Mares with severe equine asthma exhibit a narrower trachea than geldings in a retrospective radiographic study of 189 horses

Objective: To evaluate potential relationships between sex, asthma severity, and tracheal diameter in adult horses through the use of thoracic radiography. Methods: This was a retrospective case-control study of horses that underwent thoracic radiography at the Université de Montréal from 2018 through 2024. Horses were classified as control, mild/moderate asthma, or severe asthma (SEA) according to the latest consensus statement on equine asthma. To account for radiographic magnification and body size, standardized ratios of tracheal diameter relative to the fifth thoracic vertebral body length and aortic diameter were calculated. The effects of disease status, sex, and their interaction were analyzed using a 2-way ANOVA and Tukey multiple-comparisons test, with results presented as mean [SD] and 95% confidence intervals (CI) of the difference for significant pairwise comparisons. Results: 189 horses were included. The trachea-to-fifth thoracic vertebral body length ratio differed by disease status, with an interaction between sex and disease, as SEA mares had a lower ratio (0.7 [SD, 0.1]) than SEA geldings (0.8 [SD, 0.1]; 95% CI, 0.03 to 0.2), control mares (0.8 [SD, 0.1]; 95% CI, 0.05 to 0.3), and mild/moderate asthma mares (0.8 [SD, 0.1]; 95% CI, 0.03 to 0.2). The trachea-to-aorta ratio was influenced by disease status as it was lower in SEA horses than in controls. Absolute tracheal diameter was affected by disease status and sex, with an interaction between both factors, with SEA mares exhibiting the narrowest trachea. Conclusions: Tracheal diameter is influenced by both sex and asthma status in horses. Clinical Relevance: Whether this sex-related difference in tracheal diameter influences lung mechanics and exercise capacity remains to be investigated.