Horses that exhibit abnormal heart rhythms while either exercising or at rest are at higher risk for suffering fatal cardiac events, the Horseracing Integrity and Safety Authority said on Thursday while encouraging trainers to use wearable devices to periodically measure the phenomenon.
Abnormal heart rhythms – known more formally as atrial fibrillation – are commonplace among horses and other mammals, though the deviations are not often considered life-threatening. HISA said in its Thursday announcement that an analysis of data from horses experiencing the abnormal heart rhythms indicated that the arrhythmia “can worsen during exercise, causing more severe arrhythmia that leads to sudden death.”
HISA said that the findings should encourage horsemen to screen their horses for the phenomenon. HISA added that the “majority” of fatal cardiac events occurred in horses relatively early in their careers, “with the highest incidence occurring in horses with zero to five” lifetime starts.
“Integrating cardiac screening into routine evaluations – whether through wearable devices or veterinary exams – could allow trainers and veterinarians to identify at-risk horses before exercise,” said Dr. Karen Hassan, the chair of a research group put together by HISA to examine sudden deaths in horses. “This opens the door to scalable monitoring protocols and standardized response strategies that could save equine lives.”
Though rare, sudden deaths comprised eight percent of the fatalities that occurred during races in the first six months of 2025 and 18 percent of training fatalities over the same time period, according to HISA. Approximately 50 percent of those sudden deaths “were likely related to cardiac issues,” HISA said.
HISA started the working group on sudden deaths in October of 2023. The phenomenon of sudden death in horses and other large animals is not well understood, and necropsies of horses that experience sudden death are not typically able to precisely identify the cause of death due to a variety of factors.
HISA began funding two studies examining sudden death late in 2024. One is being led by Dr. Sian Durward-Akhurst of the University of Minnesota and is examining any potential genetic or biological markers that could identify at-risk horses. The other, conducted jointly by Dr. Durward-Akhurst and Dr. Cris Navas at the University of Pennsylvania, is using wearable devices to gather data on biometrics, stride characteristics, and speed, among other metrics.
HISA said that the working group is next planning to develop real-time cardiac monitoring protocols and the specific criteria to identify horses for being at-risk for a cardiac event. The group will also work to evaluate a “standardized response protocol” for horses that suffer adverse cardiac events.
In the meantime, HISA expects to organize a series of educational seminars for horsemen and veterinarians “in the coming weeks” to discuss the working group’s findings, the authority said.
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