The primary method of heat dissipation in horses is through evaporation of sweat, especially during the summer when horses are living in areas with warm to hot ambient temperature conditions. When horses are exercised, their bodies generate a great deal of energy which converts to heat. During the warm up phase of exercise a few important interactions are happening in the horse:
- The rate of heat production in horses muscles greatly exceeds the rate of heat dissipation, which causes a rapid increase in muscle temperature.
- Core blood is heated with heat storage exceeding heat dissipation, which results in the horse's core body temperature to increase in proportion to the exercise intensity.
- The horse's skin temperature decreases slightly as a result of the increased convection related to the motion, then it gradually increases.
Therefore when horses are exposed to warm temperatures and exercised they are more at risk of overheating. Horses with a thermoregulatory system impairment, such as anhidrosis, are even more at risk of overheating, due to their inability to properly sweat.
Unconditioned, older horses (due to changes to their thermoregulatory system), dark colored horses, horses in trailers, overweight horses, horses grazing on endophyte-infested fescue grass, horses that have recently relocated from colder climates, larger horses, and dehydrated or overweight horses are also at a heightened risk of developing heat stress. All horses that are exposed to a humidity index over 180 are at risk of heat stress, and should not be exercised until weather conditions improve.
Early recognition and vigorous treatment are essential to limit the consequences of heat stress in horses.