Returning after a lay-up and absence of recent starts are associated with complete humerus fractures in Thoroughbred racehorses in South Korea (2009–2022)

Objective: To identify risk factors associated with complete humerus fractures in Thoroughbreds in South Korea. Methods: A retrospective case-control design with multivariable logistic regression was used. This study included 131 Thoroughbreds diagnosed with complete humerus fractures following racetrack activity between 2009 and 2022 (inclusive). Additionally, 660 healthy Thoroughbreds (699 observations) with at least 1 racetrack activity during the same period were included as controls. Results: A total of 8 variables were associated with the risk of complete humerus fractures. Notably, horses within the first 20 days after resuming racetrack training following a lay-up longer than 60 days (OR, 13.5; 95% CI, 5.5 to 33.1), as well as those with no race starts in the 60 days preceding the event (OR, 11.6; 95% CI, 5.5 to 24.6), were associated with an increased risk of the fracture. The likelihood of fracture was higher during trials or races than during racetrack training (OR, 8.9; 95% CI, 4.3 to 18.7), and horses with only 1 to 5 prior starts were also at increased risk (OR, 4.8; 95% CI, 2.0 to 11.3). Additional risk factors included sex (female and intact males), lack of prior trial race experience, and history of a prolonged lay-up exceeding 180 days. Conclusions: This study identified risk factors for complete humerus fractures in Thoroughbreds in South Korea and emphasized the importance of preventive measures. Clinical Relevance: The implementation of risk profiling and other preventive measures (such as pre-exercise veterinary examination that incorporates diagnostic imaging) for horses returning after a lay-up or those with no recent race participation could help reduce the incidence of complete humerus fractures.