How To Recognize Heat Stress in Horses
By Heather Smith Thomas
Hot weather, especially when coupled with high humidity, can be devastating for livestock — and for horses if they must work in the heat. If the heat/humidity index becomes high, horses are at risk even when just standing in a barn or at pasture.
Extremely hot, humid weather poses a risk but is most likely to be life-threatening when a horse is exerting (creating more body heat) or hauled in an enclosed trailer with inadequate ventilation.
Normal body temperature for a horse is between 98 and 101 degrees Fahrenheit. Individual horses have a consistent resting temperature. Some are always 99 and others 101. A difference between horses and humans is that body temperature can vary more widely in horses, with a larger range of safety. A rectal temperature of 103 in a horse is considered elevated, but not dangerous.
“If the horse has been working and sweating, he just needs to be walked around, letting him have sips of water, and some washing down with cool water,” says Dr. Kent Allen, a veterinarian in Virginia who works with sport horses that compete in hot weather. “It’s normal for a horse to heat this much while working.”
In the horse, sweating is the primary means of cooling. “Horses are very efficient at sweating. They also dissipate heat via radiation, if air temperature is cooler than the body,” says Allen. “They can also pant (breathing fast and shallow), but this is the least effective cooling method, since the horse must breathe through his nose.
“A dog can open the mouth and pant (losing heat via mouth and tongue), but the horse doesn’t have that option,” explains Allen. He just breathes faster to create more air exchange via the lungs.
Riders should be aware of subtle signs of stress when working a horse in hot weather. “Many good horses have a tremendous amount of heart and will continue to work,” says Allen. “Those horses will keep going but start to show signs of stress. Where normally the horse’s ears would be pricked forward and he has enthusiasm for the job, he may become lop-eared and droopy.”
The overheated horse is listless, tired, and walks with a stiff gait (no bounce). “These are things a rider should pay attention to. Don’t keep pressing the horse or he may keep going until he collapses. If you see subtle signs of heat stress, stop and take a rectal temperature,” says Allen.
If the horse’s body has been trying to cool via sweating, breathing faster, etc., but these methods have not kept pace with heat accumulation he becomes “exhausted.” He may be unable to sweat anymore or sweats at a reduced rate. “Dehydration doubles or triples the risk for heat stroke,” says Dr. Barney Fleming, a veterinarian who has vetted endurance rides all over the U.S. The horse has run out of ways to cool himself, so his temperature shoots higher.
Skin becomes less elastic due to fluid loss from underlying tissues. A pinch of skin pulled out from the neck or point of the shoulder does not spring right back into place, but stays tented for 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 lost at least 4 gallons of fluid, which equates to about 32 pounds of body weight. If the pinch of skin stays elevated for 6 seconds or longer, he is severely dehydrated.
Mucous membranes such as gums become dry and discolored, turning brick red instead of bubble-gum pink. Heart rate increases as the body tries to pump more blood to the surface for cooling but has less fluid to do it.
The horse’s eyes seem sunken; eyelids and tissues around the eyes are wrinkled, due to loss of fluid in the tissues. Any sweat the horse produces will be thick and sticky instead of clear and watery.
Capillary refill time is longer than normal. If you press your finger into the gum, the blood does not rush right back afterward. The spot where you pressed stays pale for several seconds.
Pulse and respiration rate may remain high in spite of resting the horse. Pulse may be weak, heart rhythm may be irregular, intestinal sounds may be diminished or absent if the gut has stopped working. Anal tone is another clue. If the anal sphincter is loose and floppy, the horse is in trouble. It may be difficult to get accurate temperature on a hot, tired horse because the rectum is so relaxed. If he seems hot, but has a normal rectal temperature, be suspicious that it’s not reflecting his true temperature.
The horse looks depressed, with lower head carriage, and may not be interested in eating or drinking. He might not be sweating, but is hot to the touch. If you are riding him, watch for signs that he can’t keep going in normal fashion and begins stumbling. He may become anxious and start behaving in an irrational or erratic manner, due to overheating of his brain. He may be excited, depressed, or disoriented.
A hot horse may have rapid, shallow breathing (primary panting), but often the sign of a hot horse is blowing (taking deep gulps of air) when he stops exercising. “The horse blows (second phase panting) to try to get rid of heat,” says David Marlin, Ph.D. (Visiting Professor in Physiology, Oklahoma State University). “Second phase panting is slow and deep, with an effort. The horse is drawing in as much air as possible, trying to maximize respiratory heat loss. This is an easy way to recognize a hot horse,” he says.
A horse does not have to be exercising to get heat stroke. He may be standing outside with no shade, day after day, which is a stress if there’s high humidity and no breeze, and he’s not getting enough salt or electrolytes. Horses that don’t have free choice access to salt are more at risk.
High humidity always increases risk for heat stroke. When air is full of moisture, evaporation rate slows or ceases and the horse loses the cooling effect of sweat. “The hot sweat sits on the skin and becomes insulation, holding heat in rather than dissipating it,” says Fleming. The body doesn’t cool and signals for more sweating. Sweat covers the body and runs off in streams; the horse stays wet but doesn’t get cooler, and overheats. In an arid climate, by contrast, sweat evaporates almost as fast as it is produced, constantly cooling the body. If you put water on the horse, it evaporates quickly.
Be aware of weather conditions and weather predictions, especially 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’s risk for overheating.
If the total number is 150 or higher, this is the danger zone — any working horse will probably overheat. For instance, if temperature is 90 degrees the horse may get into trouble if humidity is 60 percent, or even at 80 degrees if the humidity is 70 percent or higher. When humidity is high, the horse may overheat at a lower temperature because he can’t get rid of the sweat and is unable to cool himself.
“One of the things we learned while doing studies for the Atlanta Olympics is that if it’s lower than 75 degrees it doesn’t matter what the humidity is,” says Allen. “When air temperature gets above 75 degrees, it does matter,” he says.
Some horses are more at risk. The larger the body, the more disadvantage in dissipating heat. “Body fat insulates and retards heat loss, but body size is also a factor. Even within the same breed, a small Thoroughbred or Quarter Horse has the advantage over a bigger one,” Marlin says. The smaller animal has more body surface per total body mass.
Large horses, fat horses, heavily muscled horses and heavily pregnant mares don’t dissipate heat as well as small, lean horses. “In any horse, heat must dissipate from the body core, all the way out — through the lungs or skin,” says Fleming. “The more bulk, the more problems.”
A fit horse usually does better than an unfit fat horse, but in some instances the fit horse might be more at risk because he has a higher capacity for exercise and may keep going longer. “Sometimes unfit horses save themselves by quitting early,” says Marlin.
The horse most at risk for heat stroke may have a high temperature, but could also be a horse with moderate temperature that has been exercised for a long time. “Or it could be a horse that’s been exercised continually or periodically (such as at a horse show, or working cattle) without enough water to drink,” he says.
“Heat Stress in Horses” is excerpted from the July 2013 issue of The Cattleman magazine.