Risk Factors for Exercise-Associated Sudden Cardiac Death in Thoroughbred Racehorses

SIMPLE SUMMARY: Understanding the causes of racehorse deaths is an important step in addressing this welfare concern. Previous studies have identified that during Thoroughbred racing, catastrophic musculoskeletal injuries, which necessitate euthanasia, account for approximately 75% of fatalities, with sudden athletic death occurring in the remaining 25% of cases. In sudden athletic death cases a post-mortem examination is needed to determine the cause of death. Approximately half of these sudden athletic deaths are attributed to a fatal cardiac arrhythmia, which is termed sudden cardiac death. In order to investigate risk factors for sudden cardiac death, we reviewed the post-mortem reports from horses that died on metropolitan racetracks in Melbourne, Australia and grouped horses into sudden cardiac death and all other fatal injuries. We found that horses with sudden cardiac death were more likely to die during training than during racing, had fewer lifetime starts and were less likely to be entire (uncastrated) males. Exercise intensity appears not to be critically important in precipitating sudden cardiac death in horses. Sudden cardiac death occurred early in the careers of affected horses. ABSTRACT: Cardiac arrhythmias resulting in sudden cardiac death (SCD) are an important cause of racehorse fatalities. The objective of this study was to determine risk factors for SCD in Thoroughbreds by evaluating a sample with a policy of mandatory post-mortem following racing or training fatalities. Risk factors were compared between case horses with SCD (n = 57) and control horses with other fatal injury (OFI, n = 188) by univariable and multivariable logistic regression. Survival in years for horses with SCD was compared to OFI using the Kaplan–Meier method with log rank test. The following variables were most important in the multiple logistic model: Horses with SCD were more likely to die during training than during racing, SCD (42/57, 74%) vs. OFI (82/188, 44%; odds ratio [OR], 95% confidence interval [CI], 2.5, 1.2–5.4; p = 0.01), had fewer lifetime starts, median (interquartile range [IQR]), SCD (3.0 [0.0–9.0]) vs. OFI (9.0 [0.0–22.8]; OR, 95% CI, 0.96, 0.9–1.0; p = 0.02 and were less likely to be entire (uncastrated) males, SCD 9/57 (16%) vs. OFI (46/188, 25%; OR, 95% CI, 0.47, 0.1–0.9; p = 0.03). Survival in years (median (IQR)) for horses with SCD was 3.6 (3.1–4.4), which was shorter than OFI (4.5 [3.1–6.0], hazard ratio, 95%CI, 1.6,1.2–2.3; p < 0.001). SCD occurs more commonly in training than racing, which suggests exercise intensity is less important in precipitating this fatality. In this study, SCD occurred early in the careers of affected horses.