JAVMA Paper: Lasix Among Risk Factors in Sudden Death

A paper published in the current edition of The Journal of the American Veterinary Medical Association calls for further study of an association between the use of Lasix in racehorses and sudden death.

The paper, "Fifteen Risk Factors Associated with Sudden Death in Thoroughbred Racehorses in North America (2009-2021)," is written by Euan D. Bennet and Tim Parkin. The paper is included in the edition of the JAVMA published Oct. 20.

Lasix (the diuretic furosemide), which has been allowed on race day in North America to prevent or reduce the severity of exercise-induced pulmonary hemorrhage, was among the 15 risk factors significantly associated with sudden death. It found that "horses racing while on furosemide medication were at 62% increased odds of sudden death."

In observing those numbers in the study, the paper calls for further study. 

"The association between furosemide and sudden death prompts further study to understand which biological processes could contribute to this result," the paper notes.

Under current medication rules, most horses in the U.S. are permitted to race on Lasix, and the majority of trainers elect to treat their horses with it. The most common exceptions of horses that race without it are 2-year-olds and stakes competitors. Numerous racing jurisdictions prohibit the use of race day Lasix for those types of horses.

The paper finds that other factors associated with sudden death include a horse's age and sex, purse of the race, race distance, and a horse's recent history of injury or lay-up. 

"Associations found between previous injury and sudden death suggests pre-existing pathology could contribute in some cases," the paper notes.

The study relied on The Jockey Club's Equine Injury Database, and Parkin is an England-based veterinary epidemiologist who has consulted on the EID since its inception. Through a study of the EID, the industry has made a number of changes in different areas to help reduce catastrophic musculoskeletal injuries, which since 2009 have accounted for the vast majority of equine deaths during racing.

While sudden deaths not related to musculoskeletal injury account for just 7.4% of all fatal incidents, the industry has improved safety through a step-by-step process as opposed to a silver bullet. On the musculoskeletal side, this has included changes in medication rules, claiming rules, purse guidelines, and increased efforts to improve surface safety—just to name a few. Many of these changes were put in place to address risk factors that were identified through the EID.

This step-by-step approach has helped racing reduce equine fatalities 30.5% since 2009. The rate of 1.39 per 1,000 starts in 2021 is a record low for the industry dating to 2009, when it started tracking rates. Another way of phrasing this number is that in 2021 equine fatalities did not occur in 99.86% of starts.

As these step-by-step changes have reduced musculoskeletal injury, Parkin, who heads the University of Bristol's veterinary school, sees potential to make further equine safety progress by addressing the smaller subset of sudden deaths—non-musculoskeletal injuries. Since 2009 these sudden deaths have occurred at a rate of 0.13 per 1,000 starts. In June, at the Welfare and Safety of the Racehorse Summit at Keeneland | BloodHorse.com Track Profile">Keeneland, Parkin outlined some of the sudden-death findings that appear in the paper.

He noted that as some high-profile horses have suffered sudden deaths in recent years, the problem has moved more into the public eye. In December, disqualified Kentucky Derby Presented by Woodford Reserve (G1) winner Medina Spirit died after a workout at Santa Anita Park. The California Horse Racing Board later announced that a necropsy could not precisely determine why the grade 1-winning colt collapsed and died after a Dec. 6 workout but said results suggested acute heart failure, without confirmation.

While neither the CHRB nor Parkin addressed any link between Lasix use and Medina Spirit's sudden death, it was one of two medications in his system at the time of his passing. The CHRB reported that omeprazole, an anti-ulcer medication, and Lasix were detected in blood and urine samples. These findings were consistent with the medication report filed with the CHRB by the colt's attending veterinarian.

Many horses are treated with Lasix before timed workouts.

Beyond high-profile instances of sudden death, the EID numbers show there has not been the same steady decline in the number of sudden deaths as seen in the musculoskeletal injury numbers since 2009. Parkin said this has resulted in sudden deaths becoming a larger proportion of equine deaths in racing—topping 10% of all equine deaths in racing in several recent years.

"People are noticing it more on the track," Parkin said, adding that compared with catastrophic breakdowns where the problem is clear, it's not always fully understood what the exact cause of death is in horses listed in the sudden death category.

Parkin outlined the association between Lasix and instances of sudden death revealed by the EID.

"We identified for the first time a relationship between the use of Lasix in the racehorse and sudden death," Parkin said. "The use of Lasix in a race increases the risk by about 62%."

Parkin acknowledged that even with regulatory changes in recent years that have seen more horses run without race-day Lasix, the EID numbers are comparing a large number of horses racing on Lasix with a relatively small number of horses racing without the diuretic. That said, he was confident that the EID numbers—which showed horses racing with Lasix experiencing sudden death at that 62% higher rate than horses who raced without it—are statistically relevant.

"I think that there is evidence that there is some sort of physiological relationship, potentially, between the use of Lasix and sudden death," Parkin said. "It certainly warrants further investigation."

Parkin said before the industry compiled such information in the EID, such relationships were elusive.

"I think the reason this wasn't identified before is purely related to statistical power," Parkin said. "This is purely driven by the fact that we now have sufficient number of years of data—starts and sudden deaths in the database—to enable us to draw this conclusion."

At the safety summit, Parkin said he looks forward to finding out more about the impact of exercising or racing horses on Lasix, how that changes their blood chemistry, and perhaps contributes to the risk of cardiac arrhythmia.