The veterinary mystery of the California cluster


Veterinary researchers try to understand a spike in sudden deaths, possibly cardiac-related, among thoroughbreds.

Earlier this year, The Blood-Horse and the Daily Racing Form both reported that California Thoroughbred racing at Hollywood Park and Santa Anita saw a spike in presumptive sudden cardiac deaths from July 1, 2011, to June 30, 2012. The fatalities occurred both during training and racing. Eleven horses died of cardiac failure during the one-year period, compared with six during the similar period in fiscal year 2011 and four in 2009, according to the California Horse Racing Board's annual reports (see "A well-documented mystery").

A well-documented mystery

"Sudden death in a racehorse is distressing for everyone involved in racing," says Peter Physick-Sheard, BVSc, MSc, FRCVS, associate professor at the University of Guelph (Ontario), Veterinary College, who has been studying the issue in both standardbred and thoroughbred racehorses, looking at electrocardiograms and heart rhythms in horses while they're racing. "It raises animal welfare, economic and safety concerns and represents horrendous public relations for the industry."

Although sudden death in thoroughbred racehorses is fairly rare, it is of greater significance than in racing standardbreds. A study published in the Equine Veterinary Journal on sudden cardiac death in racing thoroughbreds found that it occurred in 9 percent of fatalities in California—96 cases in exercising horses between February 1990 and August 2008.1 Also during that 18-year period, five cases were reported in Pennsylvania; 23 in Victoria, Australia; 16 in Sydney; four in Hong Kong; and none in Japan.

Sudden deaths in horseracing are not uncommon, but the recent cluster of inexplicable cardiac deaths in thoroughbreds in California continues to stump researchers and key figures in the industry. (GETTY IMAGES/ANETT SOMOGYVÁRI)

"Sudden deaths have been an issue in horseracing internationally for many years," says Rick Arthur, DVM, equine medical director at the UC-Davis School of Veterinary Medicine, which was assigned to advise the California Horse Racing Board. "They are typically called heart attacks, but, in reality, cardiac failure, or even suspicious cardiac failure, accounts for less than 50 percent of all fatalities."

Some of the so-called "cardiac sudden death cases" may actually be pulmonary hemorrhage, Arthur says. "It is common for horses to bleed externally due to pulmonary hemorrhage after training, or there may be a major vessel rupture internally," he says.

Though cardiopulmonary-vascular incidents of those types are some of the most common, in California we've had a number of sudden deaths from other issues. About 10 percent of them are the result of a major bone fracture. In some of those incidents, the horse goes down and exsanguinates internally, so it is at first deemed a cardiac death, though that may not be the cause at all."

Starting with speculation

Francisco Uzal, DVM, PhD, professor of clinical diagnostic pathology at UC-Davis School of Veterinary Medicine and the director of the California Animal Health and Food Safety Laboratory System is trying to determine the cause of the racetrack deaths in California. As a part of looking into the recent racetrack fatalities and those occurring over the past several years regarding sudden cardiac death, he and his team have conducted pathologic and histologic examination of cardiac tissues and extensive toxicologic studies have been conducted. "But we have not been able to find the cause of some of these recent cases of sudden deaths," Uzal says.

According to the Daily Racing Form, traces of anticoagulant rat poison were found in two of the horses. Both horses had internal hemorrhage problems, though whether this was related to the deaths has not been confirmed.

Despite this fact, Uzal does suspect that at least some of the sudden deaths seen in the general horse population may be produced by arrhythmias. "But in many cases, we do not see morphological changes in the heart muscle or anywhere," he says. "Many times we speculate that changes in the cardiac conduction system may be responsible, but it is pure speculation. What we frequently see on postmortem is almost no cardiac pathology in sudden death in horses, making these cases somewhat of a mystery."

The cluster of sudden racetrack deaths in southern California is a different issue than atrial fibrillation (AF) ("How veterinarians can address atrial fibrillation in horses," July 2013 dvm360). "With exercise, AF should not cause sudden cardiac death," says Virginia Reef, DVM, DACVIM, director of large animal cardiology and diagnostic ultrasonography at the University of Pennsylvania's New Bolton Center. "You worry that if there is nothing found at postmortem, or if they find changes in the myocardium, possibly there was a fatal ventricular arrhythmia that triggered it," Reef explains. "Exercising horses have been shown to have ventricular arrhythmias that occur immediately after peak exercise, just as they are slowing down.

"Why there was a cluster of apparent sudden cardiac death in southern California is unknown and probably very difficult to determine," Reef continues. "It may be that a toxin, virus or drug interaction was involved, or it may be an adverse consequence of the maximal effort performed. If any of these horses had ventricular arrhythmias, and if the arrhythmia was more malignant, it could have triggered a cardiac death episode."

Just as with human athletes, the circumstances surrounding the episode and the postmortem findings may provide little information on underlying cause, says Physick-Sheard. Reef agrees. "In these cases the term 'sudden cardiac death' is applied," she says. "It is assumed the primary cause is a serious disturbance in heart rhythm, but evidence is lacking because affected animals' ECGs are not being monitored at the time of death."

Applying research to the problem

To try to better understand causes, a study was recently performed in standardbred racehorses in which heart rhythm was followed from harnessing until the end of the race during normal competition. "Disturbances in rhythm immediately after the race were identified that would be capable of causing a fatal outcome, but there were no deaths," says Physick-Sheard. "However, clear indications of strategies that might reduce risk of a fatal outcome were noted. Sudden cardiac death associated with racing occurs more commonly in the thoroughbred than in the standardbred, and it is possible that ventricular arrhythmias are taking place here also."

To resolve this question in racing thoroughbreds, researchers recently monitored heart rates and rhythms during normal scheduled racing at Woodbine Racetrack in Toronto, from saddling to unsaddling and into early recovery. "The objectives were to characterize the range of usual rate and rhythm variations and to provide guidance as strategies to minimize risk are developed," explains Physick-Sheard. He says the thoroughbred work was done after the standardbred work, and data are now being processed.

During the standardbred study, the research team monitored horses' heart rates and heart rhythms during normally scheduled standardbred racing.2 "We put the monitoring gear on when the horses came into the paddock, and it stayed on until the horses came off the track at the end," Physick-Sheard says. "We were able to monitor the entire exercise response. What we identified of particular significance was that about 18 percent of horses would have significant rhythm disturbances during recovery, fairly unusual ventricular rhythm disturbances, which were not previously identified in horses."

He goes on to say that although all of the horses in the study continued to race and none died, the arrhythmias did occur at the time when there appears to be a peak in post-exercise death, as the literature shows that most horses identified as dying of sudden cardiac death die in the period immediately after racing.

"When we saw these rhythm deficits, each of them had the potential to deteriorate into a malignant or fatal arrhythmia, though none of them did," Physick-Sheard says. They all recovered and went back to normal sinus rhythm. The principal finding was that horses subjected to racing exercise will tend to show disturbances during the recovery period.

As part of the study, the team also identified factors that predisposed horses to the probability of a horse experiencing cardiac rhythm disturbances. The factors included being a trotter, getting parked at the half-mile pole and breaking in the stretch. "Our analysis of those and associated data leads us to believe that some types of physical stress, predominantly stress that is associated with psychological stress, may be predisposing these horses," Physick-Sheard explains.

They also found that the arrhythmias frequently occurred in association with what is generally thought to be a normal variation in rhythm when the heart rate is decreasing. "The heart rates do not descend smoothly," says Physick-Sheard. "They tend to come down in a stepwise fashion, and sometimes that step is quite obvious, sometimes not. But there is always a tendency for the heart rate to come down in the middle ranges in a stepwise fashion. We identified that these episodes of sudden slowing appear to be a trigger for ventricular rhythm disturbances. That is another piece of information that we are also pursuing to indicate why sudden death may occur in all athletes during recovery from maximal effort. Some deaths occur in humans under similar circumstances."

Physick-Sheard says researchers don't know for certain whether these rhythm disturbances are just a usual variation from normal, or whether they actually represent pathology. "We are pursuing that issue with another study to try to find out whether there is pathology associated with these rhythm disturbances," he says. "It's not possible for us to be there when a horse dies because we don't want any horse to be dying, especially not while we're monitoring it. But that would be the only way to prove that these rhythm disturbances were in fact the cause of death. So we may never know for sure. The suspicion is that these disturbances on occasion deteriorate and result in death."

In human cardio stress testing, echocardiography is part of the normal routine. The difficulty is using this technology in horses. "Further studies to fully understand all the pieces to the puzzle would certainly necessitate echocardiographic studies," says Physick-Sheard. "For now, knowing that we have so much more information to gather, we are concentrating on real-world (racetrack) exercise. It's not possible to do ultrasound examinations on the racetrack at the present time."

Determining the cause of the California cluster

UC-Davis' Arthur says pathologists at California's Animal Health laboratory are conservative, so they won't categorize a sudden death as one diagnosis or another unless they have a definitive explanation.

"California has an extensive postmortem necropsy program," he says. "If you look at the nonmusculoskeletal sudden death incidents, there have been about 56 of them since July 1, 2010, but only 15 of those were from racing horses. As California has approximately 40,000 to 50,000 starts per year, that's about one out every 10,000 starts—less than 5 percent of all fatalities by starts. We also had during that same period about 40 horses die suddenly during or after training. Since a horse trains about 30 to 60 times per race start, having one of these untoward events when a horse is training is more likely."

A careful look at these cases indicates that a ventricular conduction anomaly occurring after training or racing (as per Physick-Sheard and McGurrin's work) may be at work, says Arthur. "We suspect that this represents a large portion of the sudden deaths where we do not have a definitive diagnosis, which is approximately one-third of the sudden deaths.

"One point to remember," he continues, "is that the pathologist frequently finds pulmonary hemorrhage in a lot of these horses, and myocarditis/myocardial degeneration as well, even if they were put down because of a fetlock fracture. So it is often difficult to determine the cause of death in every case due to one particular finding. A finding does not mean a cause of death."

Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine with a background in horses, pets and livestock. Kane is based in Seattle.


1. Lyle CH, Uzal FA, McGorum BC, et al. Sudden death in racing Thoroughbred horses: an international multicentre study of post mortem findings. Equine Vet J 2011;43(3):324-331.

2. Physick-Sheard PW, McGurrin MKJ. Ventricular arrhythmias during race recovery in Standardbred Racehorses and associations with autonomic activity. J Vet Intern Med 2010;24(5):1158-1166.

Suggested Reading

1. Gelberg, HB, Zachary JF, Everitt JI, et al. 1985. Sudden death in training and racing Thoroughbred horses. J Am Vet Med Assoc 1985;187(12):1354-1356.

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