Heat Stress in Horses

Warmer summer days can provide the ideal weather for leisurely rides on your favorite mount, but heat can take a toll on horses, just like people. When exercising or working your horse in temperatures in the 90s and 100s, overheating and heat stress can endanger your horse—and even turn deadly. Watching for heat stress signals is vital to your horse’s health.

Below, we’ll examine what heat stress looks like, the warning signs, what to do if your horse overheats, and how to prevent dehydration from heat stress.

What Causes Heat Stress in Horses?

Horses primarily create heat while digesting their feed and during any activity that requires their muscles to work. Even light activity in extremely hot, humid weather could cause your horse to overheat.

You may notice that your horse’s blood vessels appear larger and distended when it’s hot outside. This reaction is normal, as it shunts the blood to the skin to rid the body of excess heat; however, this doesn’t provide adequate cooling if the air temperature is warmer than the horse’s body temperature.

Other than humans, horses are the only mammals that sweat. During hotter conditions, they primarily use this mechanism to wet the body and allow cooling via evaporation. The problem occurs when the sweat doesn’t evaporate to allow adequate cooling, potentially leading to heat stress.

“Horses are pretty efficient at dissipating heat compared to species that don’t sweat,” says Kris Hiney, Ph.D., associate professor and Equine Extension Specialist at Oklahoma State University. “But they can still get heat stress [during] prolonged periods of high temperatures or working in high temperatures. Very young or infirm horses are more likely to suffer from heat stress.”

Detecting Heat Stress

Hot skin and profuse sweating or less sweating than expected for the activity could be warning signs of heat stress. Hiney advises looking for high body temperature, an elevated heart rate, and a respiration rate beyond what’s expected for the workload.

“Working in hot temperatures may not be completely avoidable,” she says. “[But] it’s important to recognize the signs that your horse may be overheating.”

Hiney recommends practicing the following techniques ahead of time so both you and your horse will be familiar with them when needed.

◆ Use a rectal thermometer (keep one with you if you’re far from a barn) to monitor heat load. Body temperature will normally increase with exercise, and body temperatures of 104 degrees to 106 degrees Fahrenheit are common for some events.

However, horses with these body temperatures need to be cooled carefully. If a horse’s temperature is over 106 degrees, take immediate action to cool the horse. Temperatures reaching 107 degrees may require veterinary assistance.

◆ Check your horse’s heart rate, either with a stethoscope or manually. When the horse is allowed to rest, his heart rate should noticeably begin to drop. It should return to 40 to 50 beats per minute within 15 minutes. Failure of the heart rate to drop quickly indicates heat stress.

“Flared nostrils and heaving flanks while breathing are not only from work, but also from the increased heat load,” Hiney explains. “Allow the horse to rest and watch carefully for a reduction in breathing rate and intensity.”

◆ Assess the amount and location of sweat. Normal sweating patterns include along the neck, flanks and between the hind legs. With intense exercise or extreme temperatures, the horse may be completely covered in sweat and will need help being cooled.

A horse that has ceased sweating is most likely suffering from heat exhaustion and is in critical need of care.

How to Treat Heat Stress

If you suspect your horse is overheating, it’s vital to stop exercising and move him to a cooler setting right away. You will likely need to help him cool down to prevent heat stress from turning into heat stroke, which can have dire consequences.

“Evaporation through sweat is the major way horses dissipate heat, but high humidity levels will limit the effectiveness of sweating,” says Hiney. “Increase airflow to the skin by removing all tack and equipment and use a sweat scraper to remove excess sweat.

“Cool running water can greatly aid in cooling,” she continues. “Apply water or even ice along major blood vessels like the neck, chest and inside of the legs. Blood will cool as it passes through these areas and then return to the trunk of the body to help dissipate the heat load. Continual application of cool, running water will prevent the warming of the water on the horse’s skin. Otherwise, use a scraper to remove the warm water and increase the rate of cooling. Water left on the skin will warm rapidly and trap heat against the horse’s body.”

Hiney says that you can still help your horse cool down even when you don’t have access to running water by using a sponge to wet the major blood vessels. Again, you should concentrate on the neck and chest. She also suggests adding rubbing alcohol to the water to increase evaporation and aid in cooling.

If you need to cool your horse while riding if you’re not near running water, Hiney suggests using a squirt bottle to apply cool water along the major exposed blood vessels in the neck and chest. You can refill your water bottle from streams, lakes or stock tanks.

“Air movement is another major way that animals lose heat,” she says. “Supplying fans or keeping the horse in a breezy area is ideal. Fans with higher velocities provide more effective cooling. If you have access to electricity, put a fan near the arena to aid in cooling during rest periods. This fan can greatly assist with continuing normal training during the summer. Even so, be sure that the horse’s heart rate and respiration rate have dropped before returning to work.

“Simple strategies like keeping the horse in the shade or riding in shaded areas prevent heat gain directly from the sun,” Hiney adds. “Standing under trees is even more useful, as the horse will radiate heat up to the leaves, which are continually cooled by their own evaporation.”

She warns that heat stress can quickly turn to heat stroke. If you feel this is happening, cease work, strip tack off the horse, and get him into the shade immediately. When facing heat stroke, veterinary care with IV fluids may be necessary.

Heat Index

Through appropriate management, heat stress can be prevented. Start by paying attention to the heat index in your area to understand when it’s dangerous to work your horse. Although different horses may tolerate heat differently, follow general guidelines to ensure you’re not putting your horse in danger.

“Pay attention to heat and humidity,” Hiney advises. “A good rule of thumb is to add the values of the temperature [in degrees Fahrenheit] and humidity [relative percentage] to get the horse’s comfort index. Horses cool themselves normally—unless dehydrated or fatigued—if the combined values are below 130.

“If the combined temperature and humidity is over 150 (such as 90 degrees Fahrenheit with 60 percent humidity), horses will need assistance in cooling,” she continues. “If temperatures and humidity are expected to reach 170, it may be best to plan early morning riding or skip intense work. Even riding in the evening may not see a large drop in temperature until the sun has been down for several hours.”

Dangerous Dehydration

High heat or excessive sweating can cause dehydration in horses. If your horse doesn’t replace the fluid lost during sweating by drinking adequate water, he can become dehydrated, which can be life-threatening.

“Horses can become dehydrated in as little as two to three hours under the right conditions,” says Hiney. “Horses need to be well-hydrated before beginning to work, for example, horses that have come off of a long trailer ride.”

She explains that some warning signs of dehydration include prolonged capillary refill time (checked by pressing your finger into the horse’s gums) and tenting when skin is pulled from the neck or over the eye. The horse may also have a gaunt or tucked-up appearance.

Colic and overheating are possible, but if the horse gets an electrolyte imbalance due to dehydration, tying up and even death could occur.

“The primary mechanism horses use to dissipate heat is through sweating,” says Hiney. “Sweating leads to dehydration if the horse isn’t allowed to replenish, and the rate of sweating can outstrip the ability to keep up.

“Warm body temperatures encourage horses to drink, so allow access to water in the cool-down period,” she continues. “Allow frequent sips of cool water. Saline can also help replenish water and electrolytes, but be sure to have plain water available, too.”

Key Takeaway

Luckily, horses usually bounce back after a bout of heat stress or dehydration without any lasting effects. However, both can become very serious conditions in a relatively short span and, if left untreated, can lead to death.

It’s best for your horse’s health to not allow either to begin with by ensuring he always has access to clean, fresh water and avoiding exercise in extremely hot weather.

Ride in the morning when it’s coolest, and reduce ride time and intensity when it’s hot and humid, or refrain altogether if the temperature and humidity are above safe levels.

This article about heat stress appeared in the July 2023 issue of Horse Illustrated magazine. Click here to subscribe!