It's a steamy summer afternoon and you set out on a trail ride with a friend. After half an hour her horse is drenched with sweat, but your horse is dry. Which of you should be concerned? Surprising as it seems, you may have more to worry about than she does. Sweat is a good thing--it helps your horse get rid of excess body heat.
Sweating is a key component in a complex system that allows your horse to regulate his body temperature. If that system goes haywire, the results can be serious. How can you make sure your horse's sweating mechanism is working smoothly, and how can you help him if it's not? For this article, we asked FEI-licensed equine veterinarian Duncan Peters, DVM, MS, for help in answering those questions. A lifelong horseman, Dr. Peters heads the Hagyard Sport Horse program at Hagyard Equine Medical Institute in Lexington, Ky.
Why He Sweats
Your horse sweats to cool himself in hot weather and during exercise or periods of stress, such as a trailer ride. It's the evaporation of sweat, not its presence on the skin, that actually cools the horse. Here's how the mechanism works:
During exercise, muscles generate heat; heat is a byproduct of energy metabolism.
- Circulating blood absorbs heat from the muscles and carries it off to the lungs, where some of the heat dissipates when the horse exhales, and to the skin, where heat can radiate out from the horse's body.
- If the horse produces more heat than he can unload through breathing and radiant cooling, his core temperature begins to rise from its normal resting temperature (99-100 F).
- A part of the horse's brain called the hypothalamus (which, along with many other jobs, acts as his central thermostat) senses the increase. It sends signals rushing out to sweat glands distributed in his skin.
- The sweat glands begin to pump out sweat. It's mostly water, but it also contains dissolved minerals called electrolytes. A horse's sweat has a higher concentration of electrolytes than yours.
- As the sweat evaporates, it carries heat away from the skin, reducing the horse's body temperature.
The harder the horse works (or the hotter the weather), the more he sweats. He can produce more than twice as much sweat as you can per square inch of skin. During intense exercise (cross-country, polo, endurance racing) he can lose 10 to15 liters of fluid in an hour through sweat and through water vapor that he exhales with each breath. The loss depends on climate conditions as well as exercise levels, and it can happen even if you don't see sweat pouring off your horse. On a hot, dry day, sweat may evaporate almost as quickly as it forms; he may lose a large amount of fluid without your being aware of it.
He may be slow to replace that fluid, too, because the nature of his sweat delays his thirst response. When you sweat, you lose mainly water; the water loss leaves you with an electrolyte imbalance that triggers thirst. Because your horse's sweat has a higher concentration of electrolytes than yours does, he's slower to develop an electrolyte imbalance and slower to feel thirsty.
Losing His Cool
Even coupled with the cooling effects of breathing and radiant heat loss from skin, sweating may not be enough to keep up with increases in the horse's body temperature. This may not matter very much if the stress is brief--if he exercises only a short time in hot weather, for example--and he has a chance to cool off afterward. But if the stress is prolonged (he competes in cross-country or ships long distances), heat can begin to build up in his body. Hot, humid weather adds to the problem. When the air is saturated with moisture, the sweat doesn't evaporate quickly enough to dissipate the horse's body heat. He keeps sweating, but it doesn't help.
Weather isn't the only variable in the equation. Physical condition is another; horses become better at regulating body temperature during exercise as they become more fit. They use energy more efficiently (producing less heat), sweat more readily, and their sweat becomes less concentrated so they lose fewer electrolytes. Acclimation to the heat makes a difference, too. Research carried out before the 1996 Atlanta Olympic Games showed that horses were better able to regulate temperatures when exercising in hot weather if they had spent 10 to 14 days getting used to the heat.
Besides getting your horse fit for the work you ask him to do and acclimated to the climate you ask him to work in, these steps can help prevent overheating.
- Limit exercise in hot and humid weather. Pay attention to the Heat Index, a measure that combines air temperature and relative humidity to estimate how hot it actually feels. (The National Weather Service has a key for the index at www.weather.gov/om/heat/index.shtml.)
- Let him drink. At a show, give your horse water free-choice, or offer it at least every hour--don't wait until you get back to the trailer. Plan trail rides to take in streams or other watering spots. A 3-percent hydration loss is enough to affect your horse's performance.
- Observe his general attitude--body posture, desire to eat, freedom of movement, relaxation of muscles--to help determine if he may be experiencing problems with the heat. Do the "skin-tenting" test described in the next section to check for dehydration.
