One of the largest puzzles still to be solved by the equine medical community is the occurrence of "sudden death," which involves an abrupt fatality in an apparently healthy horse during or immediately after exercise that is not associated with a musculoskeletal injury.
Exact causes for these sudden deaths have proven hard to decipher. With high-profile cases occurring in recent years, such as the training deaths of 2021 Kentucky Derby (G1) first-place finisher Medina Spirit and 2023 Santa Anita Derby (G1) winner Practical Move, the solving of such a phenomenon has become increasingly important to the racing community.
Dr. Stuart Brown, Keeneland's vice president of equine safety, moderated a panel June 25 at the Welfare and Safety of the Racehorse Summit in Lexington to discuss the state of the equine medical field in solving the sudden death puzzle.
"We're talking about something that happens in less than one out of 10,000 starts," Brown said. "It certainly comes—through the nature of that rate of occurrence—with very tough phenomena with which to study."
Sudden death in Thoroughbreds is often compared to a human having a heart attack, though heart issues alone do not cause all sudden deaths in horses. Although cardiovascular failure is believed to be a cause in many cases, sudden death has also been known to occur due to trauma to the spine or hemorrhages associated with a fracture, colic, or infectious disease.
"We know that about half of the exercise-associated sudden deaths are due to cardiac issues," said Dr. Lynn Hovda, chief commission veterinarian of the Minnesota Racing Commission. "What we don't know is what is strictly normal in a horse. ... We have a large knowledge gap that we need to fill."
Many potential risk factors, such as cardiac arrhythmias, a condition in which the heart's electrical signals don't work properly, can be present in healthy equine athletes.
"When you look at healthy exercising Thoroughbred racehorses, about 82% of them have at least one premature depolarization during exercise," said University of Minnesota professor and researcher Dr. Sian Durward-Akhurst. "(Many) have multiple premature depolarizations and about 20% of them have some kind of complex arrhythmia. ... Trying to say which horses are safe and which horses are not is really quite difficult."
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University of Kentucky veterinarian pathologist Dr. Laura Kennedy said she is able to diagnose only about half of sudden death cases.
"There can be significant overlap between a catastrophic injury and sudden death in the amount of pulmonary hemorrhage," Kennedy said. "A catastrophic injury, that's a very dramatic event. I could see that it could cause some hemorrhage in the lungs.
"The heart is an electrical instrument. It's going to be looking at those tiny conduction differences. That's where we have to focus."
The panelists agreed that the best path forward to solving sudden deaths is communication. Discussing changes in the horse's behavior, medical history, and other factors with the trainer can help paint a bigger picture of what external factors could have led to the sudden death.
Durward-Akhurst also pointed out that atrial fibrillation, an irregular and often rapid heart rate that commonly causes poor blood flow, could be an important aspect to study.
"We know it causes problems, we know it reduces performance," said Durward-Akhurst.
Atrial fibrillation is yet to be directly linked as a cause of sudden death, but according to Durward-Akhurst, it can remodel the heart. If experienced consistently during and after exercise, it potentially could put a horse at higher risk of sudden death.
Genetics are also being researched to determine whether there are specific genes that make certain horses more at risk. She reported that many samples have been taken over the last four years.
"This is probably going to be a complex tree," Durward-Akhurst said. "There might be some families where it's simply inherited. Maybe there's one gene that is causing it, but overall the sudden death is likely going to be a combination of variants."
If genetic risks exist, they can be combined with other factors such as performance history, training history, medical history, and more to determine an overall risk factor for sudden death.
University of California, Davis distinguished professor emerita Dr. Sue Stover mentioned that the one benefit to not knowing the exact cause is having the opportunity to research a wide range of potential causes that could impact horses' health.
"The more we know about the causes, the more we can work on intervention and prevention," Stover said.
The panel closed by pointing out how far equine medical technology and research have come over the last few decades. With better resources and renewed passion and dedication by researchers, the unsolvable puzzle may start having the pieces put together within the next decade.
"We can do this," said Kennedy. "Sudden death is a trickier problem because it is not biomechanical. It is biologically functional. It's a much more difficult thing to wrestle with, but we can do it."
