Cooling Methods and Heat Stress
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.
Using Microchips to Measure Body Temperature in Horses
Horse owners, veterinarians, and researchers can measure a horse’s body temperature in multiple ways. By far the most common method is the use of digital rectal thermometers. While simple to use, these devices may offer inconsistent readings and pose a safety risk to handlers and horses.
Though less common, other ways of recording body temperature include a disposable sensor attached to the underside of the horse’s tail, primarily used for tracking temperatures over long periods, and an infrared thermometer, thought to have a weak correlation to core body temperature and to be easily influenced by external factors.1
At Kentucky Equine Research, caretakers often record body temperature several times daily. For this reason, they rely on percutaneous thermal sensing microchips (PTSMs), which provide faster and more accurate readings than traditional methods.
As their name implies, PTSMs are small devices, no larger than the microchips used for pet identification. They are implanted into specific muscles of the horse. Thirty-three horses across three farms at Kentucky Equine Research have PTSMs placed in a gluteal muscle, with an average muscle depth of 1.81 ± 0.56 cm for the horses residing in Florida at the Kentucky Equine Research Performance Center. Twelve of the 33 have a second microchip in the pectoral muscle at a muscle depth of 1.42 ± 0.31 cm.2 The gluteal muscles form the large mass over the horse’s croup, while the pectoral muscles lie between the front legs along the chest.
Once implanted, PTSMs can be scanned immediately with a HomeAgain microchip reader, which displays the horse’s temperature instantly on-screen. Users can also link the microchip to a unique identification number for each horse and keep a record of the data using the EquiTrace app. This process can be repeated as many times as needed, as the microchips have lifelong battery power, and the microchip readers are rechargeable.3
Ease of reading is especially important at Kentucky Equine Research. With PTSMs, researchers can safely track a horse’s body temperature while the horse is exercised on a high-speed treadmill, a feat impossible with traditional rectal thermometers.
The use of PTSMs has greatly influenced the welfare of horses by detecting early signs of unusual changes in body temperature, such as fever or heat stress. This is especially important in horses undergoing strenuous exercise in hot and humid conditions where heat dissipation is difficult.1 Kentucky Equine Research has put this technology to good use during research trials, allowing scientists to better understand the efficacy of different cooling methods after exercise.2
Percutaneous thermal sensing microchips represent a major advancement in equine health monitoring and research.