| Summary: | Inflammatory Airway Disease has been reported in young racehorses worldwide. The aim of this study was to determine the prevalence of airway inflammation in an actively racing population of horses and to describe and quantify the degree of lung lesions in this population of horses. The Ontario Death Registry program provided a unique opportunity to evaluate the lungs of actively racing horses that died or were euthanized due to catastrophic injuries while racing or training. Lung sections of 95 horses were included in the study and evaluated using a previously validated histological airway scoring system. Additionally, staining with Toluidine blue and immunohistochemical labeling was performed on a subset of horses to further characterize the inflammation. Inflammatory cell infiltration, smooth muscle hyperplasia and hemosiderin were commonly found. The airway lesion scores were significantly higher in the caudal and dorsal sections of the lungs than in other areas. There was no correlation between the individual scores and either breed, sex, age, cause of death or performance index. The inflammatory and smooth muscle scores were normally distributed and the hemosiderin score was not. The inflammatory cell infiltration was composed of mononuclear cells, with increased number of mast cells and eosinophils in 3/20 and 12/95 horses respectively. Immunohistochemical labeling showed the inflammation around the airway to be composed of 41.0% CD3-positive T cells and 5.8% CD79a-positive B cells. In addition to the airway findings, inflammatory cell aggregates were observed around the pulmonary blood vessels and in the alveolar septa in 67/95 and 71/95 horses respectively. In conclusion, this study provides a histologic evaluation of a population of actively racing horses in which airway inflammation is a common finding.
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