The heat is on: Risk factors of heat stress in racehorses explored

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Hugh Bowman and Cheval Grand winning the 2017 Japan Cup. Second was Rey de Oro, ridden by local-based French rider Christophe Lemaire.
Cheval Grand winning the 2017 Japan Cup. © Yuki Shimono

Two studies carried out in Japan have investigated the prevalence of heat stress or exertional heat illness (EHI) in racehorses.

Typical signs of the condition include abnormal behaviour, such as head shaking, kicking out in a random fashion, pawing the ground, reluctance to move, and ataxia. Affected horses may take longer than normal to recover after exercise, with rapid breathing, prolonged increase in heart rate and excessive sweating. Severe cases may collapse.

One of the studies, led by Motoi Nomura of the Horse Racing School of the Japan Racing Association (JRA), investigated the prevalence of post-race EHI and climate conditions at racecourses in Japan.

Overall, in races run under the JRA between 1999 and 2018, they found an overall prevalence of 0.04% (387 cases out of 975,247 starters).

There was a trend for more cases recently, with a prevalence of 0.07% in the last four years.

When climatic conditions at the three racecourses with the highest prevalence were evaluated, most of the races were found to have been held when wetbulb globe thermometer (WBGT) was between 28°C  and 33°C.

The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account: temperature, humidity, wind speed, sun angle and cloud cover (solar radiation). This differs from the heat index, which takes into consideration temperature and humidity and is calculated for shady areas.

The researchers in this study, which was published in the Equine Veterinary Journal, were all associated with the Japan Racing Association, including its Equine Research Institute, and clinics at the Ritto and Miho training centres.

The second study, also published in the Equine Veterinary Journal, reviewed records of horses diagnosed with EHI after flat races. Data from cases occurring between April and September over a 12-year period, were used for a case-control study. Each case was compared with three randomly selected control cases to try to identify risk factors for EHI.

When the WBGT index was 28⁰C or higher, the risk of EHI was much higher than when it was 20⁰C  or lower.

The risk of EHI was higher in July than August, although the temperature measured by WGBT actually reached higher levels in August. The authors suggest this indicated a lack of acclimatisation to the heat.

Researchers Y Takahashi and T Takahashi also found that female horses or geldings were at greater risk for EHI than were uncastrated males, races longer than 1600m presented a greater risk, and horses aged four or older were at greater risk than younger horses.

They stressed the importance of taking measures to cool racehorses immediately after races, especially when the WBGT index is ≥28°C.

Prevalence of post-race exertional heat illness in Thoroughbred racehorses and climate conditions at racecourses in Japan. Nomura M, Shiose T, Ishikawa Y, Mizobe F, Sakai S, Kusano K. J Equine Sci. (2019);30(2):17-23. doi:10.1294/jes.30.17.

Risk factors for exertional heat illness in Thoroughbred racehorses in flat races in Japan (2005-2016). Takahashi Y, Takahashi T. Equine Vet J. (2019) doi:10.1111/evj.13179.

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