Heat stress in horses
Hot weather is a challenge for hard-working horses – on a trail ride, gathering cattle, or competing in performance classes. Heat is produced by working muscles, and is usually dissipated by sweat evaporation into cooler air. If air temperature/humidity is high, however, it’s more difficult to get rid of body heat and the horse becomes hotter.
Dr. David Marlin, a British equine exercise physiologist and visiting professor at Oklahoma State University, says that if air is cool and dry about 85 percent of the heat loss will be through the skin surface. About 15 percent is lost through the respiratory tract. A hot horse may pant – for more air exchange.”
A small, light muscled horse that doesn’t have much body fat does better in the heat than a bigger, fatter horse. “Body fat insulates and retards heat loss. The bigger the body, the more disadvantage. A small horse has more body surface per total body mass,” explains Marlin.
“Horses can get heat stroke after intense exercise for short periods, but the most serious heat stroke occurs after prolonged exercise,” says Marlin. A fit horse is usually at less risk than a soft, fat horse because he has more efficient muscles and produces less body heat while working, and sweats more efficiently without dehydrating so much. Access to salt is important to replace what’s lost through sweat.
Heat stroke can lead to unsteadiness, and then the horse may collapse – unable to get up. “He may go into convulsions due to swelling/damage in the brain, become comatose and die,” Marlin says.
Since dehydration (from continual sweating) is a big factor in risk for heat stroke, make sure the horse drinks plenty of water when working. He can’t produce sweat if he doesn’t keep drinking. Offer water between classes at a show. Let him drink at every stream or water source when out on the trail or gathering cattle. Drinking water will not cause colic or founder, if it’s not ice-cold. If the only water available is extremely cold, limit his intake and give him several opportunities to drink.
“The more he drinks, the more he replenishes his reserves for sweating, and the more urine he produces,” explains Dr. Barney Fleming (a South Dakota veterinarian who monitors horses on endurance rides). “The more he urinates, the more heat goes out from the body core; the urine brings a lot of heat with it,” he says.
Avoid strenuous work during the hottest part of the day. Ride early morning or late evening, if possible. Mornings are usually best; the air is not as warm and humid. Do strenuous work in short stints, alternating with walking or rest so the horse can stop sweating so heavily. He can work longer that way, with less risk for heat exhaustion.
Monitor vital signs and sweating, and know when to slow down. “Find out what your horse’s normal resting body temperature is,” says Fleming. “Some horses are 99 degrees while others are always 101. The horse that is 99 all the time has a better chance to go farther before he overheats into a danger zone. A horse that’s 101 all the time doesn’t have as far to go before he gets into trouble.”
Fleming uses electronic human thermometers. “You punch a button and stick it in the horse’s rectum until it beeps. Just remember that rectal temperature is about a degree lower than the horse’s core temperature. If his rectal temperature is 103 he’s borderline on safety because his core temperature is hotter than that. This is still a safe point to stop, however, and try to reverse the climbing temperature,” says Fleming.
Caring for an overheated horse
Using lukewarm water all over the horse works fine in a hot, dry climate. Water evaporates quickly into the dry air, cooling the body.
“If it’s hot and humid, use cold water,” says Marlin. “Some people don’t use enough water. They pour a little down the neck but not over the rest of the horse because they think it might damage the muscles. But the neck is only one-sixth of the total area you could be using to cool the horse. Don’t just focus on areas where veins run close to the surface. It used to be popular to hold bags of ice over the jugular vein. But you are cooling only a small amount of blood going past one part of the body,” he explains.
“It’s not as important to scrape off the water (as it heats up on the horse) as it is to keep putting more water on – getting more volume moving over the horse. As it flows off it takes heat with it,” he says.
Keep monitoring the horse’s temperature, to see if starts dropping. “It’s hard sometimes to get an accurate temperature reading from a hot, exhausted horse because the anal sphincter is loose. If he has a normal rectal temperature, be suspicious that it’s not reflecting the true temperature of the horse,” Marlin says.
If the horse is hot and seriously dehydrated, he needs IV fluids. “If he isn’t responding to cooling, get veterinary help. Heat stroke can progress rapidly into a life threatening situation. Horses that have had heat stroke are also at risk for laminitis, severe muscle damage or organ damage and these can kill them even if the heat stroke doesn’t.”
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