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During racing, Thoroughbred racehorses are at risk of overheating due to the exertion required, which can result in exhaustion and even death in extreme cases. Racing authorities in countries such as Japan and Australia, where temperatures are often high, have explored various factors that contribute to EHI, but no conclusive evidence has been found to predict which horses are at greatest risk.
Dr. Leah Trigg from the Bristol Veterinary School, Professor Siobhan Mullan, of the University College Dublin School of Veterinary Medicine and colleagues at the British Horseracing Authority (BHA), examined data on 704,434 runners at British racecourses recorded in the BHA database between July 2010 and April 2018.
The research is published in Scientific Reports.
There were 702 EHI incidents (defined in the database as either heat stress or heat exhaustion), accounting for 0.1% of runners.
The authors modelled the probability that a racehorse would present with EHI based on factors including age, whether the horse had had a previous EHI incident, race distance, race start time, ground conditions (going), average temperature in the five days prior to a race, and a measure of weather conditions (based on temperature, humidity, windspeed, and solar radiation) called wet bulb globe temperature index (WBGT).
Overall, the model correctly predicted 83.5% of EHI events, although the authors cautioned it produced a high number of false positives. Longer race distances increased the probability of EHI – the odds of a horse developing EHI in a two-mile race was 5.66 times higher than in a one-mile race. WBGT was also a predictive factor, with a horse running when it was 30 degrees Celsius 10.14 times more likely to develop EHI than a horse running at 20 degrees.
However, higher temperatures during the five days preceding a race were associated with reduced risk of EHI – the odds of EHI were 0.33 times lower when the preceding temperature averaged 25 degrees compared to 15 degrees. This suggests that horses may acclimatise to ongoing warmer temperatures and lose heat more effectively when they come to race.
Horses that had experienced previous EHI incidents were 18.59 times more likely to present with EHI, compared to horses who had not experienced EHI previously. Running on soft or heavy ground or in races before 5pm also increased the risk of EHI.
The research findings highlight the significance of ensuring that adequate cooling facilities are available, as well as identifying horses with a history of EHI to allow for early intervention if required. The authors of the study emphasize that these results are particularly relevant given the evolving climate, which is characterized by more frequent and intense periods of hot weather, and the need for the sport to address this challenge.
Dr Trigg, Honorary Research Fellow at the Bristol Veterinary School, said: "Racecourse officials should monitor WBGT at race meetings to help decide whether racing should go ahead, or if it does go ahead whether additional resources such as extra cool down areas should be provided. This data should be used to develop evidence-based policy to protect the welfare of racehorses in current and future climates."
For more details, see:
Risk factors for, and prediction of, exertional heat illness in Thoroughbred racehorses at British racecourses
Leah E Trigg, Sally Lyons & Siobhan Mullan
Scientific Reports (2023) vol 13, Article number: 3063
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