By Mark Andrews 

A drug that has been widely used in Thoroughbred racehorses in North America could increase the risk of sudden death, according to a new study. The research also identified other risk factors associated with sudden death, relating to the circumstances of the race and individual histories of the horses. 

The research, conducted by Dr. Euan Bennett of the University of Glasgow’s School of Biodiversity, One Health & Veterinary Medicine, and Professor Tim Parkin of the University of Bristol Vet School, is the first large-scale study of sudden death in Thoroughbred racehorses. 

The work was funded by the Grayson Jockey Club Foundation. A report is published in the Journal of the American Veterinary Medical Association. 

The study analysed over four million starts in horse racing over a 12-year period, using data recorded in the Equine Injury Database (EID), which contains details of nearly all official race starts made in the United States and Canada. About one in 10,000 race starts resulted in a racing-related sudden death for a horse. 

For this study, “sudden death” was taken to include any fatality occurring within three days of racing where the cause of death recorded in the EID was sudden death, pulmonary haemorrhage, exercise-induced pulmonary haemorrhage (EIPH), post-exertional distress/heatstroke (PED), or cardiac arrhythmia. Fatalities due to catastrophic musculoskeletal injury were not included.

The researchers identified a notable risk factor related to race day medication. Horses that were recorded as being administered furosemide were 62 percent more likely to experience sudden death compared to horses that weren’t on furosemide. 

Furosemide (also known as frusemide and by the trade name Lasix) has been used to prevent exercise-induced pulmonary haemorrhage (bleeding in the airways). It is also associated with enhanced racing performance. As a result, 94 percent of horse starts in the study were on furosemide. 

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The ethics of race day medication are controversial. Furosemide is already restricted or prohibited on the day of racing in certain circumstances, depending on jurisdiction. 

The results also suggest that it might be possible to identify horses at risk of sudden death before they experience it — for example, due to previous injury and interruption to training or racing. 

Amongst other findings were that the risk of sudden death was greater for stallions compared with mares, and for horses five years or older compared with horses three years old or younger. Other risk factors identified include season and value of race, and race distance. 

“Over the last 12 years, the overall risk of fatality within three days of racing has decreased by over 30 percent, but the incidence rate of sudden deaths has not changed significantly,” says Dr. Bennett. “This suggests that while interventions have been made which have contributed to a reduction in catastrophic injury, there are different sources of risk for sudden death which have not yet been identified.  

“This study suggests that a risk profile, identifying which horses are at the greatest risk of sudden death, may be possible. Given how rare the outcome is, further work is required to establish any potential interventions which might contribute to a reduction in sudden deaths. 

“On the association between furosemide use and sudden death, the fact that furosemide use is so common makes this result particularly remarkable given the statistical power of this large-scale study. Discussions around the ethics of race day administration of drugs should factor in potential risks such as those identified here, and further work is required to understand exactly why we identified this association.” 

For more details, see Fifteen risk factors associated with sudden death in Thoroughbred racehorses in North America (2009–2021). Euan D. Bennett PhD, MSci and Tim D. H. Parkin, PhD, DVSc. Journal of the American Veterinary Medical Association (2022) 

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Published with the kind permission of Mark Andrews, Equine Science Update

Photo: Dreamstime/Cheryl Quigley