Study: Horses On Lasix At Increased Risk of 'Sudden Death'

A new study published this week in the Journal of the American Veterinary Medical Association and funded by the Grayson Jockey Club Foundation, has found that horses racing on Lasix were 62% more likely to die within three days of racing than were horses running without the diuretic.

Fatalities due to catastrophic musculoskeletal injury were not included in the study.

The use of Lasix was one of 15 risk factors identified in the report.

The study, relying on information from the Equine Injury Database, examined starts made by 284,387 Thoroughbreds at 144 racetracks in the U.S. and Canada from 2009 to 2021. A total of 4,198,073 starts were included in the study, which was conducted by Dr. Euan Bennet and Dr. Tim Parkin. The number of starts equals 92.2% of all races in North America run during the period of the study.

Among the starters, there were 536 instances where a horse died within the three-day window after racing, a rate of  0.13 per 1,000 starts.

Horses falling into the “sudden death” category had to have died within three days of racing and have had one or more of five fatal injury descriptions:  (1) sudden death (recorded as “SUD” in the EID); (2) pulmonary hemorrhage; (3) exercise-induced pulmonary hemorrhage (EIPH); (4) postexertional distress/heatstroke (PED); and (5) cardiac arrhythmia.

The study included 233,276 starts made by horses without Lasix. Eighteen or 0.08 per 1,000 starters died within three days. There were 3,964,797 starters who ran with Lasix and the result was 518 sudden deaths or 0.13 per 1000 starters. Ninety-four percent of all horses in the study raced with Lasix.

“…it could be hypothesized that furosemide administration in horses may increase the risk of sudden death through fatal arrhythmogenesis,” the study reads. “However, as the exact cause of sudden death (cardiac vs non-cardiac) was not determined for the horses of this study, we can only speculate at possible mechanisms. Further investigation is required to understand which, if any, pathophysiologic mechanisms could underlie the association between furosemide use and sudden death, as this finding raises further concerns about the ethics of race-day administration.”

The researchers also concluded: “…identifying any potential causal link between furosemide administration and sudden death should be a priority.”

The study looked at 49 risk factors and concluded that 15 of those factors could have led to a horse dying following a race. The list includes:

(*) Horses racing in the winter were at a 28% reduced risk of dying when compared to horses racing in the summer;

(*) There were fewer fatalities in longer races. In races contested at a mile or longer horses were 32% less likely to die than horses competing in races at six furlongs or shorter;

(*) Older horses were more likely to die than younger ones. Horses five or over were 44% more likely to die than horses who were three or younger;

(*) Horses that had been racing frequently were less likely to die than horses who had their races spaced out. For each additional race start made in the previous 0 to 30 days, individual horses were at reduced risk of sudden death. Horses that had made two or more race starts in the period 90 to 180 days before the current race were at 19% decreased risk of fatality compared to horses that had made 0 or 1 start in the same time period;

(*) Horses racing for the first time in a claiming race were at an increased risk. They were at 38% increased odds of sudden death compared to horses that were not making their debut in the claiming ranks;

(*) Horses that had previously been on a vet list, were 31% more likely to die compared to horses than had never been on a vet list;

(*) Horses whose last starts were made on a synthetic track were at 33% reduced odds of sudden death compared to starts made on dirt tracks;

The researchers hoped their information could lead to fewer sudden deaths.

The study reads: “Further work is required to determine which, if any, clinical signs are potential indicators and, indeed, whether such a rare outcome could be reliably predicted. The thoughtful application of big data predictive modeling would be helpful here; routine biometric monitoring of racehorses throughout their training and racing careers could provide valuable insights into horses' physical well-being.”