A study co-conducted by an epidemiologist who analyzes Thoroughbred fatality data has found a significant statistical correlation between the use of raceday furosemide and sudden death, a finding that is sure to influence the ongoing debate over the raceday use of the diuretic to mitigate bleeding in the lungs. The study, which was authored by Drs. Euan D. Bennet and Tim H. Parkin, found that horses that raced on Lasix had a 62 percent greater risk of dying of sudden death – usually cardiac events – than horses that did not race on the drug. Parkin is an epidemiologist at the University of Bristol in England who is a consultant to the EID and conducts a yearly examination of its data. The study analyzed more than four million starts at races at U.S. and Canadian racetracks from 2009-21, using the Equine Injury Database. The database was launched in 2009 to assist the racing industry in identifying risk factors for all fatalities at U.S. tracks, and examinations of the data have led to various reforms in the racing industry over the past decade. Over that same time period, racing fatalities have dropped 30.5 percent. Sudden deaths are far less likely to result in a fatality than musculoskeletal injuries and represent approximately 10 percent of the fatalities at North American racetracks. The study identified 15 factors that were associated with sudden deaths, with Lasix use standing out as the most significant correlation. The study found that horses that ran on Lasix accounted for 518 of the 536 sudden deaths recorded during the 13-year period. Because approximately 94 percent of the starters in the study raced on Lasix, and because sudden deaths were statistically rare – 0.13 percent of all starters – the study’s authors cautioned that the correlation “merits more research.” :: DRF Bets members get FREE DRF Past Performances - Formulator or Classic. Join now! “Identifying any potential causal link between furosemide administration and sudden death should be a priority,” the authors wrote. Debate over raceday Lasix use has cleaved the North American racing industry for decades. Raceday use of the drug is not permitted in most racing countries, and over the past five years, many North American racing jurisdictions have begun prohibiting the raceday use of the drug in stakes races and 2-year-old races. This year’s Breeders’ Cup races will be conducted with Lasix prohibitions in all races for the second straight year. In 2020, the event’s five 2-year-old races had raceday Lasix bans, but the other nine races allowed Lasix use. Lasix is a diuretic, and although no studies have conclusively determined how the drug mitigates bleeding in the lungs, researchers have theorized that Lasix lowers blood pressure in pulmonary tissue. Numerous studies have determined that horses administered the drug on raceday suffer less severe episodes of bleeding. Other studies have shown that horses administered the drug on raceday have better performances than those that race without the drug. Debate on that aspect of the drug’s impact centers on whether horses compete to the best of their ability due to the mitigation of bleeding or whether Lasix is a performance enhancer, perhaps due to significant water loss after its administration. The study’s authors speculated that the “diuretic action” of Lasix could be affecting electrolyte levels in horses, with an associated risk of suffering cardiac events due to that imbalance. However, they stressed that the study did not seek to examine any causes for risk factors for sudden death. “The diuretic action results in a loss of sodium, potassium and chloride into the urine and hence predisposes to electrolyte abnormalities,” the authors wrote. “In humans, these electrolyte disturbances may predispose to arrythmias and arrhythmic death. Therefore, it could be hypothesized that furosemide administration in horses my increase the risk of death through fatal arrhythmogenesis [the sudden onset of cardiac arrythmias].” The authors also noted that the significant correlation between sudden death and Lasix use “raises further concerns about the ethics of raceday administration.” Sudden death in racehorses generated headlines late last year when Medina Spirit, the now-disqualified winner of the 2021 Kentucky Derby, died shortly after completing a workout at Santa Anita. Because sudden deaths comprise a significant share of racing fatalities, the racing industry has been seeking more data on the incidents. The study found several other significant risk factors, including a higher risk in the summer compared to the winter; a higher risk in races with purses greater than $10,000; a higher risk for horses 5 years old or older compared to 3 years old or younger; a higher risk for intact males over geldings and females; and a higher risk for horses that had been added to the vet’s list at least once in their career. :: Want to learn more about handicapping and wagering? Check out DRF's Handicapping 101 and Wagering 101 pages.