Airway endoscopy and bronchoalveolar lavage (BAL) are the two tools many equine vets reach for when a horse has a cough, a nasal discharge, or poor performance. The scope shows you the airway. The lavage fluid shows you the cells. Each finding on its own can mislead, and the real skill is reading them together with the history.
This guide covers how those findings are interpreted, using the 2016 ACVIM consensus statement (Couëtil et al., 2016), and what to ask when you’re choosing an equine scope. It doesn’t cover how to perform a BAL or pass a scope. That belongs in a clinical reference or hands-on instruction. Here we stay with what the findings mean.
A quick vocabulary update
The 2016 consensus statement grouped inflammatory airway disease (IAD) and recurrent airway obstruction (RAO, or heaves) under the term “equine asthma,” with IAD at the mild end and RAO at the severe end (Couëtil et al., 2016). In today’s nomenclature:
- IAD is now mild to moderate equine asthma (mEA).
- RAO, or heaves, is now severe equine asthma (sEA).
You’ll still see the old terms in older papers and in conversation, so it helps to know both. The panel was careful about one point: the new classification shouldn’t be read as a continuum in which every horse with IAD goes on to develop RAO. They also noted that the majority of horses with IAD appear to recover (Couëtil et al., 2016).
How mild and severe asthma tend to present
The statement describes severe asthma, or heaves, as marked lower airway inflammation and obstruction with frequent coughing, increased respiratory effort at rest, and exercise intolerance. Signs and obstruction can be reversed with corticosteroids, bronchodilators, or a change in environment, and it principally affects horses over 7 years of age (Couëtil et al., 2016). Mild to moderate asthma can affect horses of any age, although it is more commonly reported in young horses (Couëtil et al., 2016). Horses with similar presentations, such as chronic cough, excess mucus, or poor performance, can vary widely in severity (Couëtil et al., 2016), which is why the findings from the scope and the lavage matter.
What the consensus statement looks at
The statement builds a diagnosis from several findings read together with the horse’s history and clinical signs. Two of them come directly from the work you do with a scope: the endoscopic mucus score and the BAL fluid cytology (Couëtil et al., 2016). For severe asthma, the case definition also includes evidence of airflow limitation that reverses with a bronchodilator or an environmental change, and airway hyperresponsiveness (Couëtil et al., 2016).
Endoscopy: the mucus score
On airway endoscopy, the consensus statement identifies excess mucus in the tracheobronchial tree as a diagnostic finding for mild to moderate asthma. The thresholds differ by type of horse, on a 0 to 5 scale (Couëtil et al., 2016):
- Racehorses: a mucus score of 2 or higher out of 5
- Sport and pleasure horses: a mucus score of 3 or higher out of 5
Mucus matters for performance. The statement notes that poor racing performance in racehorses, and reduced willingness to perform in show-jumpers and dressage horses, are associated with excess tracheal mucus, although an association with tracheal wash neutrophilia has not been detected (Couëtil et al., 2016).
For severe asthma, the statement says a combination of pronounced BAL neutrophilia (neutrophils above 25%) and tracheal mucus accumulation (a grade above 2 out of 5) may indicate it, alongside severe exercise intolerance (Couëtil et al., 2016).
BAL cytology: the numbers
The consensus statement gives reference values for healthy horses and criteria for the two categories. These are based on studies that used a 250 mL infusion volume (Couëtil et al., 2016).
| Neutrophils | Mast cells | Eosinophils | |
|---|---|---|---|
| Healthy horses | 5% or less | 2% or less | 1% or less |
| Mild to moderate asthma | above 10%, or | above 5%, or | above 5% |
| Severe asthma | above 25% | typically below 2% | typically below 1% |
For mild to moderate asthma, one or more of the three thresholds is enough (Couëtil et al., 2016). The healthy-horse total nucleated cell count in the statement is 530 cells per microliter or less (Couëtil et al., 2016). The Merck Veterinary Manual’s page on asthma in horses shows the same cytology ranges in a table (Merck Veterinary Manual, n.d.), which is a handy free reference.
Values in between are equivocal
The statement says results between the healthy range and the asthma thresholds are likely technique dependent (Couëtil et al., 2016). It also notes that differences in method, including infusion volume, prevent definitive cut-offs, which is why the reference values are tied to the 250 mL protocol (Couëtil et al., 2016). Interpret the numbers against your own technique and the horse’s signs, not as a standalone diagnosis.
Endoscopy and BAL don’t always agree
It’s tempting to expect a high mucus score to go hand in hand with a high neutrophil percentage. The consensus statement points out that tracheal mucus accumulation correlated positively with BAL neutrophil percentage in some studies but not in others, and negatively with BAL mast cell percentage in at least one (Couëtil et al., 2016). In other words, a horse can have a convincing mucus score and a modest BAL, or the reverse. That’s one reason the statement treats these as separate pieces of evidence.
The statement also links abnormal BAL cytology with poor performance and exercise intolerance in both racehorses and non-racehorses (Couëtil et al., 2016).
Reading a result: an illustration
The following is a hypothetical example, not a real case, to show how the thresholds work. Suppose a sport horse has a history of cough and reduced performance, an endoscopic tracheal mucus score of 3 out of 5, and a BAL with 12% neutrophils taken using a 250 mL infusion. Against the consensus criteria, both findings meet the thresholds for mild to moderate asthma (mucus 3 or higher in a sport horse, neutrophils above 10%) (Couëtil et al., 2016). If the same horse had 7% neutrophils, that would fall in the equivocal range, and the mucus score and the history would carry more of the weight.
What to look for in an equine scope
The scope doesn’t change the thresholds above, but it determines what you can see and whether you can see it consistently. When you’re comparing equine endoscopes, ask about:
- Working length. Equine and large animal endoscopes are longer than small animal scopes. Ask which length suits the procedures you actually do.
- Insertion tube diameter. It has to fit the airway you’re working in.
- Instrument channel size. It determines which accessories you can pass.
- Field of view and image quality. You’ll be judging mucus on the screen, so a clear, well-lit image matters.
- Image and video recording. Saved images let you document findings and compare over time.
- Processor compatibility and ruggedness. A barn is hard on equipment. Ask how the scope and processor handle dust, moisture, and travel.
- Reprocessing. Ask for the manufacturer’s cleaning and disinfection instructions before you buy. Our endoscope cleaning and reprocessing guide covers the principles.
Our Aohua line includes large animal endoscopes. The VET-FHD33 veterinary video endoscope is listed with a 3,300 mm working length, a 9.8 mm insertion tube, a 2.8 mm channel, and a 145° field of view. The OR-160 touch screen video processor works with both flexible and rigid scopes. You can browse the full range on our veterinary endoscopes page, and see our posts on the top veterinary endoscopes, what to know before investing in one, and endoscopy for diagnostics and treatment.
Which length and diameter suit lower-airway work in your patients is a clinical question. Ask the manufacturer and a colleague who does it regularly.
A short checklist for your next case
- Is the history compatible, such as cough, nasal discharge, or reduced performance?
- Did you record a tracheal mucus score, and does it meet the threshold for this type of horse?
- Do you know your BAL volume and technique, so the numbers can be compared with the 250 mL reference values?
- Are the cytology results clearly abnormal, clearly normal, or in the equivocal range?
- Does the picture fit mild to moderate or severe asthma, or does it point somewhere else?
- Is your scope cleaned, disinfected, dried, and stored according to the manufacturer’s instructions before the next horse?
Service and support
Our repair team provides phone support and repairs, our inspection service covers preventative maintenance, and the Just Works service plan puts it on a schedule. Financing is available, and you can request a quote for any scope.
Frequently asked questions
What is the new name for IAD in horses? Mild to moderate equine asthma. Recurrent airway obstruction (RAO, or heaves) is now called severe equine asthma (Couëtil et al., 2016).
What endoscopic mucus score suggests equine asthma? The 2016 ACVIM consensus statement identifies excess tracheobronchial mucus as a score of 2 or higher out of 5 in racehorses and 3 or higher out of 5 in sport and pleasure horses (Couëtil et al., 2016).
What BAL cytology is normal in horses? Based on a 250 mL infusion, the consensus statement gives healthy reference values of neutrophils 5% or less, eosinophils 1% or less, and mast cells 2% or less (Couëtil et al., 2016).
What BAL neutrophil percentage indicates severe equine asthma? Severe asthma typically shows neutrophils above 25%. The consensus statement says a combination of pronounced neutrophilia and an endoscopic mucus grade above 2 out of 5, with severe exercise intolerance, may indicate it (Couëtil et al., 2016).
Can BAL results be borderline? Yes. Values between the healthy range and the asthma thresholds are equivocal and likely depend on technique, so interpret them with the horse’s history and clinical signs (Couëtil et al., 2016).
Do endoscopy and BAL always agree? No. The consensus statement notes that tracheal mucus correlated with BAL neutrophil percentage in some studies but not others (Couëtil et al., 2016).
Does the consensus statement apply to every BAL technique? Not exactly. The reference values are based on studies that used a 250 mL infusion volume, and the statement notes that differences in method prevent definitive cut-offs (Couëtil et al., 2016).
References
Couëtil, L. L., Cardwell, J. M., Gerber, V., Lavoie, J.-P., Léguillette, R., & Richard, E. A. (2016). Inflammatory airway disease of horses—Revised consensus statement. Journal of Veterinary Internal Medicine, 30(2), 503–515. https://doi.org/10.1111/jvim.13824
Merck Veterinary Manual. (n.d.). Asthma in horses. Merck & Co. Retrieved November 14, 2026, from https://www.merckvetmanual.com/respiratory-system/respiratory-diseases-of-horses/asthma-in-horses
