Written by: Teresa Pitman

The summer heat can be deadly to your horse

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A hot, humid day. One rider, one horse. Both are exercising at a moderate level, but who is more likely to overheat?

It might surprise you to know that your horse gets hotter much faster than you and is more susceptible to the negative effects of heat stress. Prof. Michael Lindinger, an animal and exercise physiologist at the University of Guelph, explains, “It only takes seventeen minutes of moderate-intensity exercise in hot, humid weather to raise a horse’s temperature to dangerous levels. That’s three to ten times faster than in humans. Horses feel the heat much worse than we do.”

And the effects can be serious. If a horse’s body temperature shoots up from the normal 37-38ºC to 41ºC, temperatures within working muscles may be as high as 43ºC, a temperature at which proteins in muscle begin to denature (literally cook). Horses suffering excessive heat stress may experience hypotension, colic and renal failure.

Lindinger, a faculty member in the Department of Human Health and Nutritional Sciences, became involved in studying the effects of heat on horses when he was a lead researcher on the Canadian team that contributed information on the response of the horse to heat and humidity for the Atlanta Summer Olympics in 1996. Horses are more susceptible to heat for several reasons, explains Lindinger. First, they are larger and have a higher percentage of active muscle than humans do during exercise. When muscles are being used, they produce a lot of heat. Horses rely to a significant extent on sweating to cool off. They can sweat 15 to 20 litres per hour in cool, dry conditions and up to 30 litres per hour in hot, humid conditions, but only 25 to 30 per cent of the sweat produced is effective in cooling the horse by evaporation. “Because so much more sweat is produced than can be evaporated, the rest just drips off the horse’s body,” says Lindinger. “By comparison, up to 50 per cent of the sweat people produce is evaporated from our bodies during exercise and helps to cool us.”

Sweating results in the loss of not only water, but also electrolytes (sodium, chloride, potassium, calcium and magnesium) and may create a significant deficit in the body if the sweating is excessive. These salts in horse sweat are also four times as concentrated as in human sweat. Lindinger refers to a scene he witnessed where endurance horses had been standing while their sweaty bodies were repeatedly scraped and cooled with water. As the liquids evaporated from the ground, the soil surface was left white because of the electrolytes in the horses’ sweat.

“Those salts have to be replaced,” he adds, as horses depend on electrolytes to maintain vital body fluid balance, transmit nerve impulses, and for healthy muscle and circulatory system function. “Just giving the horse water will not rehydrate a dehydrated horse. When horses drink plain water, it dilutes their body fluids, and their bodies respond by trying to get rid of more water and more electrolytes.” Horses also pant to dissipate heat, but Lindinger says this is only effective if the air is at least five degrees cooler than the horse’s body temperature.

Staying Cool

There are a number of ways to protect horses from the harmful effects of summer heat. Begin by teaching your horse to drink an electrolyte solution to replace sweat losses. “Start with a small amount in the water, allowing the horse to get used to the taste, and gradually increase it over days and weeks until you have reached the manufacturer’s recommendation.” Keeping your horse properly hydrated is the most important step in protecting it against the harmful effects of heat, he says. A salt lick should also be readily available in the stable and paddock.

If you’re preparing for a competition, Lindinger recommends trying to acclimatize your horse to the heat by spending four hours daily, at least five days a week for three weeks, in hot conditions. For best results, exercise the horse for an hour during the second hour of each of those days.

“Many riders will train their horses in the mornings or evenings when it’s cool, then go to a competition held during the hottest part of the day. You need to get horses used to being ridden in the heat and allow them to develop the full spectrum of beneficial adaptations that come with heat acclimation.” Lindinger says that horses who have been through a process of heat acclimation will lose more heat through sweating and respiration and will be better able to stay hydrated because they are more likely to drink.

When your horse is hot, look for shade and breezes to help cool it down, but never use a blanket or “cooler” on a horse that is sweating, he adds, suggesting the best way to cool a horse quickly is to rinse the horse’s body repeatedly with cold water and scrape off the excess water. “You can cool the horse two degrees in ten minutes this way: pour on the water, scrape it off, pour on more, and just keep repeating it,” says Lindinger. “The scraping part is important, because otherwise the water will be trapped in the horse’s hair and will quickly warm up. By scraping and pouring on fresh, cold water, you keep the cooling process going.”

Signs of Heat Stroke

If, despite your best efforts, your horse displays any of the following symptoms, he may be suffering from heat stroke:
• panting (elevated respiration of up to 80 breaths per minute; normal range is 8 to 16).
• elevated pulse rate (28-40 bpm is normal), especially if it does not drop after several minutes of rest, or increases after exercise ceases.
• profuse sweating or no sweating (anhidrosis).
• body temperature over 39ºC / 103ºF (37-38ºC / 99-101ºF is normal)
• ‘thumps,’ a spasmodic jerking
of the diaphragm and/or flanks resembling hiccups
• sluggishness or depression
• dehydration, which can be determined via a skin pinch along your horse’s neck. If the skin takes longer than a second to snap back, the horse is dehydrated.

Call a vet immediately and get your horse into the shade and/or breezy area (a fan will work), hose or douse him with buckets of cool water (use described method), apply ice packs to the large blood vessels along the inside of the legs and belly, or even stand him in a pond or stream if one is nearby. In pre-Atlanta Olympics heat and humidity studies, the old horseman’s warning to never put cold water on the large muscles of the rump and croup was dispelled; it turns out that you can very safely and effectively cool a horse this way. Offer cool water in small sips. Delaying treatment in severe cases can lead to brain damage and/or death.

Your horse doesn’t have to be working hard to be affected by heat stress; he may just be standing in a poorly-ventilated trailer or stall on a hot, humid day. One US trailering study showed that when temperature readings outside were 80ºF (26.6ºC), it was 107º (41.6ºC) inside the horse trailer. Add to this the humidity created by a couple of horses and the situation can quickly become critical.