Severity and effects of pharyngeal lymphoid hyperplasia vary with age and racetrack location in Thoroughbred racehorses

This study evaluated the severity of pharyngeal lymphoid hyperplasia (PLH) in relation to age, performance and geographic location of Thoroughbreds racing at 13 American racetracks. PLH severity (graded 1-4 with validated criteria related to size, appearance and coalescence of hyperplastic follicles) and tracheal mucus score (0-5) were determined from videoendoscopies recorded 30-60 min post-racing. There were 198 from 2-year-olds, including following graded stakes races, and 116 were from ≥2-year-old horses following graded races. Performance data was obtained from Thoroughbred racehorse databases. Associations between PLH and speed index, average race speed, mucus score, age, and track location were separately investigated for the 2-year-olds and for the stakes horses using Pearson’s Chi-squared independence test, ANOVA or an ordinal logistic regression model. was significant. There were age (PLH Grade ≥3 in 92% 2-year-olds, 88% for 3-year-olds and 52% in ≥4-year-olds; ) and track location () effects on PLH grade. In 2-year-olds, PLH varied with track location () and race day air quality index (AQI; ). Horses housed at the racetrack had higher PLH grades than horses transported to the race (). Speed index (), tracheal mucus () and average speed () were not associated with PLH severity in 2-year-olds. In stakes horses, severity of PLH was not different between 2- and 3-year-olds but decreased with age in ≥3-year-olds (). Higher PLH grades were associated with decreased speed index () but not average speed (). Track location () and mucus scores () were also associated with PLH grade in all stakes horses. PLH was most severe in 2- and 3-year-old stakes horses but did not impair their performance. Of stakes horses ≥4-years-old, those with PLH ≥3 performed worse than those with PLH ≤2. Racetrack location and AQI were associated with PLH severity in racehorses of all ages for reasons that require further study.