by Heather Smith Thomas

The horse is better adapted for heat retention (keeping warm in cold weather) than he is for dissipating heat in hot weather, but he has several ways to cool himself when overheated. These include radiation of heat from the skin surface (which works best when the air temperature is cooler than his body temperature), air exchange in the lungs (bringing in cooler air and exchanging it for warm air, dissipating some of the body heat from the blood that is constantly coming into the lungs), and sweating. Sweat glands open up and release fluid from the blood stream-from tiny capillaries near the skin surface-to help cool the horse by evaporation.

During hot weather, ability to keep cool is vitally important. Horses or humans can only function within a very narrow temperature range; the body must maintain a fairly stable temperature. If body temperature climbs too high, the body's "engine" can literally burn itself out. Its metabolic rate increases about seven percent for each degree of temperature rise. The heart muscle loses its ability to contract properly, beats more rapidly in an effort to move more blood to the skin for cooling, then beats weakly and erratically. If the high temperature persists, the body wears itself out from the increased metabolism and attempt to cool itself.

In the horse, whose normal range of temperature is 99 to 100.5 degrees, a temperature that stays above 103 becomes serious. Above 105 is dangerous, and a temperature that stays beyond 107 becomes life threatening. Without the body's cooling mechanism (sweat), even moderate exercise would raise the horse's temperature about three degrees per hour, and this exertion would soon kill him.

During hot weather or exertion, the skin acts to release and dissipate heat from the body. The hair lies down flatter and loses much of its insulating quality, allowing more heat to escape. The small capillaries bring overheated blood to the surface for cooling. The sweat glands open up and allow some of the fluid from the bloodstream to escape to the body surface for evaporation.

Sweat is the main source of water loss in a horse's body during hot weather or exertion, and he can lose up to four gallons per hour from his 70 gallon supply. Some animals (dogs, sheep, cattle) have few sweat glands and must rely on air exchange through the lungs for cooling. They pant open-mouthed when hot, for faster air exchange. The horse has more sweat glands than humans or other animals and only rarely pants to cool himself (when extremely overheated or when the air is too humid for sweat evaporation).

Sweat glands are activated by heat and exercise, and sometimes by fear, anxiety or illness. They secrete water (filtered from the small capillaries near the skin surface) mixed with salt and other electrolytes, along with small amounts of protein, urea and ammonia (body waste products). Most sweat glands open into the hair follicles just below the outer skin layer. If air is dry, sweat evaporates as soon as it is produced, and the cooling effect on the skin brings body temperature down to normal levels. In humid air, however, evaporation is reduced, sweat covers the body and may run off in streams. Without evaporation, the body does not cool so it signals for more sweating. Sweat production increases but does little good; the horse stays wet but does not become cooler. If he continues to exert in humid conditions, he quickly dehydrates.

Lather, as shown here on the neck is a sticky, salty, sweat

Horses lose significant amounts of fluids and electrolytes in sweat, and if they are not monitored carefully, these losses can result in serious complications.

A horse unaccustomed to hot weather or heavy exertion will sweat profusely, losing not only water but also many valuable electrolytes and proteins. If exercising, he "lathers," producing a sticky, salty sweat. By contrast the fit or acclimated horse has a conditioned sweating reflex and loses less water and minerals. He starts to sweat more gradually, and his sweat is clearer, containing fewer proteins and salts. Horses that sweat a lot become fatigued more quickly than fit horses and must have those important minerals replaced to avoid exhaustion and collapse. Horses that exert in hot weather should always have access to salt; a horse exercising moderately may lose 50 to 60 grams of salt in sweat daily, and a horse exerting strenuously will lose even more.

Body fluid and electrolyte loss in horses doing strenuous exercise may lead to excessive fatigue, muscle spasms and cramps, dehydration or thumps (spasm of the diaphragm muscle, triggered by decrease in calcium and potassium). Under ordinary conditions the horse gets an adequate amount of these minerals in ordinary feeds, but during periods of stress and exertion when the horse sweats heavily, these minerals are lost more quickly than they can be replaced by feed. The horse may temporarily lose more electrolytes than its big system can afford, and must receive a supplement in order to prevent the effects of serious electrolyte shortage.

Extensive electrolyte loss can be fatal. When a horse dies of exhaustion he actually dies from effects of severe dehydration and electrolyte depletion. The body cannot continue to function when it gets too short of water and the salts that aid proper movement of water in and out of the body cells.

The dehydrated horse develops several obvious symptoms. His temperature rises because he is no longer able to sweat efficiently to cool himself. His skin becomes less elastic because of fluid loss from the tissues. A pinch of skin pulled out from his neck or shoulder does not spring right back into place but stays pinched up for several seconds. The mucous membranes in his mouth become dry and discolored, turning angry red instead of shiny and pink. Heart rate increases as the body tries to get more blood to the surface for cooling, but has less blood fluid to do it with. The horses eyes seem to sink into the head, eyelids and tissues around a eye wrinkle due to loss of fluid. Sweating decreases and what little sweat he does produce is thick, sticky and short of fluid.

ANHIDROSIS Some horses suffer from a sweating disorder called anhidrosis which literally means without sweat. Horses subject to tenuous activity in hot climates sometimes develop this problem. Usually the sweating reflex automatically goes into action during hot weather or exertion, but in hot, humid weather there may not be much difference between the air temperature and the horse's body temperature. Sweat doesn't evaporate properly and the horse's cooling system is therefore unable to bring his body temperature down. After sweating profusely for a time, the horse may quit sweating.

This condition was first discovered in British Thoroughbreds taken to tropical countries for racing, polo and cavalry. It is a problem for horses in tropical countries and in some areas of the U. S (particularly Gulf Coast states). Anhidrosis is not restricted to these areas, however; cases have been reported as far north as Minnesota and Michigan, and in arid climates like Arizona and California. It has been estimated that 20 to 30 percent of horses in hot, humid regions suffer from some degree of anhidrosis. Even horses that grow up in these hot areas can lose ability to sweat, though the problem is most common in horses brought from other regions. Anhidrosis usually develops during the hottest part of the year, and may come on quickly or develop gradually over several weeks; the problem then continues until cooler weather of Fall.

A mild case may go unnoticed at first. The horse may sweat some, but not enough to adequately cool himself. A rider or trainer might notice that the horse's performance suffers as the weather gets hotter and more humid. The horse's sweating may be patchy, with moisture only on a few parts of his body.

The shut-down of sweat glands is due to various stresses, particularly heat and humidity. The sweat glands quit responding to stimuli for awhile, then when the weather cools off, they start working again. If a horse has to sweat continually to cool himself, as during a hot humid summer or when confined to a hot, stuffy stall, his sweat glands work overtime and eventually shut down. The horse becomes dry-skinned, pants with mouth open, and has an elevated temperature (103 degrees or higher). He may have a few patches of sweat behind his ears, under his mane, at the elbows and flanks, but no moisture over his body. Other signs of anhidrosis are dry flaky skin and hair falling out, especially around the eyes. The dry coat and hair loss can result when oils from the sebaceous glands are no longer taken to the skin surface by sweat. The dry skin may become itchy.

Typical depressed stance of an exhausted horse.

Prolonged inability to sweat can prove fatal; the horse is at risk for heat stroke and other problems. If discovered early, however this condition can be reversed by cooling the horse and giving his body's cooling system a break. The horse should be bathed with cool water from a garden hose or sprayed with a fine mist of cool water to bring his temperature back down to normal. Then his temperature should be kept low enough he doesn't need to sweat. He must stay out of the sun, but not in a hot, humid stall. Use of a portable fan may be adequate to stabilize his body temperature until his sweating reflex can recover. If anhidrosis is treated as soon as it occurs and the horse has not suffered this problem before, his sweating ability may recover in a few days. But if the problem is longstanding, he may need help for several weeks to maintain proper body temperature while he recovers. Misting fans can be helpful, for they can lower stall temperature by as much as 15 degrees. The horse will not be able to exert in hot weather until he recovers. Some horses do not regain their sweating ability and must be moved to a cooler climate.

A survey of four central Florida Thoroughbred breeding farms, conducted by the University of Florida a few years ago to determine the extent of anhidrosis in that region, found that 25 percent of the 91 horses in training suffered from this condition to some degree. Of the 74 non-pregnant mares, 15 percent were anhidrotic, but only four percent of the 217 pregnant mares. The young animals (401 foals or youngsters that had not yet begun training) were less affected, with only two percent suffering from anhydrosis. The young horses were probably least at risk because of their smaller body mass (more efficiency in getting rid of body heat) and not being in training where they had to exert in the heat. Horses doing their training in the early mornings before sunup tend to suffer less than horses having to exert when the day is hotter.