Exertional heat illness

Heat stress or exertional heat illness (EHI) in competing horses seems to have become more of a problem in recent years.

There are, however, few published studies into how common the problem is in racehorses and the factors that underlie it.

Typical signs 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.

Two studies from Japan have investigated the prevalence of EHI and risk factors for the condition in Thoroughbred horses racing on the flat.

In a paper published in the Journal of Equine Science, Nomura and colleagues (1) investigated the prevalence of post-race EHI and climate conditions at racecourses in Japan.

Overall, in races run under Japan Racing Association (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 wet bulb globe thermometer (WBGT) was between 28°C  and 33°C.

In the Equine Veterinary Journal a paper by Takahashi and Takahashi (2), 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 20 ⁰C  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.

Other risk factors identified were:

• Female horses or geldings were at greater risk for EHI than were uncastrated males.
• Races longer than 1600m presented a greater risk.
• Older horses (Age 4 years or more) were at greater risk than younger horses.

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

For more details, see:

(1)    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

(2)    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