Sport horse IAD - moderate equine asthma?

Many sports horses show occasional coughing and/or poor performance, but no increased respiratory effort at rest; markedly, but not severely increased BALF neutrophils, an increased tracheal mucus score and/or airway hyperresponsiveness. This presentation, which could be classified as « moderate equine asthma », was found in approximately 10-20% of sport horses in several studies investigating show jumpers or dressage horses in Switzerland and Germany. However, examination protocols were not consistent between studies, which were based on convenience samples, so the results allow only a rough estimate and may not be representative of different sport horse populations.

“Moderate” obviously implies a distinction from “mild” and “severe”. Findings as described above could potentially reflect incomplete remission from underlying severe equine asthma, i. e. recurrent airway obstruction (RAO). To further investigate this, a challenge with (poor quality) hay may be necessary, which is relatively straight forward at least in theory. In contrast, making the distinction between milder forms is more difficult, since inflammatory airway disease (IAD) spans mild and moderate equine asthma according to the recently revised consensus statement.

The studies in sports horses referred to above indicate that at least another 10-20 % of sports horses show « mild equine asthma ». This could be characterised by the same clinical signs as moderate equine asthma, but with only mild neutrophilic, eosinophilic, and/or metachromatic airway inflammation in the range defined as « equivocal » in the revised consensus.

Comparing sport horses vs. racehorses, we can expect that milder forms of lower airway disease are more relevant and therefore more often investigated and identified in the latter (see “racehorse IAD – mild equine asthma?”), resulting in reporting bias towards mild IAD in racehorses. The younger mean age could further lead to a larger proportion of milder forms of “racehorse IAD”, since there is evidence that severity of equine asthma is often progressive. Direct comparison of representative populations of racehorses vs. sport horses of different disciplines would be required to substantiate these speculations.