Exercise-associated sudden death (EASD) is defined as the spontaneous death of a closely observed, apparently clinically normal horse that occurs during or within one hour of exercise. This timeline includes active exertion, returning to the barn from racing or training, and while the horse is being cooled out or bathed after the activity. These dramatic events often occur in the presence of racetrack workers, media professionals, and the public.
When high-profile deaths or clusters of these events occur, scrutiny becomes closer, more directed, and sharper. The death of the disqualified Kentucky Derby winner, Medina Spirit, during a morning workout at Santa Anita in 2021 generated a media firestorm. Likewise, during a spate of 12 racehorse deaths at Churchill Downs in the spring of 2023, the exercise-associated sudden deaths of two horses generated controversy, as well as wild speculation.
Postmorten Examinations for EASD Cases
In Kentucky, as in many other racing jurisdictions, horses that succumb to EASD are subject to a rigorous and detailed postmortem examination protocol. In addition to a standard postmortem examination, an extensive visual and microscopic examination of the heart is performed, the spinal column is examined, and toxicology testing is performed as indicated by the individual horse’s history. Despite this extensive testing, pathologists reach a clear diagnosis in fewer than 50% of cases.
EASD is divided into two broad categories: those with significant postmortem examination findings and those without, termed autopsy-negative. From March 2017 through December 2024, 55 EASD examinations were performed at the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL). A diagnosis was reached in 45% of cases, while 55% were considered autopsy-negative. Autopsy-negative cases are presumed to be of cardiac origin and are termed sudden cardiac deaths (SCD).
Efforts to Mitigate EASD in Horses
In 2008, the Jockey Club and industry stakeholders, including racetrack management, trainers, private and regulatory veterinarians, and Thoroughbred breeders, initiated a coordinated effort to address musculoskeletal injuries in racehorses. Through these and other efforts, the rate of catastrophic musculoskeletal injuries (breakdowns) has decreased by 34%. With the decreased rate of catastrophic musculoskeletal injuries, EASD has gained more prominence as a cause of exercise-associated mortality. As occurred in 2008, a focused approach to EASD and associated risk factors is being undertaken through multi-institutional research groups and regulatory bodies.
In October 2023, the Horseracing Integrity and Safety Authority (HISA) founded an EASD working group that includes veterinary professionals from a wide range of disciplines, including regulatory veterinarians, clinicians, diagnosticians, and researchers. Current members of the working group represent the University of California-Davis, the University of Kentucky, the University of Minnesota, the University of Pennsylvania, the Virginia Racing Commission, and the Minnesota Racing Commission.
The goals of this working group include the development of monitoring tools to prospectively identify horses at risk, on-track emergency management plans, analysis of variations in genetics and biomarkers of horses that suffer EASD, and the development of standardized postmortem examination protocols.
Final Thoughts
The successful development and implementation of risk-mitigation strategies for the prevention of catastrophic musculoskeletal injuries provides a roadmap for the development of mitigation strategies to decrease the prevalence of exercise-associated sudden death. With a concerted effort among owners, trainers, private veterinarians, regulatory bodies, diagnosticians, and researchers and a dedication to the welfare and safety of racehorses, this goal can be achieved.
Related Reading
- EVJ Highlights Ongoing Work to Improve Racehorse Safety
- Ambient Air Pollution’s Effects on Racehorse Performance
- Gene Doping in Horse Sport: Concerns and Perspectives
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