Study questions value of positive TW cultures in equine asthma
Bottom line
Positive tracheal wash bacterial cultures in horses with asthma weren’t linked to evidence of bronchial infection, airway remodeling, or more intense airway inflammation in a new Equine Veterinary Journal study highlighted by EquiManagement. The study evaluated whether horses with asthma and a positive aerobic tracheal wash, or TW, culture actually showed signs of bacterial involvement deeper in the airways, and the answer was essentially no: positive cultures did not track with bacteria identified in endobronchial biopsies, worse histologic remodeling, higher bronchoalveolar lavage fluid inflammation, or higher tracheal mucus scores. The findings add to a growing body of work suggesting that bacterial growth on TW in equine asthma may reflect colonization or sampling context rather than clinically meaningful lower-airway infection. (beva.onlinelibrary.wiley.com)
Why it matters: For equine veterinarians, the study reinforces a practical diagnostic point: a positive TW culture, by itself, shouldn’t be read as proof of bacterial pneumonia or a reason to default to antimicrobials in horses with asthma. Current AAEP guidance separates sampling priorities for suspected infectious respiratory disease versus asthma, noting TW when bacterial or fungal pneumonia is suspected and BAL when asthma is suspected. Earlier research has also found that adding antimicrobials to standard severe equine asthma treatment in horses with positive TW cultures did not improve outcomes, which supports a more cautious, stewardship-minded interpretation of culture results. (aaep.org)
What to watch: Expect this paper to further shape how clinicians interpret TW culture results in asthmatic horses, especially around antimicrobial decision-making and the relative roles of TW versus BAL in workups. (aaep.org)
Key facts
- Study type
- Equine Veterinary Journal study
- Topic
- Positive aerobic tracheal wash cultures in horses with asthma
- Main finding
- Positive TW cultures were not linked to bronchial infection, airway remodeling, BAL fluid inflammation, or tracheal mucus scores
- Biopsy finding
- No association with bacteria in endobronchial biopsies
- Organisms noted
- Streptococcus species and Pasteurellaceae
- Clinical takeaway
- A positive TW culture alone should not be read as proof of bacterial pneumonia or a default reason for antimicrobials in horses with asthma
- Sampling guidance
- AAEP guidance uses TW for suspected bacterial or fungal pneumonia and BAL for suspected asthma
A new Equine Veterinary Journal study is challenging a common clinical assumption in equine respiratory medicine: that a positive aerobic tracheal wash culture in a horse with asthma signals a meaningful bacterial lower-airway infection. As summarized by EquiManagement and the journal listing, researchers found no association between positive TW culture results and evidence of bronchial infection or colonization, airway remodeling, bronchoalveolar lavage fluid inflammation, or tracheal mucus burden in horses with asthma. (beva.onlinelibrary.wiley.com)
That question matters because TW culture results can complicate case management. In practice, horses with equine asthma may have bacterial growth detected on tracheal samples, leaving clinicians to decide whether that growth is incidental, secondary, or clinically actionable. The broader equine asthma literature has been moving toward a more cautious interpretation for years. A 2020 study on tracheal microbial populations in horses with moderate asthma did not support bacterial overgrowth as a common feature of chronic disease, and an earlier treatment study found no outcome benefit from adding antimicrobials to standard therapy in severe equine asthma cases with positive TW cultures. (onlinelibrary.wiley.com)
The new study specifically asked whether a positive TW aerobic bacterial culture in horses with asthma correlated with deeper evidence of infection or more severe airway pathology. Investigators assessed bacteria and remodeling in endobronchial biopsies, along with bronchoalveolar lavage fluid cytology and tracheal mucus scores. According to the journal summary, positive TW culture status did not correspond with worse semiquantitative histologic scores, more inflammation on BALF, or more mucus. The same lack of association held even when researchers looked at organisms commonly recovered in these cases, including Streptococcus species and members of the Pasteurellaceae family. (beva.onlinelibrary.wiley.com)
The findings also fit with how major equine organizations frame respiratory diagnostics. AAEP’s field diagnostic guidance recommends TW when bacterial or fungal pneumonia is suspected, while BAL is the preferred sampling approach when asthma is suspected. Separately, AAEP educational material notes that tracheal mucus scoring and TW cytology can be useful adjuncts in equine asthma, but BAL remains central to characterizing lower-airway inflammation. Taken together, that context suggests the new paper is less about dismissing TW altogether and more about using it in the right clinical question. (aaep.org)
Direct expert reaction to this specific paper was limited in publicly accessible coverage, but the industry direction is clear. Recent AAEP materials and prior reviews emphasize that respiratory samples need to be interpreted in light of the horse’s history, exam findings, and the suspected disease process, rather than in isolation. That’s especially relevant in asthma, where environmental exposure, mucus stasis, and chronic inflammation can muddy the meaning of a culture result without proving active bacterial disease. (aaep.org)
Why it matters: For veterinary professionals, the paper supports a more disciplined antimicrobial stewardship approach in equine asthma. If a horse has clinical findings consistent with asthma, a positive TW culture alone may not justify antibiotics, particularly in the absence of fever, systemic illness, imaging changes, or cytologic evidence pointing to pneumonia. It also reinforces the value of matching the sample type to the suspected diagnosis: BAL for asthma phenotyping and inflammatory assessment, TW for suspected infectious lower-airway disease. That distinction can help avoid overtreatment, reduce unnecessary antimicrobial exposure, and sharpen case communication with pet parents and trainers. (pmc.ncbi.nlm.nih.gov)
What to watch: The next step will be whether this evidence changes day-to-day prescribing behavior and diagnostic algorithms in referral and field practice. Watch for future consensus updates, conference discussion, and follow-on studies that define when a positive TW culture in an asthmatic horse does indicate true coinfection, and which combination of cytology, endoscopy, imaging, and clinical signs best separates colonization from disease. (beva.onlinelibrary.wiley.com)