If you detect the symptoms of heat stroke in your horse, you should call your veterinarian immediately. Here are some things to look out for and what to do while you wait for your vet:
Symptoms
If it’s an extremely hot and humid day, it’s not hard for your horse to get overworked in a short amount of time. Just a few signs to look for to tell you that your horse is over-heated are if he has an elevated breathing and pulse rate, a lot of sweating and salivation, an irregular and hard heartbeat and a high body temperature.
Ways to battle it
The same ways to battle it are some of the same things that should be used for prevention. Give your horse sips of cool water from a bucket, move him to a shady place and sponge him off with water on his legs, stomach and neck. If your horse already has heat stroke, these should only be used as a temporary fix until the vet arrives. He is the best option to figure out a solution to the problem.
Source: PetMD