Abstract: Background – Steaming hay reduces respirable particles and is commonly used to feed horses with asthma. However, it showed inconsistent benefits in clinical studies.Hypothesis/Objectives – We hypothesized that horses with severe equine asthma (SEA) would develop airway obstruction when fed dry hay but not steamed hay. Animals – Nine horses with SEA from a research herd, in remission on a pelleted diet.Methods – Horses were fed steamed and dry hay for 4 weeks in a prospective, cross-over study. Lung function (impulse oscillometry) and blinded weighted clinical scores (WCS) were recorded before and after 4 weeks of hay feeding. A linear mixed model with post-hoc tests was used.Results – Resistance at 5 hertz (R5) increased over the 4-week period (time effect and post-hoc End versus Baseline: p< 0.001), with no difference between treatments (mean (SD), kPa/L/s) (Baseline dry: 0.065 (0.014); End dry: 0.079 (0.019); Baseline steamed: 0.063 (0.009); End steamed: 0.078 (0.014)). There was a treatment-sequence interaction (p< 0.001) with higher R5 during the second treatment. WCS did not change significantly (median (range)) (Baseline dry: 2 (1-5); End dry: 2 (1-4); Baseline steamed: 3 (1-6); End steamed: 1 (1-4), time and treatment effects p >0.1).Conclusion and clinical importance – Steamed hay induced a mild but significant deterioration of lung function in horses with SEA. The lack of differences with dry hay could be due to the unexpectedly mild exacerbation during this study. The treatment-sequence interaction suggests a cumulative effect of stabling and hay feeding on airway resistance.
