The United Kingdom experiences continued record breaking highs in temperature and humidity. Horse competitions continue regardless to the welfare of the horse. Horses are exposed to Heat Stress and Heat Stroke, both are extremely dangerous and can lead to suffering, death and debilitating illness.
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It is left to sport governing bodies to cancel events, a high reluctancy exists due the financial losses incurred.
Evidence from The FEI World governing association:-
https://inside.fei.org/news/cancellation-endurance-competition-fei-world-equestrian-games%E2%84%A2-tryon-2018
The British climate is variable, this does not provide horses time to acclimatise to the conditions. Humidity can be higher than in other countries and is serious contributor to heat related conditions for horses.