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.
The tipping point varies, but for any horse, a buildup of body heat leads to fatigue and can have serious consequences. The combination of prolonged hard work and hot, humid weather can be particularly deadly.
In The Danger Zone
If your horse isn't able to regulate his body temperature, he'll overheat and may even suffer heat stroke. (In heat stroke, your horse's sweating mechanism fails, and his temperature rises quickly to 106-110 F.) If he sweats excessively he will be at risk for dehydration, which disrupts key functions like circulation and digestion and can damage organs. In extreme cases, dehydration can be fatal.
Here are five danger signs:
- He's breathing hard--and he keeps breathing hard. Rapid, shallow breathing is common after exercise in warm weather, but breathing should slow fairly quickly to your horse's normal resting rate (below 20 breaths per minute for most horses). If the rate stays high, he may be panting to cool down.
- His temperature stays high. A horse's temperature can reach 106 F during intense work but should fall quickly after work ends. Be concerned if it doesn't drop a degree or two within 20 to 30 minutes.
- He seems depressed and exhausted. He may turn down food; horses generally don't eat when they're dehydrated, but they'll usually go for water. However, a horse who's severely dehydrated may be so depressed that he refuses to drink.
- His skin lacks resiliency. Test by pinching a fold of skin in three places--at midneck, high on his shoulder and low on his shoulder. If he's well hydrated, the skin will snap back promptly; if fluids are low, it will stay folded up for a few seconds and then slowly flatten.
- His gut is quiet. Listen by placing your ear or a stethoscope on his flank, checking both sides. You want to hear lots of healthy bubbles and gurgles; little or no noise means trouble. The horse's intestines typically hold large amounts of water, and he draws on that water as a reserve. When he begins to dehydrate, gut motility slows. This can lead to digestive problems, impaction or ileus (in which material stops moving through the intestine).
Act fast to help your horse recover. Get him into the shade and
- hose or sponge him with cold water to help him cool down--but remember that water must evaporate to have a cooling effect. In hot, dry weather, the water you apply may evaporate quickly. But in humid air, it will sit on your horse's skin, quickly heating up and forming an insulating layer that actually slows cooling. Scrape it off, hose again and keep repeating until the water no longer heats up. (In some cases, a bath with alcohol may help dissipate heat more efficiently than water.)
- set up a fan, if there's no breeze--blowing air will help dissipate the heat. Even better, use a misting fan. These fans (which you see at some three-day events and other competitions) use water vapor to lower the temperature of the air.
- let him drink to replace fluids lost in sweat. You won't hurt him by giving him some water after exercise. It's practically always safe to let your horse drink a small amount--up to a gallon--walk him for a few minutes and offer water again. If you let him drink intermittently, a gallon at a time, you may stay ahead of dehydration.
- give him a dose of electrolytes (see "When Does He Need Electrolytes?"). This will help him restore his losses from sweat and trigger thirst.
Should you call your veterinarian? That depends on the severity of the signs and your horse's general attitude. If his vital signs are improving and he's alert, drinking water and interested in green grass, you can probably handle the situation. If he seems dull or disoriented, turns down food or especially water, or hasn't cooled down despite your efforts, consult your veterinarian. He can assess your horse's condition, administer fluids by stomach tube or intravenously, and provide other supportive treatment as necessary.
While excessive sweating can leave a horse dehydrated, a horse who doesn't sweat may be in greater danger because he has no effective way to unload the heat that builds up in his body. Without help from you, his body temperature may stay high. And it can reach dangerous levels--106-110 F after exercise, putting him at risk of heat stroke.
The veterinary term for this condition is anhidrosis. It can affect any horse, although it seems to be more common in hot, humid climates. It can come on gradually or appear all at once. It's not well understood--environmental and exercise stress, along with metabolic or even hereditary factors, may be involved. The important thing is to recognize it and deal with it. Here are signs to watch for:
- A dry coat after work in warm weather. The severity of anhidrosis varies, so your horse may be completely dry or a little damp under his tack or between his hind legs. Except in those patches, his skin is dry and hot.
- Labored breathing during and after exercise. His nostrils flare and his sides heave as he takes rapid, shallow breaths in an effort to lose body heat. He breathes harder than you would expect for the amount of work he does, and he continues to do so long after exercise stops. He may even breathe hard when at rest on a hot day as he tries to compensate for his failure to sweat.
- Poor performance. He may seem lethargic and reluctant to work, and he becomes quickly exhausted.
- Poor coat. Thinning and loss of hair on the face or body sometimes accompanies anhidrosis. This can be an early sign of the condition.
The signs may be enough to diagnose anhidrosis. There's also a "sweat test," in which a veterinarian injects a small amount of the drug terbutaline under the skin of the horse's neck. This produces local sweating in a normal horse, but not in a nonsweater.
If your horse has anhidrosis, you must find ways to limit heat buildup in his body and dissipate the heat he can't lose through sweating.
- Exercise him early in the morning or in the evening, when it's not so hot. Take frequent breaks, allowing his breathing to recover before you ask for more effort.
- Cool him out aggressively after work--with cold water and fans, as described on page 46. Monitor his vital signs, and don't stop your efforts until they're normal.
- When he's turned out, be sure that he has shade. Or turn him out at night and keep him in the barn--with ample ventilation and a fan--during the hottest part of the day.
- A supplement, One AC (Miracle Powder Company, www.nonsweater.com), has proved helpful for some anhidrotic horses. It's a combination of vitamins, amino acids and minerals. The manufacturer says that horses generally start sweating again within 10-14 days of starting the supplement.
Horses who have stopped sweating in hot and humid climates often recover after spending time in cooler climates. One thought is that constant heat and humidity overstimulate the sweating mechanism to the point that it shuts down. Cool, dry conditions give the mechanism a chance to revive.
When Does He Need Electrolytes?
Electrolytes are minerals that are dissolved in the horse's body fluids, where they help protect him from dehydration and regulate muscle function and other processes. These minerals (the major ones are sodium, chloride, potassium, calcium and magnesium) are lost in large quantities and redistributed within body tissues when the horse sweats. That means that your horse may run low on electrolytes just when he needs them most.
Should an electrolyte supplement be part of his daily regimen? Probably not. Horses can't store these minerals and promptly eliminate what they don't immediately use. "Most people likely oversupplement with electrolytes," says Duncan Peters, DVM. "If the horse is getting good forage and a commercially formulated feed that contains balanced vitamins and minerals, he is probably getting enough electrolytes to replenish what he loses even if he works an hour or an hour and a half in warm weather. The horses who need supplements are those who compete and train at high levels--eventing at Preliminary and above, combined driving, polo, endurance--and in very hot or humid conditions."
There are times when supplementing with electrolytes makes sense based on sweat loss:
- If your horse has worked hard enough to sweat significantly (his neck and chest and sides are wet, not just a patch under the saddle), a single dose will help make up lost electrolytes. "If your horse sweats and you only replace water, you dilute your horse's electrolyte pool," Dr. Peters says.
- If you haul the day before a big show, a dose of electrolytes that night will make up any deficit he may have from the stress of transport. It may also encourage him to drink after the trip, which will help him rehydrate before competition. But you can't "preload" your horse with electrolytes the night before an event, because he'll eliminate the excess.
- If a horse isn't drinking well, supplemental electrolytes can help by triggering thirst. However, Dr. Peters says, horses who willingly tuck into good-quality forage and have free access to clean water and a salt block generally drink enough without extra electrolytes.
- Many endurance competitors give electrolytes half an hour before the start of a race and at intervals during competition, depending on environmental conditions. The goal is to prevent dehydration by maintaining fluid balance and by prompting thirst. "But the horse must still get fluid, so he must have opportunities to drink," Dr. Peters notes.
Give electrolytes orally, as a paste, dissolved in water (in a separate bucket from the horse's drinking water) or mixed in feed. You can use a quality commercial supplement (it should be at least 60 percent electrolytes and no more than 40 percent sugar), following the dosing recommendations on the package. Or make your own by mixing three parts table salt (sodium chloride) and one part "lite" salt (potassium chloride). One rule of thumb is to give 2 ounces of the mixture per hour of hard work, depending on heat and humidity levels. Always provide clean water alongside. Forage or feed is important, too, says Dr. Peters; a slurry of straight electrolytes can irritate the horse's mouth or an empty stomach.
This article originally appeared in the July 2009 issue of Practical Horseman magazine.