A new study of data from the Equine Injury Database has revealed that horses medicated with furosemide (Lasix) on race day were at 62 percent increased odds of sudden death compared to horses that were not racing on furosemide.
Funded by the Grayson Jockey Club Foundation, the study was published by Dr. Euan Bennet and Dr. Tim Parkin on Oct. 20, 2022, in the Journal of the American Veterinary Medical Association. It examined the 4,198,073 race starts made by 284,387 Thoroughbred horses at 144 racetracks in the United States and Canada between 2009 and 2021; those numbers represent 92.2 percent of all official race starts during that period.
Of those nearly 4.2 million starts, 536 resulted in a horse's sudden death, an incidence rate of 0.13/1,000 starts. Sudden death was defined as any horse that was recorded as a fatality within three days of racing, along with one or more of the following fatal injury descriptions or (presumptive) diagnosis, as provided by each participating track to the EID: (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.
In total, the study examined 49 risk factors for sudden death, and found that 15 of those risk factors were significantly associated with sudden death, including horse age and sex, season and purse of race, race distance, and horses' recent history of injury and lay-up.
The most startling statistic was that associated with Lasix; of the 536 sudden deaths examined in the study, 518 were recorded as racing on the anti-bleeding medication. Only 18 horses suffering sudden death had raced without the medication. Researchers determined horses racing on Lasix had a 62 percent higher risk of sudden death than horses racing without it.
Study authors explained that 94 percent of starters in the recorded data raced on Lasix, so it is an extremely common risk factor and merits more research.
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Want to support our journalism while accessing bonus behind-the-scenes content, Q&As, and more? Subscribe to our Patreon stream.They wrote: “Identifying any potential causal link between furosemide administration and sudden death should be a priority. Furosemide administration, alongside water restriction, is common race-day practice for the management of EIPH and has been shown to also result in improvements in race performance. Although evidence supports the use of furosemide to reduce the severity of EIPH, the ethics of permitting any race-day medication is controversial and problematic for the sport.
“Furosemide is a diuretic and reduces the severity of EIPH by reducing blood volume and hence blood flow and pulmonary arterial pressure. The diuretic action results in a loss of sodium, potassium, and chloride into the urine and hence predisposes to electrolyte abnormalities. In humans, these electrolyte disturbances may predispose to arrhythmias and arrhythmic death. Therefore, it could be hypothesized that furosemide administration in horses may increase the risk of sudden death through fatal arrhythmogenesis.
“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.”
Other risk factors in the study were as follows:
- Compared to horses racing in summer, those racing in winter were at a 28% reduced risk of sudden death.
- Starts made on synthetic track surfaces were at 33% reduced odds of sudden death compared to starts made on dirt tracks.
- Longer race distances were associated with a significantly reduced risk of sudden death.
- Horse starts in state-bred races (in which only state-bred horses were permitted to race) were approximately 41% less likely to experience sudden death than horses in other races.
- Horse starts in races with purse values > $10,000 were at approximately 40% greater risk of sudden death compared to starts in races with purse values < $10,000.
- Compared to female horses, geldings were not significantly different with respect to their risk of sudden death, but intact male horses were at 39% increased risk of sudden death.
- Older horses were at increased odds of sudden death compared to younger horses, with horses 5 years old and above at 44% increased risk relative to horses 3 years old and younger.
- 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 making their first start as a claimer were at 38% increased odds of sudden death compared to horses that were not previously claimers.
- Compared to horses that had never had a lay-up (defined as a period of longer than 60 days with no race starts), horses whose previous lay-up was within 30 days of the current start were at 37% increased odds of sudden death.
- Horses that had previously been added to a vet list were at 31% increased odds of sudden death compared to horses that had never been on a vet list before.
- Horses that had raced a longer cumulative distance in their prior career were at reduced odds of sudden death compared to those with less cumulative distance.
- Horses with more previous wins in their prior career were at increased odds of sudden death.
The study's authors also wrote that the risk factors were most helpful in identifying an at-risk horse when layered together; that is, if they removed one variable from the model (like number of starts in 90 to 180 days) the statistical significance of other factors like season and furosemide use changed. The final model used for the paper included all three variables because it produced the most accurate prediction of a horse's risk. This, they believed, was likely due to the rare nature of a sudden death compared to the overall number of race starts in the database.
Sudden deaths are also difficult to study because even with a necropsy, it's not uncommon for the exact cause to remain unknown.
According to the study: “Some of the factors identified suggest that the possibility exists to identify at-risk horses well before they experience sudden death. 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.”
See an abstract of the study here.
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