Your horse’s normal temperature should be between 99° and 101°F. If your horse’s temperature exceeds that range, he is becoming overheated and needs to be cooled down. It’s a good idea to bring a thermometer along with you to any shows or events that you’ll be riding at so that you can monitor your horse’s temperature.
Increased Respiration and Heart Rate
A horse who is too hot will experience a higher than average respiration and heart rate. A heart rate greater than 50 beats per minute, and a respiration rate of more than 30 breaths per minute is cause for concern.
Horses sweat to cool themselves, but high humidity and high heat can make their sweating less efficient. If your horse is sweating profusely, it’s a likely indicator that he is struggling with the heat.
Lack of Sweat
If exercised too heavily in extreme heat and humidity, a horse may stop sweating. This is a symptom of heatstroke and may be accompanied by overall weakness, uncoordination, and even collapse. Horses with anhidrosis suffer from an inability to sweat; if you suspect your horse has anhidrosis, then speak with your veterinarian about ways to keep him comfortable and safe during the summer.
Tremors in your horse’s abdomen or hind end can indicate that he is overheating. In some cases you may see his hind and muscles twitch in time with his heartbeat. You might also notice that your horse is experiencing muscle spasms in his hind end and legs. These are serious symptoms and indicate that your horse needs to be cooled down immediately.
You can test your horse’s hydration in two ways. The skin test refers to pinching a small tent of skin from your horse’s neck with your fingers. Hold it for a few seconds, then release - your horse’s skin should quickly snap back into place.
More accurate than the skin test, testing your horse’s capillary refill time provides you with a better idea of whether or not he may be dehydrated. Press a finger against your horse’s gum for a few seconds, then release it. Watch the color of the gum where you have just had your finger - the white, pale color should be replaced by the healthy pink normal color of your horse’s gum within three seconds.
If your horse is showing signs that the heat is too much for him, immediately bring him out of the sun and cease exercising him. Bring your horse into the shade of some trees or your barn, and immediately cool him. Remove his tack, hose him continuously with cool water, and offer him water to drink. Always call your veterinarian if you suspect your horse is overheating.
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Original Source: Is Your Horse Telling You That He's Too Hot?