Environmental Exposures and Airway Inflammation in Young Thoroughbred Horses

Abstract

BackgroundInflammatory airway disease (IAD) in horses is a widespread, performance‐limiting syndrome believed to develop in response to inhaled irritants in the barn environment.ObjectivesTo evaluate changes in bronchoalveolar lavage fluid (BALF) cytology and exposure to particulates, endotoxin, and ammonia during horses' first month in training.AnimalsForty‐nine client‐owned 12‐ to 36‐month‐old Thoroughbred horses entering race training.MethodsIn 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.ResultsMedian 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 r s = 0.37, P = .012, 28 days in training, r s = 0.38, P = .017).Conclusions and Clinical ImportancePulmonary eosinophilic inflammation develops in response to respirable particulate exposure in young Thoroughbreds, indicating a potential hypersensitivity to inhaled particulate allergens.

Inflammatory Airway Disease of Horses—Revised Consensus Statement

“…Eosinophilic IAD is less commonly observed, and this phenotype appears to be related to respirable dust exposure but not internal parasitism, in young horses 42, 49. However, the implications for athletic performance are unclear.…”

“…The presence of high eosinophil or mast cell counts, and of Th‐2 cytokines such as IL‐4 and IL‐5, in BALF of some horses with IAD suggest a role for aeroallergens in this syndrome 27, 54. Indeed, exposure to airborne particles has been linked to BALF eosinophilia49 and tracheal mucus51 in young racehorses in training. Older horses without clinical evidence of airway disease that are exposed to high level of organic dust and endotoxin respond with a mild to moderate BALF neutrophilia 3, 16, 55.…”

“…The IAD phenotype has been associated with horses’ age in particular, BALF eosinophilia is more commonly encountered in young horses (<5 years old)27, 49, whereas BALF neutrophilia is more frequently diagnosed in older horses (>7 years old) 13, 27, 39. Eosinophilic IAD is less commonly observed, and this phenotype appears to be related to respirable dust exposure but not internal parasitism, in young horses 42, 49.…”

“…Activity in the barn and ventilation affects dust exposure with peak levels occurring during the morning or midday, especially at the time of feeding and cleaning of the stalls 45, 131. Opening of barn doors and more open stable design, regardless of season, improve ventilation, and decrease exposure to dust 43, 49, 51…”

An observational study of environmental exposures, airway cytology, and performance in racing thoroughbreds

“…However, in young horses, numbers of mast cells and eosinophils in BALF are often increased, suggesting hypersensitivity 1, 3, 4, 5, 6, 7. Different cytologic manifestations of equine asthma might reflect differing disease mechanisms with different implications for performance.…”