Learn how your horse’s internal thermostat works in extreme heat and ways to keep him cool

Maybe it was his name. The 18-hand one-ton Norman Cob named Texas found himself in a very uncharacteristic heat wave last summer in northwestern France.

“He can go 35 kilometers (22 miles) on an average day, and I usually drop him to 25 (15 miles) when it’s hot out,” says owner Fanny Chatel, who drove Texas in harness 1,550 miles across northern France last year. “But 40°C (104°F°)? That’s just too much to ask.”

Like many European countries, France experienced a major heat wave in 2019, bringing temperatures so high that the national equestrian federation had to adjust competition times to avoid putting horses at risk of heat stress.

Owners across the affected zones took measures to protect their horses, providing shade and easing exercise routines. For Chatel, that meant a four-day hiatus from her four-month escapade in Saumur—France’s equitation capital.

“I’m the one asking Texas to drive me on this exciting journey,” Chatel says. “It’s up to me to make sure he’s comfortable enough to do it.”

A Primer on Thermoregulation

Like most mammals, horses come equipped with ­complex—and very effective—ways to thermoregulate, or self-adjust their internal body temperature like a thermostat. The primary goal of thermoregulation isn’t comfort but, rather, survival, keeping the internal organs functioning properly at the ideal equine body temperature of around 99.5 degrees Fahrenheit, says Kristina Dahlborn, PhD, professor in the Department of Anatomy, Physiology, and Biochemistry at the Swedish University of Agricultural Sciences, in Uppsala.

Ambient temperatures affect body temperature significantly. When it’s cold out, the equine body works to conserve and increase internal heat. On hot days it dissipates and reduces heat, she says, through its interface with the air: skin. Thermal receptors in both the internal organs and the skin send messages to the hypothalamus—a small part of the brain responsible for many hormonal functions—which then activates various thermoregulatory mechanisms to trap or release heat as necessary.

While sweating is a good tool for dissipating internal heat, it is less effective in horses than in humans—simply because pound for pound and inch for inch, humans have much more skin surface area for their body size, says Dahlborn.

Further, due to their great muscle mass, horses produce much more heat than we do and must dissipate the surplus despite the skin area disparity, says Cecilie M. Mejdell, DVM, PhD, of the Norwegian Veterinary Institute’s Department for Health Surveillance, in Oslo.   

The horse can also use the respiratory system as a cooling mechanism. “Horses can’t pant like a dog, but they can increase their respiratory rate to dissipate heat from the airways,” as they exchange more air, says Mejdell.

The first line of approach for thermoregulation, though, is the skin’s efficient and expansive vasodilation system, she explains. When it’s cold, horses can “close up” the blood vessels in their skin so the heat in the blood stays deeper inside the body. As temperatures increase, the diameter of those vessels also increases, routing as much as a third of the horse’s blood at a time to the skin alone, close to the surface and away from the internal organs, which get the chance to cool off.

This system, however, comes at a risk: When too much blood gets pumped away from the internal organs, they can suffer oxygen deficiency—the basis of heat stroke, says Bénédicte Ferry, DVM, equine preventive health expert at the French Institute of the Horse and Equitation, in Saumur.

All About Sweat

When horses sweat, it evaporates off their skin into the air, pulling with it body heat, resulting in a cooling effect. Horses have a lot of sweat glands, and they can sweat profusely, ­producing 5 gallons of sweat in only an hour, says Ferry. However, that excessive production doesn’t help cool the horse much, since only about 25-30% of it evaporates. The rest sits on the body and runs down the legs.

On humid days the air is already saturated with water, so sweat doesn’t evaporate as well, says Grete H.M. Jørgensen, PhD, of the Norwegian Institute of Bioeconomy Research, in Tjøtta.

While the horse produces even more sweat (up to 7.5 gallons, says Ferry) than he would in the same conditions on a less humid day, the cooling process is less effective because the laws of physics interfere. Because less sweat evaporates, less heat dissipates from the body.

Horse sweat is dense with salts, which horses lose considerable amounts of when they perspire, explains Mejdell. To help horses replenish these reserves, provide a salt block and/or add salt to feed and supply unlimited fresh, clean water.

However, if a horse is sweating ­intensely, a salt block alone won’t suffice, says Dahlborn. “Sweat contains 9 grams of salt per liter, and a horse can lick about 20 grams per day,” she says. This means a salt lick prepares them for losing about half a gallon of sweat, no more.

Water, Dehydration, & Water Balance

Water is crucial to thermoregulation. Because of the gallons of sweat horses produce in extreme heat, it’s critical they have a constant supply of cool, clean water to replenish, says Ferry.

When the equine body gets low on water, it still produces sweat, pulling water from throughout the body to create it. Eventually, as their water reserves deplete, horses start to dehydrate.

Providing a constant supply of fresh water might seem obvious, but it can get overlooked at often the worst times—­specifically, during exercise and transport, Ferry says. Horses are exposed to much more heat in these conditions, so it’s important to take regular exercise and travel breaks (at least once an hour in hot summer) to offer fresh drinking water.

Spraying or sponging cool water onto the horse can mimic and even surpass sweat’s effect. Cool tap water has a much lower temperature than sweat. It will draw more body heat to the surface to evaporate the water. The caveat, however, is the water can accumulate in the horse’s coat and create a layer that’s too thick to evaporate quickly, trapping body heat beneath. Remove excess water with a sweat scraper to avoid this effect, says Ferry.

Vital Electrolytes

Electrolytes are charged chemical compounds that have critical roles in causing muscles to contract, transmitting nerve impulses, and completing other kinds of cellular functions. Their “electricity” provides the “power” needed for these functions to occur.

The main electrolytes lost through sweat are chloride, sodium, and potassium. Usually, electrolyte loss sets off a series of bodily events that trigger the brain to tell the horse, “I’m thirsty,” thus driving him to replenish the water supply by drinking.

Normally, horses find electrolytes in plants or soil, says Mejdell. But when horses are performing in the heat, being transported, or otherwise dealing with hot conditions, they might need ­supplementation.

Electrolyte supplements come as powders or pastes, which are particularly useful for athletes needing to replenish electrolytes lost during exercise. Otherwise, horses should have regular access to a salt/mineral block or loose salt, especially during extreme heat, says Ferry.

The Exercise Factor

Horse muscle is powerful stuff, burning a lot of energy. That energy comes from the muscle cells’ high metabolic capacity, which generates physical activity, as well as all that heat, says Dahlborn.

“When horses exercise, about 20% of the metabolism in the muscle cells is used for work, and the remaining 80% becomes heat,” she says. The added heat isn’t wasted, however; it heats the body when it’s cold outside, and it keeps muscles warm while working.

Still, this means most cases of heat stress occur following intense and/or long exercise sessions in hot environments, Ferry explains. Good planning can help protect your horse from overheating. “Avoid intense work during the hottest times of the day,” she says. Aim for early mornings when the air is still cool, and hose and sweat-scrape your horse after.

The Airflow Factor

Horses thermoregulate better in environments with good airflow, because fresh air can remove heat from the horse, says Kathy Holcomb, PhD, retired professor from the University of California, Davis, School of Veterinary Medicine.

Free-roaming horses or domestic horses on large pastures with varied terrain migrate during severe heat to spots with good airflow. But confined horses have less choice, she says.

Fans can keep air circulating within barns, says Jørgensen, as can air inlets and outlets, mesh stall fronts, and open windows and doors.

Pay attention to stagnant hot air in trailers and poorly ventilated stalls, says Ferry. Only transport your horses during the coolest times of the day or at night, and ensure good air circulation. If you must tie your horse outside on a hot day, such as to the trailer at a competition, try to find a place with shade and a breeze.

Shade Structures

Studies have shown that when given the choice while at rest, horses seek shade on hot days. “We saw frequent, brief (about six-minute) bouts, with horses going in for short stays about 17 times a day, mostly in the mornings,” says Holcomb.

However, poorly ventilated shaded areas might not be cooling enough to tempt horses to spend time in them, she says.

Both man-made buildings and natural vegetation (trees) can provide appropriate shade, says Jørgensen.

“For best practices, shade should be made available at all hours of the day during hot, sunny weather,” says ­Holcomb.

Tipping the Body’s Limits

Generally speaking, horses have evolved to live in ambient temperatures ranging from -40 to 104 degrees F, says Dahlborn. But the scope of temperatures they can tolerate is probably more restricted, depending on their breed, health, body condition, and even color—with darker horses absorbing more of the sun’s rays.

Still, if we consider this range as within two extremes, we find that most parts of the world likely reach the upper (hotter) limit more than the lower. While many owners stress about their horses getting cold in winter, Mejdell says the heat of the summer is the greater risk.

Measure if a horse is getting too hot by checking his temperature with a rectal thermometer, say Holcomb. It will rise if he’s experiencing heat stroke because the body can no longer dissipate heat through vasodilation, sweating, and breathing. Consider body temperatures up to 101.3 degrees F normal; beyond that, the horse is starting to experience hyperthermia, or heat stroke, says Ferry.

At 105.8 degrees F vasodilation calls excessive blood away from the organs to the skin. “This is a major emergency and can lead to death,” she says.

Above 108.5 degrees F, proteins in the brain (and other organs) risk coagulation, says Dahlborn. If the brain’s thermoregulation control centers fail, thermoregulation fails, she says.

Ferry says other warning signs of heat stress or stroke include:

• Constantly increasing respiratory rate, even 30 minutes after exercise.
• Dry, sticky gums.
• Gums that take longer than one second to turn pink again from white after you’ve pressed them with a fingertip (capillary refill time).
• A general unwell appearance, possibly with muscle soreness or colic signs.
• In advanced stages, increasing heart rate, red mucous membranes, loss of consciousness, and falling down. 

If you notice any of these signs, stop working the horse immediately and place him in a cool, shady, well-ventilated area, ideally with fans, says Ferry. Offer fresh water and cool him however you can, hosing or pouring cool water on the neck, chest, legs, head, and poll, which houses the nerves responsible for sensing thermoregulation. Use a sweat scraper to avoid reheating the body, she adds. “If you follow these steps, the horse’s temperature should drop 2 degrees C (3.6 degrees F) within 10 minutes,” says Ferry.

If the horse doesn’t recover within that period, call a veterinarian immediately. He or she will probably provide intravenous fluids and/or administer rehydration liquids by nasal tube.

“If the organs were affected, expect them to take awhile to recover,” says Ferry. “The horse will probably have to be off work for a prolonged period of time.”

Take-Home Message

Most horses can cope with hot conditions if they have access to self-cooling opportunities, says Mejdell. Our role as owners is to provide our horses with ways to thermoregulate properly while at rest and stay cool when in work.