Epidemiology of Airway Inflammation and Mucus in Horses by S.J. Holcombe; P2662.1205

Etiology Young horses commonly develop upper airway inflammation. The location of the nasopharynx at the entrance of the airway exposes it to multiple types of allergens, irritant particles, and viral or bacterial agents. The local lymphoid tissue responds to these stimuli by secreting mucus to entrap inhaled particles and by producing local immunoglobulins. As young horses start their performance careers, enter training barns, and travel, they become exposed to multiple new antigenic stimuli. Thus, pharyngitis is very common in young horses, although its prevalence decreases as horses age and develop immunity. This has been documented by Hobo et al., who reported a 37% prevalence of pharyngitis (grades 3–4) in 2-yr-old Thoroughbred racehorses that decreased to nearly 0% in horses 6 yr or older. Auer et al. reported that 68 of 70 young Thoroughbred racehorses had evidence of pharyngeal lymphoid hyperplasia or pharyngitis. Similar to the work of Hobo et al., they found that 2-yr-old horses had the most severe inflammation compared with other age groups. In the study by Auer et al., none of the horses had a history of diminished racing performance. The results of this study suggest that pharyngeal lymphoid hyperplasia may be a normal response to new environmental antigens in young horses. Therefore, because pharyngitis is frequently self-limiting and has not been definitively associated with poor performance, this disease is usually not treated. However, there are anecdotal concerns that pharyngitis may be a prelude to dynamic upper airway obstruction, and the sequelae of nasopharyngeal inflammation may be more performance limiting than the initial bout of pharyngitis. Accumulating evidence suggests that regional inflammation of the upper airway may predispose individuals to obstructive upper airway disease, such as nasopharyngeal collapse, dorsal displacement of the soft palate, and aryepiglottic fold collapse.