AbstractBackgroundSteaming hay reduces respirable particles and is commonly used to feed horses with asthma. However, it showed inconsistent benefits in clinical studies.Objectives(1) To assess the effects of steamed hay on lung function and airway inflammation in horses with severe equine asthma (SEA) in remission; (2) To compare these effects with a dry hay diet.Study DesignCross‐over in vivo experiment.MethodsHorses were fed steamed and dry hay for 4 weeks in a prospective, cross‐over study, with a 4‐week washout period. Lung function, bronchoalveolar lavage (BALF) cytology, and a 23‐point weighted clinical score (WCS) were recorded before and after 4 weeks of hay feeding. A mixed linear model with post hoc tests was used.ResultsResistance at 5 Hz (R5) increased over the 4‐week period (time effect and post hoc end vs. 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 significant increase in BALF neutrophil percentages over time (end vs. baseline: p < 0.001) (baseline dry: 6.7 [5.4]; end dry: 13.1 [6.0]; baseline steamed: 5.6 [2.6]; end steamed: 10.5 [4.3]). WCS did not change significantly (baseline dry: 2.6 [1.5]; end dry: 2.2 [1.2]; baseline steamed: 2.9 [1.5]; end steamed: 1.9 [1.3]).Main LimitationsThis study involves small number of horses in a research setting. Hay dust content and particles in the breathing zone were not measured.ConclusionsSteamed hay induced a mild but significant deterioration of lung function and inflammation in horses with SEA. The lack of differences with dry hay could be due to the unexpectedly mild exacerbation during this study.