Sudden Death In Racehorses: A Way To Screen And Diagnose At-Risk Runners?

[Wandering Eyes]

Though rare, sudden deaths in racehorses have long proven a source of great frustration for being just that–a sudden event with virtually no way to predict it, and oftentimes no way to explain it.

Emerging research, however, offers real hope that perhaps soon, the industry will have a way to both zero in on the small number of horses at heightened risk of apparent sudden death, as well as a better understanding of just what might be happening to them.

Using data from more than 21,000 horses who wore the StrideSAFE biometric sensor in just over 60,100 races over a four-year period, researchers were able to zero in on just 0.7% of the runs as the ones containing those horses most at risk of suffering sudden death. And within that group, it caught 83% of those horses that indeed would eventually go on and suffer a sudden death event during the study period.

Horses at elevated risk of sudden death were flagged as far out as eight months before dying.

“There's a lot of horses out there and people are busy. You can't screen them all,” said Warwick Bayly, dean of Washington State University's College of Veterinary Medicine, who described as “preliminary” the status of the StrideSAFE research.

“But if veterinarians have to act or focus their efforts, this is telling them, based on the current state of knowledge, 'go focus on these horses,'” Bayly said.

But focus their attention on what exactly?

Up to now, experts have largely speculated over sudden death causes, their assumption being they're mostly heart related. But what heart murmurs and arrhythmias are the primary culprits? And how impactful are other factors, like genetic predisposition, underlying infections and other triggers?

A key part of the problem is that sudden deaths often leave no discernable physical sign for pathologists to piece together a clear diagnostic picture.

Since the launch of Horseracing Integrity and Safety Act (HISA), 214 horses have suffered an exercise associated sudden death (EASD), 75 in racing while 139 were in training. According to available postmortem findings, around 56% of cases have been deemed inconclusive, meaning they're associated with cardiac death.

Late scratch | Sarah Andrew

Thanks in part to a huge ongoing study led by a team of researchers put together under HISA's auspices, a picture is emerging as to what kinds of heart irregularities might need special attention. This includes horses in atrial fibrillation (a type of arrhythmia) while resting, and arrhythmias in the late recovery period after exercise.

According to Sian Durward-Akhurst, assistant professor of genetics, genomics and large animal internal medicine at the University of Minnesota College of Veterinary Medicine, the goal with HISA's sudden death working group is to get enough data to be able to answer the question, “are there any ECG characteristics that are predictive?”

 

Study Findings

The StrideSAFE biometric sensor–a small iPhone shaped device that slips into a horse's saddle cloth–records with each stride thousands of different data points like its acceleration and deceleration, its up and down concussive movement, and its movement from side-to-side. Importantly, it is not a heart monitor.

As part of a study soon to be published, the StrideSAFE team used the sensor on over 21,000 horses who made 60,103 starts from between July 2021 and August 2025.

During this four-year study period, 29 horses from this population suffered a sudden death event during exercise. StrideSAFE had captured 77 starts these 29 horses had made prior to the fatal event.

The StrideSAFE team then ran all 60,100-plus starts through an algorithm–which is built around 16 different variables–specially designed to capture a horse's likelihood of suffering a sudden cardiac death. Once done, the algorithm places each start into one of four different categories:

1 – Normal
3 – Low Risk
4 – Moderate Risk
5 – High Risk

The StrideSAFE team wanted to see if the algorithm could flag the 77 starts made by the 29 horses who would go on and later suffer a sudden death during exercise (we'll call them the EASD horses).

Vet care in the barn area | Sarah Andrew

Furthermore, if the algorithm worked as planned, it would also mark as “normal” the remaining 60,026 starts made by the other horses who didn't suffer an EASD event. Here's what the researchers found:

Of the 29 horses that would go on and suffer a sudden death event, 27 of them received a category 3-5 flag at least once. The number of starts assigned to these three elevated risk categories constituted just 3.5% of the total starts (2,098 of 60,026 total starts).

Within that group, 24 of the horses that would later die a sudden death received a category 5 flag at least once. Category 5 horses have a one-in-ten chance of dying from sudden death, researchers found. The percentage of runs assigned to category 5 constituted just 0.7% of the overall starts (426 of 60,026 starts).

Furthermore, EASD horses assigned a category 3, 4 or 5 start received their respective flag at least once during the 8 months prior to dying.

If this continues to play out in ongoing research, this could mean that sudden deaths aren't as sudden as we thought, similar to how a “bad step” was once used to explain a musculoskeletal injury but has since been disproved.

Which begs the question: why is a biometric sensor able to pick up horses at heightened risk of sudden death?

 

What Does StrideSAFE Pick Up?

With such limited scientific understanding of equine sudden death to hand, any answer to that question would right now be largely “speculative,” said Dave Lambert, who founded StrideSAFE.

“But it's reasonable to assume there's some kind of cardiac impairment that is having an effect on blood flow and therefore the vitality of the work-rate of muscles. And once their muscles are going to change, their stride is going to change,” Lambert added.

He compared this effect to humans with cardiac issues like heart disease, especially in the elderly. They more easily get fatigued, and when they do, their muscles twitch and tremble.

“If the blood flow to a muscle that is working very, very hard is in any way impaired, then presumably that muscle isn't working quite as well,” said Lambert. “And all a horse has to do is weaken for a hundredth of a second at some point during the stride and we're going to pick that up.”

Broadly speaking, what Lambert's findings suggest is that StrideSAFE could provide trainers and veterinarians a way to zero in on those horses who need additional scrutiny without “having to do an ECG on every horse in training,” he said.

Device | StrideSAFE WEB

According to his calculations, at a track with 80 runners per-day, his algorithm would flag about one category 5 horse every two days. If all category 3, 4 and 5 horses are selected for extra veterinary scrutiny, that will come out to around two or three candidates each day, according to Lambert's numbers.

That's not a lot of horses. Even so, what do you do with them once flagged?

In an ideal world, there would be a set of protocols to follow akin to the sorts of diagnostic assessments given to horses with suspected musculoskeletal injuries-think nerve blocking for lameness, and PET and MRI scans.

No such clear set of protocols currently exist in the U.S. for horses suspected of having cardiac problems. That said, Hong Kong provides something of a guide.

For four racing seasons, official veterinarians in Hong Kong have listened with a stethoscope to every runner's heart during a pre-race inspection on the day before the race, as well as a second examination between 15 to 30 mins before the race start time.

Only horses assessed to have a normal heart rhythm are allowed to start. Those that exhibit an arrythmia during a pre-race examination are withdrawn so that a more thorough clinical examination can be performed, and to gauge whether they're safe to return to racing.

According to Bronte Forbes, head of veterinary regulation at the Hong Kong Jockey Club, an average 0.09% of runners are withdrawn for heart rhythm irregularities pre-race. During officially requested post-race examination, 0.27% have heart rhythm irregularities.

“Although there are some common arrhythmias in racing horses that have known levels of risk, and prognosis for returning to racing, some may be more complex and in those cases we commonly consult experienced specialist equine cardiologists to confirm whether an arrythmia should be considered as significant enough to warrant restrictions to training and racing and inclusion in the Official Veterinary Examination system with further monitoring and examinations,” Forbes added, in an email (more on this in a bit).

The thing is, from jurisdiction to jurisdiction, clinician to clinician, there remains much debate over what constitutes a problematic murmur or arrythmia in racehorses. Some clarity on the matter, however, might soon be arriving.

 

Ongoing Study Under HISA

About one year ago, HISA launched a voluntary study for trainers to equip their horses with Arionoe's Equimetre sensor during morning exercise-mainly fast works, but also gallops and jogging days. The project is chaired by Karen Hassan, compliance and research veterinarian for HISA.

Unlike StrideSAFE, Equimetre is both an ECG electrocardiogram (ECG) monitor and a biometric sensor that fits under the girth.

The aim of the study is to see if electrocardiograms employed during exercise can be used to determine at-risk horses along a sliding scale, from the healthy end to those horses whose welfare is compromised by cardiac issues.

To date, 1491 horses have delivered 7740 recordings. The aim is to ultimately collect data from 10,000 horses.

It takes time to go through the data, which is being done by hand rather than through artificial intelligence. So far, 2050 recordings from 708 horses have been analyzed.

Horse with an Equimetre device | Arioneo WEB

Among the recordings already evaluated, about 88% of the horses fitted with the Equimetre illustrate some kind of arrhythmia. About 31% show complex arrhythmias.

While that ongoing study is ongoing, Cristobal Navas De Solis, associate professor of cardiology/ultrasound and internal medicine at the University of Pennsylvania School of Veterinary Medicine–and a member of HISA's sudden death research group–has also been involved in a retrospective study that has yielded fascinating results.

Navas and his fellow researchers have dug down into a list of 13 horses from the U.S., France, Australia and Britain that suffered a sudden death and had a history of wearing the Equimetre sensor leading up to the event.

Five of these 13 horses were wearing the device the day they died. Of these five, three of them left the stall in atrial fibrillation before they exercised and subsequently died.

What this suggests is that particular attention needs to be paid to horses not engaging in high-intensity exercise while on atrial fibrillation.

Asked about what happened to these three horses' hearts just prior to dying, Navas explained their heart rates during or after high-speed work became excessively high before the atrial fibrillation then deteriorated into very unstable rhythms “like ventricular fibrillation or tornado-like ventricular tachycardia that causes death.”

Of the other seven horses with Equimetre recordings within one month of the sudden death, one was in atrial fibrillation one-week prior to death (but was not wearing the sensor the day of the event).

What's known about atrial fibrillation in racehorses? For one, racehorses in atrial fibrillation rarely exhibit any outward symptoms at rest. It's also pretty rare. Of the horses in HISA's ongoing study, between 0.25% and 0.75% of them have shown atrial fibrillation during training.

Navas explained that most horses with atrial fibrillation will be just fine if not asked to exercise. “It might not impact mild to moderate exercise, but it will impact racetrack performance and safety during high-intensity exercise like racing if it persists,” he wrote, alluding to a 2021 study out of Hong Kong.

That same study also showed how 80% of horses continue racing and training after one episode of atrial fibrillation but after a second episode, most horses (66% of them) were retired.

Arrhythmias during the recovery period after exercise appear to be another target of attention in racehorses, said Navas.

“It is interesting because traditionally, recovery arrhythmias are considered less concerning than arrhythmias during the high-intensity exercise period,” wrote Navas in an email, before stressing that a larger population is required to draw firm conclusions (while at the same time extolling those trainers who have already participated in the ongoing study).

Despite the preliminary nature of so much of this research, Bayly urged the veterinary community to look at and perhaps harness some of these emerging findings rather than wait for concrete answers to be nailed down-what could take years.

“There are people who will always want more information,” said Bayly. “And in many of these cases, I'd say it's the classic example of the enemy of good being better or best.”

HISA's research group is recruiting more horses for 2026. Anyone interested in participating should contact Navas at: navasdes@vet.upenn.edu.

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