Relationship of post-race tracheal mucus scores to performance and the environment in 2-year-old Thoroughbreds

Post-race tracheal mucus is a common finding in 2-year-old Thoroughbreds without signs of respiratory disease, and its clinical relevance is unclear. We evaluated the relationship between mucus score (0-5) and racing performance, racetrack location, surface and condition and some environmental factors in this age-group of horses. 1,071 post-race videoendoscopies were independently assigned mucus scores by three observers blinded to horse identity, and the consensus score was recorded. Pearson’s Chi-squared independence test and ANOVA or the Kruskal-Wallis test analysed relationships of mucus scores to performance and racetrack environmental variables, respectively. Significance was set at . Tracheal mucus was observed post-racing in 98% of videoendoscopies; 54% had mucus scores ≥3. Race distances (), horses’ race times (), and horses’ speed index () were not associated with mucus score. Lower mucus score was not associated with a better finishing place (). The association between racetrack surface material and mucus score was not significant (). Racetrack location () was associated with mucus score, but no single racetrack factor could be causally linked to this locational association. Neither stall construction material () nor bedding type () had a significant association with mucus score. Orientation of the horse’s stall within a barn (facing inwards vs facing outwards, n = 151) was not significantly associated with mucus score (). There was no combined effect of stall orientation and bedding type on mucus score (orientation and shavings: ; orientation and straw: ). Air quality index (AQI) was related to mucus score () with the likelihood of mucus score ≥2 increasing as AQI worsened (). While these results do not demonstrate an association between mucus score and performance or a cause-and-effect relationship between mucus scores and the racetrack environment, they do justify further investigations of factors influencing mucus scores in 2-year-old racing Thoroughbreds.