Heat Stress in Horses

Summer is here, and with it hot and often humid weather that can cause horses to become overheated.

The most important single factor in preventing heat stress is providing plenty of clean, fresh water and trace mineralized salt to all horses. Journal photo.

Horses produce large amounts of heat, mainly through digestion of feed and muscular activity during exercise. If the air is cooler than the horse’s body temperature, blood is shunted to the skin where the horse easily rids itself of the excessive heat. However, if the air temperature is warmer than the horse’s body temperature, blood shunting is not enough, and sweating becomes the primary means by which the horse cools itself. The horse is the only mammal, other than man, that cools itself primarily by sweating. This wets the body so cooling, due to evaporation, can occur.

Problems develop and the cooling system breaks down during hot, humid weather when sweat doesn’t evaporate and adequate cooling cannot take place. The result is heat stress, which often leads to heat exhaustionor life-threatening heat stroke.

What It Looks Like

Symptoms of heat exhaustion include profuse sweating, muscle weakness, stumbling, rapid breathing (normal breathing ratefor an adult horse is 8-18 breaths per minute) and an increased body temperature of 102 degrees to 106 degrees Fahrenheit (normal temperature is 98-101 degrees Fahrenheit).

The horse continues to sweat profusely, and despite the fact that its skin often feels cool, its body temperature is elevated and may remain elevated even after the horse has rested. A much more serious condition is heat stroke, where the affected horse’s skin is hot and dry, breathing is extremely rapid and the horse’s body temperaturemay reach 106-110 degrees Fahrenheit. If untreated, horses experiencing heat stroke may collapse, go into convulsions and die.

If a horse becomes overheated, the horse owner should not attempt to differentiate between the two syndromes but should immediately stop riding the horse, place him in the shade and call a veterinarian, as the situation may quickly become an emergency. If there is no breeze, provide air movement with a fan, if possible. Starting with the feet and legs, gradually wet the horse’s entire body, including the head, with cool water. Avoid using cold water and never attempt a cold-water enema. Small amounts of drinking water can be given at 15-minute intervals until the veterinarian arrives.

Preventing It

Heat stress can be prevented with proper management and conditioning. Calculating the heat index may alert the horse owner to unfavorable weather conditions where the horse should not be worked. To calculate the heat index, add the temperature in Fahrenheit and the percentage of relative humidity. If the sum is below 120, there should be no problem exercising a horse. If the sum is between 130 and 150, the horse will probably sweat but should not experience any problems if he gets plenty of water to replenish fluid lost during sweating. When the heat index exceeds 180, do not exercise horses because their body?s heat dissipation systems will not be adequate to prevent heat stress.

Certain horses are more susceptible to heat overload. Poorly conditioned horses, overweight horses, geriatric horses, horses in direct sunlight when the temperature is more than 100 degrees Fahrenheit, or animals in hot, poorly ventilated stalls or trailers are always at risk.

Horses not consuming enough waterand not getting sufficient salt or electrolyte supplements are especially prone to overheating. Horses transported from cooler climates that have not had time to adjust to hot weather are especially vulnerable and should be provided adequate time to adjust to the warmer weather conditions. Rations high in protein generate extra body heat during the digestion process and make horses more susceptible to overheating.

Horses produce large amounts of body heat during chewing, digesting and metabolizing feed, so avoid riding them too close to feeding time during hot weather. Heat generated during food digestion added to the heat generated by exercise could push them into heat stress. As a rule of thumb, feed horses three to four hours before exercising and wait at least two hours after they have been ridden before feeding.


The most important single factor in preventing heat stress is providing plenty of clean, fresh waterand trace mineralized salt to all horses. The average American Quarter Horse will normally drink 8-10 gallons of water a day. High temperatures, even without exercise, will double this amount. Therefore, if water is being supplied in buckets, add an additional bucket per stall during the summer. Check the buckets at least three times per day to ensure that the horse has plenty of water. Recent research has shown that horses working hard, such as endurance horses, can sweat nearly four gallons per hour under conditions of high heat and humidity.

Under normal conditions, a balanced ration and free access to mineralized saltare sufficient to replace electrolytes lost during sweating. However, equine sweat contains a large amount of sodium and potassium, and horses sweating excessively might not be able to replenish these electrolytes by merely consuming a salt supplement. Therefore, provide a good electrolyte supplement to the feed of any horse working hard in hot weather.

Heat stress can cause serious injury or even death. Making sure your horse is in shape, acclimated and properly hydrated are the keys to a successful and safe summer of riding.

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