Novel risk factors associated with fatal musculoskeletal injury in Thoroughbreds in North American racing (2009-2023).

Background: The Equine Injury Database (EID) is a census-level record of Thoroughbred racing in North America, currently recording 95.6% of all race starts in 2023, along with partial training and veterinary histories of each horse.

Objectives: To identify horse-, race- and track-level risk factors associated with race-related fatal musculoskeletal injury (MSI) of Thoroughbred racehorses in North America.

Study design: Retrospective cohort study.

Methods: The study cohort included all race starts made by horses born after 31 December 2006, at tracks that fully report to the EID and consisted of 3,851,659 race starts made by 250,840 Thoroughbred racehorses (median [IQR] starts per horse 11 [5-22]) at 115 racetracks in the USA and Canada between 2009 and 2023, inclusive. Ninety-seven potential risk factors were investigated using univariable and multivariable logistic regression modelling.

Results: Exactly 5733 fatal MSIs were recorded, an incidence of 1.49 fatal MSIs per 1000 starts. Twenty risk factors had statistically significant associations with increased or decreased odds of fatal MSI. Previously unidentified risk factors included claiming race-related variables and void claim rules (VCR). Horses racing as claimers were at increased odds compared with those who were not (odds ratio 1.31, 95% confidence interval 1.19-1.45, p < 0.001 for the lowest claim prices). Starts in races with the strictest VCR were at reduced odds compared with starts in races with no VCR (OR 0.76 [0.67-0.85], p < 0.001).

Main limitations: Availability of new data sources increased substantially during the 15-year time period of the study, meaning some new risk factors are limited in scope compared with others.

Conclusions: Thoughtful integration of new data sources with race-level data can lead to new insights into risk factors for deleterious outcomes affecting racehorses. Results can inform ongoing efforts to mitigate the risk of fatal MSI, through direct regulatory intervention and through building a risk profile based on individual history and track-level factors.