Ask A Vet: Why Do Equine Heart Attacks Happen? - Horse Racing News | Paulick Report
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Ask A Vet: Why Do Equine Heart Attacks Happen?

Homeboykris in the winner’s circle before he died of a heart attack (@campotres photo)

The sudden death of Homeboykris on the Preakness undercard in May shocked many people, including the horse's connections. Necropsy results on the horse's body are expected to be released at the July 12 meeting of the Maryland Racing Commission, but veterinarians suspect the horse died of a heart attack. Homeboykris's connections took to social media in the days afterward to express their sorrow and surprise. The horse, as far as they could tell, was perfectly healthy on the day he won his final race at Pimlico, collapsing minutes later.

An article published in the journal Equine Veterinary Education in February of this year on the prevalence and detection of heart conditions in horses underscored how elusive the condition can be. Researchers believe some types of cardiac abnormalities could be common in Thoroughbreds, but that doesn't tell the whole story. One type of abnormal function, called supraventricular premature contraction, during exercise has been shown in 10 to 50 percent of Thoroughbreds studied, but it does not increase the risk for sudden death. The study found the rate of overall deaths on the track to be one to three out of 10,000 starts. About half of those could have origins in the heart.

That information has people wondering: Why do heart attacks happen to some horses but not others? And what can we do to reduce the number of them?

Dr. Sophy Jesty is a veterinarian at the Charleston Veterinary Referral Center in South Carolina, and is board certified in both large animal internal medicine and cardiology. Much of Jesty's research has focused on equine cardiology, so she tackled a few basic questions about heart attacks in horses.

How common are heart attacks in performance horses, generally speaking?
Strictly speaking, horses don't suffer from heart attacks in the same way that people do. A heart attack refers to a blockage of a coronary artery which supplies the heart. This causes part of the heart muscle to die, and if the blockage is big enough, a heart attack can cause sudden death in a person.

Horses don't get blocked coronary arteries, but they do experience sudden death for other reasons. Sudden death in horses is a rare event as a percentage, but because so many horses are raced, a fair number die each week at racetracks. Not all of these deaths are truly cardiovascular sudden death; many are instances of catastrophic musculoskeletal injury. Every instance of sudden death in a horse affects us deeply, and there is a push in veterinary medicine to better understand the causes of sudden death in horses in the hopes of decreasing the incidence.

Is there a particular population in terms of breed or sport that's thought to see more heart attacks?
Although very unlikely, any horse could experience sudden death. But the risk is greater in equine athletes because exercise increases the work of the cardiovascular system. The general public is more aware of sudden death in racehorses because certain races are aired on TV. But sudden death occurs in a variety of equine sport disciplines, including eventing, cross country, show jumping, and dressage, among others.

Do we know what the most common causes of heart attack are in horses?
The most common cause of sudden death in horse athletes is a rupture of the aorta, which is the biggest artery in the body. The horse subsequently very quickly bleeds out into its chest cavity, much like Marfan's syndrome can cause in men. Some of these horses probably have an aneurysm of the aorta, or a prior weakening of the blood vessel wall, which predisposes to rupture when blood pressure increases during exercise. Research is underway to try to find the cause(s) of the underlying arterial wall weakness in certain horses.

The next most common cause is probably an arrhythmia, or abnormal heart rhythm during exercise. Arrhythmias are often triggered by adrenaline, and can be fatal because they can cause the heart to stop suddenly. Unlike aortic rupture, an autopsy performed on a horse that died from an arrhythmia would not necessarily show any abnormalities. This is because the arrhythmia is a transient electrical event which can't be seen. Other causes for sudden death in horses are less common, but include acute heart failure caused by abnormalities of the structure or function of the heart (either that the horse was born with or that the horse acquired), upper airway obstructions, or toxins.

What symptoms or warning signs could handlers watch for to anticipate a heart attack? My understanding has always been that they're quite sudden. Sudden death can occur in horses without any warning whatsoever. But if a horse suddenly pulls himself up, or feels weak or wobbly during exercise, the rider should allow the horse to stop. If a horse takes a long time to cool down after exercise, is less capable of working than they used to be, or exhibits an upper airway noise during exercise, he should be worked up for abnormalities of the musculoskeletal, respiratory, and cardiac systems.

When Homeboykris collapsed and died of a heart attack on this year's Preakness undercard, a lot of people worried that he had the heart attack because he had had a long race career. Is there any reason these two things should be related? Homeboykris was nine years old, which is certainly old for a racehorse. Although there are no studies to my knowledge addressing the question of how age affects the risk of sudden death in horses, theoretically, advancing age could increase the risk. At some point, the heart and lungs no longer work as strongly and efficiently as when the horse was younger. As animals age, their arteries lose elasticity, which could increase the risk for aortic rupture during exercise. However, nine years old is not geriatric for a horse, and in many sport disciplines, nine years old would only be considered mid-career.

I've read that at least one racing jurisdiction is planning on doing ECGs in an attempt to screen horses who may have undetected heart problems; are there treatments available for those that do? Yes, there is more and more interest within the veterinary community in elucidating the causes of sudden death in horses, and much research has already been undertaken concerning the evaluation of ECGs in exercising horses. As far as I know, at least one racing jurisdiction is planning on acquiring ECGs in racehorses to screen for important arrhythmias that could be life threatening. The trouble is that frequently dangerous arrhythmias are only present during exercise, which means that a screening ECG with the horse at rest would be unlikely to predict many instances of sudden death.

Nonetheless, if screening ECGs at rest detects only a single horse at risk, it might be worth the effort. It's just that other screening would also be necessary to significantly reduce the incidence of sudden death in horses. If a potentially dangerous arrhythmia is detected, management can be pursued. Certain arrhythmias are sustained and can be fixed by converting the heart back to a normal rhythm in an equine veterinary hospital. Other arrhythmias are intermittent and therefore the heart doesn't need to be converted.

Often, resting the horse is essential to see if the arrhythmia goes away over time because exercise would continue to inflame the heart muscle. Sometimes corticosteroids are used to decrease heart immolation as well. Horses that have persistent arrhythmias that are not well controlled with conversion, rest, or corticosteroids, can be treated with long-term antiarrhythmic drugs, but these horses should not be used for exercise (and in many sport disciplines, they would not be permitted to compete due to drug regulations).

I feel like we hear about heart attacks in horse sport more often than we used to, although that may be because of social media; is there any reason to think they're on the rise, and if so, do we know why? I don't know if the incidence of sudden death in horses is actually increasing. My feeling is that it is just our awareness of sudden death in horses that is increasing. With TV, YouTube, and social media, the dissemination of information is explosive compared to a couple of decades ago. Still, there are many episodes of sudden death in horses that go unreported. Lower level competitions are not always well regulated and are not well covered by the news; horses competing in these events are likely at higher risk for injury and sudden death, but their plight goes largely unnoticed.

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