Comparative cardiac histopathology and toxicology screens in horses succumbing to exercise-associated sudden cardiac death and control horses.

Horse racing has faced increased scrutiny because of concerns about equine welfare and the safety of both equine and human participants. Sudden death (SD) in racehorses is relatively uncommon, and its causes are still unclear. Little information and considerable confusion exist regarding the clinical significance of microscopic lesions in the hearts of horses with SD. We examined the hearts of 29 horses that suffered exercise-associated sudden cardiac death (EASCD) and 29 clinically healthy control horses that were euthanized because of catastrophic musculoskeletal injuries. Acute cardiomyocyte injury (ACI) was found at a significantly higher overall prevalence and with a significantly higher severity score in the left atrial appendage in horses with EASCD than in control horses. ACI may be a very early stage of cardiomyocyte degeneration, perhaps driven by myocardial ischemia during high-intensity exercise. We found no significant differences in the prevalence, severity, and distribution of inflammation, fibrosis, or miscellaneous lesions between the 2 groups. Inflammation, fibrosis, and miscellaneous lesions in the heart may have clinical significance in individual cases, but they can also be confounding background lesions. Postmortem confirmation of sudden cardiac death (SCD) remains difficult, given that fatal arrhythmias can occur with no associated microscopic cardiac lesions. Elevated hepatic cobalt concentrations were overrepresented in SCD horses. Toxicology results for other performance-enhancing drugs or xenobiotics were negative or inconclusive.