You know just from standing close beside your horse that he, like every living animal, generates heat. But have you ever considered where that energy comes from? Just going about the daily business of staying alive, the cells of the body convert the sugar glucose into energy. But the cells do not use their fuel very efficiently--only a portion is turned into useful energy; the rest becomes waste heat.
A horse at rest generally maintains a body temperature between 99 and 101 degrees Fahrenheit (F). But when his muscles are called into action, be it for a cross-country run or a stroll to the water trough, the cells begin to metabolize glucose at higher rates, which in turn, increases the amount of excess heat produced.
"The harder a horse works, the hotter he gets," says David Marlin, PhD, of Hartpury College in Gloucester, England, an equine exercise physiologist who helped prepare standards of care for horses competing in the 2008 Olympic equestrian games in Hong Kong. "It's the intensity of the exercise that matters, not simply the running speed. The way we normally define how hard a horse is working is by heart rate." Running at high speeds or over long distances, of course, significantly increases a horse's heart rate, but so does slower work in soft footing, for example, or carrying heavier weights, or working too hard when the horses is generally unfit.
A horse may safely experience a temporary climb in body temperature to as high as 103 degrees F. But to prevent his body temperature from spiking to dangerous levels, any extra heat must be dissipated. That process begins with the blood, which heats up as it travels through the working muscles, then carries that extra heat out to the skin.
"Blood is equivalent to the cooling fluid in a car," says Marlin. "Cooler blood is flowing to the muscles, picking up heat, taking it to the skin and getting rid of it, then circulating through the muscles again. This is similar to cooling fluid coming from the car radiator, going to the engine, picking up heat and going back to the radiator to get rid of some of the heat before going round again."
If the outside air is not too humid and the temperature is cooler than the horse's body temperature, the excess heat will simply radiate into the outside air; a smaller amount will also be dissipated as the horse breathes, as he exhales heat and draws cooler air into his lungs. "About 85 percent of the heat loss will be through the skin surface all over the body, and about 15 percent through the respiratory tract," Marlin says.
To boost the rate of heat loss, a horse will first increase his respiratory rate--flared nostrils are a sign that he's trying to draw in as much air as possible to aid that process. When the horse is generating more heat than can be dissipated via breathing and radiation alone, his body goes to the next stage of cooling--he sweats. That is, fluids from the bloodstream will pass through the sweat glands to the surface of the skin, where evaporation increases the rate of cooling.
"Horses are very efficient at sweating," says Kent Allen, DVM, a sports-medicine veterinarian in Middleburg, Va., and a veterinary coordinator for the Atlanta Olympics in 1996 as well as the 2010 World Equestrian Games.
A horse can produce a prodigious amount of sweat--"a quarter of a liter per minute," says Marlin. "If you multiply that by 60 minutes of exercise, that equates to about 16 liters per hour--a huge amount." Horses also have ample reservoirs of fluids in their systems to allow for relatively high levels of fluid loss, but there is a cost: "The body uses energy to produce sweat," he adds. "It also loses electrolytes and water, which causes its own problems. [Horses] become dehydrated and their electrolyte balance becomes disturbed."
Most of the time horses recover from exertion quite readily--all they need is time to rest and access to ample water, salt and proper feed, and perhaps an electrolyte supplement, and they'll be back to normal within hours or maybe days, if the work was especially intensive. However, if the horse is overworked and/or overheated to an extent that seriously taxes his capacity to cool himself, then his temperature can spike to dangerous levels, and if the process is left unchecked, his body will begin a sequence of events that become progressively more dangerous.
What You Can Do to Help
Intervene immediately if you suspect that your horse is overheated. The extent of the measures you need to take depends on how hot the horse has gotten. If he's still alert, still sweating normally, and his rectal temperature reads 103 degrees F or lower, he is overheated but not in danger. "He just needs walking around, letting him drink, and some washing down with cool water," says Allen. "It's normal for a horse to heat this much while working."
But, when a horse starts edging toward overheating, more extreme cooling measures are necessary: