UK DIAGNOSTIC ROUNDS Exercise-associated Sudden Death in Racehorses.
Laura A. Kennedy, DVM, Dipl. ACVP Associate Professor, University of Kentucky Veterinary Diagnostic Laboratory
Postmortem investigation of exercise-associated sud- den death (EASD) is the bane of the racehorse patholo-
gist. Tese dramatic events occur in the presence of any number of people, including racetrack workers, media professionals, and the public. When high profile deaths or clusters of these events occur, scrutiny becomes closer, more directed, and sharper. Te death of the disqualified Kentucky Derby winner Medina Spirit during a morning workout at Santa Anita Park in 2021 generated a media firestorm. Likewise, during a spate of 12 racehorse deaths at Churchill Downs in the spring of 2023, the exercise-as- sociated sudden deaths of two horses generated contro- versy as well as wild speculation.
EASD is defined as a sudden, nonviolent, nontraumatic, unexpected death that occurs during or within one hour of exercise. Tis timeline includes active exertion, return- ing to the barn from racing or training, and while the horse is being cooled-out or bathed after the activity.
When an EASD occurs during active exertion, horses dis- play a typical pattern. Rather than “falling in their tracks” these horses tend to slow down and become wobbly or ataxic, before sinking to the ground, and expiring within minutes. Often these horses are described as having white mucus membranes; rather than reflecting a severe hemor- rhagic event, this is generally due to shock and cardiovas- cular collapse. If there is any comfort in this presentation, it is that riders are rarely injured when a horse suffers an exercise-associated sudden death, as there is time for them to detect a problem and dismount quickly.
Horses that succumb to EASD are subject to a rigorous and detailed postmortem examination protocol. In ad- dition to a standard postmortem examination, an exten- sive gross and histologic examination of the heart is per- formed, the spinal column is examined, and toxicology testing is performed as indicated by the individual horse history. Despite this extensive testing, a clear diagnosis is reached by the pathologist in less 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 May of 2022, 32 EASD examinations were per- formed at the University of Kentucky Veterinary Diag- nostic Laboratory (UKVDL). Significant lesions were identified in 14 cases (44%) while 18 cases were consid- ered autopsy-negative.
38 KVMA News
Diagnoses related to the cardiac system included myocar- ditis (5 cases), myocardial fibrosis (1 case), and chronic pericarditis and pleuritis (1 case). Four cases involved rupture of a blood vessel, including one case of aortic rupture and hemopericardium, and three cases of mesen- teric vessel rupture and hemoperitoneum. Two cases were attributed to severe exercise-induced pulmonary hemor- rhage (EIPH) and one case presented with massive body wall hemorrhage of undetermined cause.
In 2008, Te Jockey Club (the official registry of Ameri- can Toroughbreds) and industry stakeholders including racetrack management, trainers, private and regulato- ry veterinarians, and Toroughbred breeders initiated a coordinated effort to address musculoskeletal injuries in racehorses. Trough these and other efforts, the rate of catastrophic musculoskeletal injuries (breakdowns) has decreased by nearly 40%. With the decreased rate of cata- strophic musculoskeletal injuries, EASD has gained more prominence as a cause of exercise-associated mortality. As occurred in 2008, a focused approach to EASD and asso- ciated risk factors is being undertaken through multi-in- stitutional research groups and regulatory bodies.
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