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Copyright 2010 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;


Preventing Heat Stress in Horses

By Heather Smith Thomas, Salmon, ID


June 2010 Issue


     Heat stress, heat exhaustion, and heat stroke are terms that refer to dangerous conditions that may occur when horses are overworked in hot weather. Extremely hot weather is most likely to be life threatening when a horse is exerting (creating more body heat) or being hauled in an enclosed trailer/van with inadequate ventilation.

     Barney Fleming, DVM ( Custer , South Dakota ) vets endurance rides around the U.S. and has seen many heat-related problems in hard-working horses. He says the danger for heat stroke can be minimized by proper care and conditioning of the horse. Every rider should know the signs of trouble, and be aware of subtle changes in the horse—to be able to stop working and reverse the condition before it’s too late.


How the Body Dissipates Heat

     “The heat the body must get rid of comes from metabolism—muscle contractions and energy production. Horses can eliminate heat several ways,” says Fleming. Horses and humans cool themselves mainly by sweating, but also via respiration—dissipating heat from the lungs.

     “The lungs have a huge surface area inside. The horse blows hot air out and draws in cooler air that absorbs heat, and blows it back out. He also flares his nostrils, to aid that process. If his nostrils are flared, he’s attempting to get rid of heat,” explains Fleming.

     “There are some horses that don’t sweat much, or sweat in patches, and this makes them more prone to overheating. When sweat evaporates from the body surface, it cools. The horse sweats in order to cool off, but if he loses too much fluid during the course of a ride, he dehydrates—and that leads to problems,” he says.

     One difference between humans and horses is that temperature can vary more widely, with a larger range of safety, in horses. “A horse’s temperature can safely be between 99 and 103 and he’s not in trouble. Between 103 and 104 is borderline.”


Conditions that May Lead to Heat Exhaustion

     “If his temperature continues to climb, this is when he gets into trouble. If the horse’s temperature gets up to 106-107 degrees, it can’t stay there long or the condition becomes irreversible,” explains Fleming.

     Factors that can increase the risk include high humidity during hot weather. When the air is full of moisture (humid) instead of dry, evaporation rate slows or ceases, and you lose any cooling effect of sweat.

     “If the air is dry you get a swift cooling effect; the sweat comes out hot, dissipates into the air (taking heat with it), and that cools the body surface.

     “If sweat can’t evaporate because the air is already full of moisture, it just sits there on the skin, hot. It becomes an insulation, holding heat in rather than dissipating it,” he says.

     The body doesn’t cool, and signals for more sweating. Sweat may cover the body and run off in streams because it can’t evaporate. The horse stays wet but doesn’t become cooler. If he continues to work, he quickly dehydrates and overheats.

     In an arid climate, by contrast, sweat evaporates almost as fast as it is produced, constantly cooling the body. And if you put water on the horse to help him cool off, it evaporates quickly.

     But on a hot humid day, moisture just sits there, and you have to scrape it off to take any heat with it.

     “You put the cool water on, and take the warm water off; then put more cool water on, etc.” he explains.

     “In areas with high humidity, you must help the horse get rid of the heat, by getting rid of the water. Here in the arid West, however, if the air is dry and there’s a breeze, it evaporates so fast you don’t even see the sweat. This is an ideal situation, because the horse will cool very well,” says Fleming.

     Be aware of weather conditions and weather predictions, especially the heat/humidity index. Don’t work a horse hard when temperature and relative humidity together are dangerously high.

     A rule of thumb: when temperature and humidity numbers are added together and the total exceeds 130 (as when it’s 80 degrees with 50 percent humidity) there is risk of overheating. If the total number gets up to 150 or higher, any working horse will probably overheat. If temperature is 90 degrees the horse may get into trouble even if humidity is only 40 to 50 percent, and may also be in trouble if it’s only 75 degrees and humidity is 75 percent.

     Another factor that influences overheating is body mass. Large horses, fat horses, heavily muscled horses and heavily pregnant mares don’t dissipate heat as well as small, lean horses.

     “Quarter horses, draft horses, or any big, heavy horse will overheat much faster than a trim, well-conditioned Arab, for instance. A lot of pleasure riders like to ride Quarter Horses, but they are more likely to overheat,” he says.


Signs of Heat Stress and Dehydration

     If the body’s methods for regulating heat have not kept pace with the heat accumulation, the animal becomes weak and “exhausted.” When the horse runs out of body fluid for sweating, dehydration complicates the process of heat regulation.

     “He can’t sweat anymore, or sweats at a very reduced rate. This greatly increases the risk for heat stroke,” says Fleming. His temperature will shoot higher.

     The “pinch test” is a clue to how dehydrated he is. Skin becomes less elastic due to fluid loss from underlying tissues, and a pinch of skin pulled out from the neck or point of shoulder will not spring back into place but stays tented a few seconds.

     If it takes 2 or 3 seconds for the skin to sink back into place, the horse is moderately dehydrated and has probably lost at least four gallons of fluid. If the pinch of skin stays elevated 6 seconds or longer, he is severely dehydrated.

     Mucous membranes in the mouth (such as gums) become dry and discolored, turning brick red instead of healthy “bubble gum pink.” Capillary refill time is longer than normal; if you press your finger into his gum, blood does not rush right back afterward. That spot where you pressed out the blood stays pale for several seconds.

     The heart rate increases as the body tries to pump more blood to the surface for cooling, but has less body fluid to do it. The horse’s eyes seem sunken; eyelids and tissues around the eyes are more wrinkled, due to loss of fluid in the tissues. What sweat the horse does produce will be thicker and sticky instead of clear and watery.


     His pulse and respiration rate may remain high in spite of rest. His pulse may be weak, heart rhythm may be irregular, intestinal sounds may be diminished or absent (the gut has stopped working), and the muscles of his anus may become relaxed and floppy.

     Important salts have been sweated out with the fluid, creating critical changes in electrolyte balance of the body. This can interfere with nerve signals since electrolytes are crucial for proper nerve and muscle function. Erratic nerve signals can contribute to digestive tract malfunction, irregular heartbeat, muscle cramps, etc. Some horses develop “thumps” (the abdomen jerks each time the heart beats).

     The horse will be depressed, and won’t have much interest in eating or drinking. If a tired horse will eat, this is always a good sign. Green grass is the best feed for a tired/dehydrated horse. DO NOT feed grain.

     An alert rider can begin to sense subtle signs of fatigue before the horse is in trouble. If you stop working the horse at the first signs, and take time to let him drink and cool out, the condition won’t progress to the point of heat exhaustion or heat stroke. Check your horse’s pulse and respiration periodically during and after a workout. Take his temperature to see how overheated he is and how quickly he recovers.

     If a horse suffers heat stroke, rectal temperature may rise as high as 106 to 110 degrees. Skin will be hot and dry, since he has run out of fluid for sweating. He may be oblivious to his surroundings, and has difficulty moving. Unless the condition is quickly reversed, he will collapse, go into convulsions or coma, and die.


How to Deal with an Over-Heated Horse

     If the horse is mildly overheated, halt your ride and remove the saddle. Sponge him with water, especially over the major blood vessels, and provide shade. Walk him in a big circle if there’s a breeze, to cool him on all sides.

     Once he seems cool, check him again 15 minutes later, and again 30 minutes after that.

     If he is not completely cooled out (temperature still elevated, body retaining internal heat), he’ll break out in a sweat again. If that happens, walk and cool him again. After any strenuous workout in hot weather, check him again several hours after you’ve cooled him and put him away, since some horses may also be at risk for colic or laminitis after working in the heat.

     If the horse is severely overheated, stop riding as soon as he starts showing signs of trouble and move him into the shade if possible. Call a veterinarian—to give the horse large volumes of IV fluids to restore what’s been lost and to help restore proper blood circulation. While waiting for the vet, try to lower his temperature. Keep air moving around him with fans, or manual fanning. One of the quickest ways to cool him is to put cold water where major blood vessels are close to the surface—like the jugular groove.

     “Keep the jugular groove wet with cool water—sponge it on, sponge it off, sponge it on, etc. Another good place to keep wet is the big blood vessels on the belly. These are very close to the surface, especially if the horse is hot. They will be standing out prominently, dissipating heat,” says Fleming.

     It’s never a good idea to wet the whole horse in an arid climate. If you chill the big muscles, they’ll contract and constrict the blood vessels, hindering dissipation of heat.

     “In the East, where humidity is always higher, you can dunk the whole horse in water and get away with it, or pour cool water over his big muscles, because it’s not evaporating very fast. The big muscle masses have a lot of heat in them and it needs to be dissipated, but in low humidity don’t speed heat loss too much or the horse will cool too fast,” explains Fleming.

     In a humid climate, however, keep applying cool water all over the horse and keep scraping it off, since the water warms up immediately on the horse. After a short session of cooling, walk the horse briefly then apply cool water again. Keep alternating walking and cooling, since moving helps promote blood flow to the skin and air movement aids evaporation (unless humidity is too high for evaporation). Keep checking the horse’s temperature. It should drop about 2 degrees within 10 minutes. Once it starts to drop, slow down on the cooling. Stop using cold water as soon as his temperature comes down to 101, or when skin over his hindquarters feels cool after a walking period, or when his respiration rate drops below 30. Definitely stop if the horse starts to shiver; this means you’ve gone too far with the cooling.

     “Continually monitor the horse. Once his temperature starts dropping, slow down on whatever it is you are doing to cool him, or you may go too far and get a collapse of the blood vessels, and put the horse in worse shape,” says Fleming.

     Don’t use ice packs over the muscles, since this constricts surface blood vessels and hinders blood flow to the skin, which retards the cooling process. If you use wet towels over the neck or head, continually pour cold water on them. A wet towel left in place without constantly adding cold water will soon warm up and act as insulation, retaining heat.

     The legs also have a lot of exposed blood vessels. “Wetting the legs with cool water, including the feet, can help cool an overheated horse. If there’s a stream nearby, walk the horse into the stream. Stand him in the water and use water to keep his jugular vein and abdominal veins wet and cooling. A lot of blood goes through the feet, and cooling them will also help reduce the chance of founder,” says Fleming.

     “Overheating is a very serious problem in horses, but caught soon enough it is very easily corrected. If you stop the horse’s activity, you are stopping the production of heat, and then all you have to do is get rid of it,” he says. But if you keep pushing the hot horse, you may push him over the edge.


     Heather Smith Thomas is the author of numerous articles and 20 books. She and her husband ranch near Salmon, Idaho . For more information about her books, visit

     Heather’s blog online is:

Copyright 2010 Rocky Mountain Rider. ALL RIGHTS RESERVED. Reproduction of any editorial material, artwork and photos is strictly forbidden without express written permission of the publisher. For information about reprint rights, please contact the editor;


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