Sudden Cardiac Death in Racehorses What We Know and Still Don't

POSTED ON  |  05-10-2018

The sudden death during training hours this summer of Bobby Abu Dhabi (Macho Uno) was a tragic incident. Connections lost a horse they loved. Racing lost one of its sprint stars. His death, however, opens a window into a still relatively obscure corner of equine fatalities. While Bobby Abu Dhabi was originally reported to have suffered cardiac arrest, it was later reported he died of other causes. What this confusion betrays are some common misunderstandings surrounding “sudden deaths,” namely that the term comprises a variety of different health issues and injuries, some of them completely unrelated to the heart.

Another problem, said Dionne Benson, executive director of the Racing Medication and Testing Consortium, is that while we can speculate why racehorses’ hearts suddenly give out, we’re still unable to provide definitive answers, like we can for humans. “This is such a hard area to dig into,” Benson said. “We have so little information out there.”

Defining “Sudden Death”

Veterinary medicine defines exercise-induced “sudden death” in racehorses as the collapse and death in an apparently healthy horse during, or within one hour after, exercise. The term comprises many different causes, not just sudden cardiac death. Massive bleeding in the lungs or abdomen; fractures of the skull or neck; hemorrhaging from a pelvic fracture–all these injuries can prove swiftly fatal in a manner that, outwardly, resembles a cardiac issue.

Of paramount importance, then, is that a thorough post-mortem is conducted swiftly, to identify, if possible, what happened. While some jurisdictions have comprehensive necropsy programs for all horses fatally injured during racing and morning training, this is far from a blanket requirement across the nation, meaning that many sudden deaths, which happen relatively rarely anyway, go unexamined.

What’s more, even if a post-mortem is performed, when it comes to sudden cardiac death, oftentimes there are no lesions, ruptured arteries or damaged heart tissue that pathologists can point to with authority and say this or that caused the heart to stop.

How Frequent Are “Heart Attacks”?

Nevertheless, there have been attempts to get to the bottom of the issue, beginning at the frequency with which sudden deaths occur. In a 10-year period between 2007 and 2017 in California, 8.2% of all training and racing related fatalities were sudden deaths. From data collected between 2007 and 2013 in California, approximately one sudden death occurred per 9,000 starts, and about one sudden death per 160,000 training days.

California’s numbers stack up against this 2011 study, considered by many experts to be the largest and most comprehensive review of exercise-related sudden death post-mortem findings in racehorses. All told, 268 different cases of sudden death from six racing jurisdictions around the world were reviewed. According to the study, the proportion of overall racing fatalities classified as sudden death was 19% in the UK, and in the Australian state of Victoria, that number was 19% in flat races and 3.5% over jumps.

Most tellingly, pathologists were only able to make a definite diagnosis in 53% of cases, a presumptive diagnosis in 25% cases, while 22% of cases were left unexplained. Indeed, “when they call me in the middle of the night and tell me we’ve just had a sudden death, it really wrecks my night because it’s so frustrating–chances to get an answer will be slim,” said Francisco Uzal, coordinator of the California Animal Health and Food Safety Laboratory post-mortem program for the California Horse Racing Board (CHRB).

Arrhythmias, Murmurs

So, what do experts suspect are the causes of sudden cardiac death? “It’s not like human heart attacks where they have too much cholesterol, blocked arteries,” Benson explained. “Because the whole equine heart is so big, it’s very susceptible to electrical irregularities.” By “electrical irregularities,” Benson means arrhythmias (an irregular heartbeat), and heart murmurs (the presence of irregular heartbeat sounds).

Perhaps surprisingly, electrical irregularities are far from uncommon in racehorses. But Peter Physick-Sheard, a professor at the University of Guelph’s Department of Population Medicine, explained that the presence alone of certain arrhythmias shouldn’t always cause alarm.

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