Heat Stress in Horses: An Overview

Knowing when horses are at risk for heat stress is just as important as knowing the clinical signs. This allows horse owners to adjust the time of day when they will work or postpone work altogether.

Signs of heat stress in a horse include elevated body temperature (105-107° F; 40.5-41.6° C) , rapid breathing, rapid pulse, general weakness, dry skin, and dehydration.

The most commonly used predictor of heat stress is adding the ambient temperature (in degrees Fahrenheit) and relative humidity (percent). The sum of these two numbers is the heat index. The following summary of heat stress risks uses this scale.

When the heat index is:

Less than 130: heat loss should not be a problem. Horses can be worked normally.

Greater than 150: heat loss is severely compromised, especially if humidity is greater than 50% of the total. Work horses but be mindful of the signs of heat stress.

Greater than 170-180: little heat loss can occur, with some experts recommending not working or competing horses. Consider riding or working horses during a different part of the day, as norma cooling methods may be inefficient.

Horses maintain and produce body temperature through several metabolic processes, including the normal chemical reactions that maintain life, fermentation in the large intestine, and muscle activity. Up to 80% of the energy created during exercise is lost as heat. Horses that are dehydrated, depleted of electrolytes from sweat loss, or cannot sweat (anhidrosis) are more susceptible to heat stress.

Horses dissipate body heat through vasodilation (the widening of blood vessels in the skin), sweating, and exhaling warm air. When the blood vessels of the skin dilate in response to exercise, it allows the warm blood to lose heat into the air. If the air temperature is near body temperature, this exchange of heat is reduced. Sweating can remove 25-30% of increased body heat. Horses working in hot, humid conditions can produce up to 30 liters (approximately 7.5 gallons) of sweat per hour. Horses not only lose water in sweat; they also lose electrolytes such as sodium, chloride, calcium, and magnesium. If the relative humidity is high, then the surrounding air already contains much water, and sweat, which produces a cooling effect, cannot evaporate. Horses additionally lose about 25% of metabolic heat through their respiratory tract when they exhale.

A horse’s normal body temperature is less than 101.5° F (38.6° C). Exercising horses often have rectal temperatures around 103° F (39.4° C). Body temperature greater than 105° F (40.5° C) is dangerous, and body temperatures as high as 106-107° F (41.1-41.6° C) can be fatal due to multiple organ failure. Clinical signs of heat stress in horses include panting, stumbling, not sweating as much as expected with elevated body temperature, depression or unresponsiveness, seizures, coma, and death. In cases of heat stress, cooling and rehydrating the horse are priorities. The most effective way to cool the horse is to repeatedly pour cool water over the horse and remove it with a sweat scraper.

Severe heat stress or heat stroke are veterinary emergencies and require intravenous fluids and intense supportive care. Anhidrosis is an often overlooked problem in poorly performing horses. Veterinarians can perform a sweat test by injecting different concentrations of epinephrine or terbutaline under the skin and evaluating sweat production at the injection sites.

For more information on heat stress, consider reading this in-depth report, first published in the proceedings of the 2024 Equine Health and Nutrition Conference, hosted by Kentucky Equine Research: Heat Stress: A Comparative Approach Between Species and Disciplines. Another article from that proceedings that might be of interest includes Should You Sweat It: Anhidrosis in the Equine Athlete.