially in settings where performance pressure can push clinicians toward overtreatment. It also reinforces the value of BALF cytology, clinical history, and environmental assessment when differentiating asthma from infectious lower-airway disease. (beva.onlinelibrary.wiley.com)

What to watch: The next question is whether these findings change prescribing behavior, guideline language, or diagnostic sequencing in practice. Expect attention on larger prospective studies, especially those that can better define when bacterial findings in airway samples are incidental, when they reflect contamination or upper-airway carryover, and when they truly justify antimicrobial treatment in horses with asthma. (beva.onlinelibrary.wiley.com)