Environmental Exposures and Airway Inflammation in Young Thoroughbred Horses

Background Inflammatory airway disease (IAD) in horses is a widespread, performance‐limiting syndrome believed to develop in response to inhaled irritants in the barn environment. Objectives To evaluate changes in bronchoalveolar lavage fluid (BALF) cytology and exposure to particulates, endotoxin, and ammonia during horses' first month in training. Animals Forty‐nine client‐owned 12‐ to 36‐month‐old Thoroughbred horses entering race training. Methods In this prospective cohort study, a convenience sample of horses was assigned to be fed hay from a net (n = 16), whereas the remaining horses were fed hay from the ground (n = 33). BALF was collected at enrollment and after 14 and 28 days in training. Respirable particulate, inhalable particulate, respirable endotoxin, and ammonia concentrations were measured at the breathing zone of each horse weekly. Results Median respirable particulates were significantly higher when horses were fed from hay nets than when fed hay from the ground (hay net 0.28 mg/m3, no hay net 0.055 mg/m3, P < .001). Likewise, inhalable particulate (hay net 8.3 mg/m3, no hay net 3.3 mg/m3, P = .0064) and respirable endotoxin (hay net 173.4 EU/m3, no hay net 59.2 EU/m3, P = .018) exposures were significantly higher when horses were fed from hay nets. Feeding hay from a net resulted in significantly higher BALF eosinophil proportions over time (P < .001). BALF eosinophils were significantly related to respirable particulate exposure (14 days in training rs = 0.37, P = .012, 28 days in training, rs = 0.38, P = .017). Conclusions and Clinical Importance Pulmonary eosinophilic inflammation develops in response to respirable particulate exposure in young Thoroughbreds, indicating a potential hypersensitivity to inhaled particulate allergens.