Heather Smith Thomas
June 3, 1988
Published in “The Chronicle of the Horse”
An athlete sweats when he exerts. Working muscles produce heat as one of the by-products of energy production. Sweating is one way to “keep the engine cool” and make sure the horse’s temperature stays within healthy limits. In order to restore fluid lost through sweating, the horse must increase his water intake to combat hydration – loss of fluids and minerals.
Symptoms of dehydration appear when the loss becomes great enough to interfere with normal body function. Water is one of the most important ingredients of the body. A horse is actually 60 percent water. Water is crucial to digestion, and makes up most of the bloodstream and other body fluids – his brain is 85% water, muscles 75%. Even the bones are 30% water.
About 25% of the horse’s body weight makes up the flexible fluid in his body – fluid that can be shuttled around to compensate for losses. A horse can lose about five percent of his body weight in fluids (50 pounds or six gallons) before he shows much sign of dehydration. Some horses will lose up to 10 % on a hard endurance ride, which is enough fluid loss to severely reduce muscle function and cause other problems as well.
The body tries to keep a constant water level and a stable temperature. Normal temperature ranges from 90 degrees to 100.5 degrees and rises with exertion. After hard work, it’s perfectly normal for a horse to have a temperature of 102 degrees. But if it climbs much higher, the body will start to show stress. The horse usually sweats in an attempt to cool himself. When faced with the choice of overheating or dehydrating, the body chooses to dehydrate, using up water to keep the engine cool. A temperature that stays over 103 degrees or keeps on climbing puts the horse in serious trouble.
Evaporation from sweat cools the body. As the liquid evaporates, it draws dome surface heat with it, cooling the horse. But there is a limit to the horse’s water reserves, and as he begins to run out of body fluid, his temperature climbs. A dehydrated horse without sufficient fluid to stay cool, may suffer 3 degrees increase in temperature per hour of moderate exercise.
When the body has plenty of water, excess fluids and salts are excreted as urine. But when the body is short on water, it conserves fluid. The horse urinates less often and the urine is scant, dark and concentrated.
Sweating can use up as much as four gallons per hour when the horse works hard in hot weather. If air is dry, sweat evaporates quickly and cools the horse. But if the air is humid, sweat won’t evaporate; it covers the horse and runs off in streams. He stays hot and clammy and sweats even more in an attempt to cool himself.
A horse that is not fit will sweat more profusely than a well-conditioned horse and will also lose more electrolytes (mineral salts) and proteins through his sweating. The soft horse has sticky, smelly sweat that lathers easily because it contains more waste products.
Electrolytes are the same elements found in seawater – sodium, chlorine, potassium, calcium, phosphorus and magnesium. These minerals made life possible for the first sea animals.
These important minerals are all well supplied in the horse’s diet except for salt, which must be supplemented. However, prolonged sweating can deplete his reserves. The horse that dies of exhaustion actually ran out of fluids and electrolytes.
The horse has several sources of water reserves. About 18 to 20 gallons is stored in the gut, half of which can be rapidly absorbed by the bloodstream, if necessary. (Blood loses water first during sweating.) When the bloodstream becomes short of fluid, it is replaced by water from between the body cells, which in turn is replaced by water from within the cells. Cell fluids are eventually replaced by water from the digestive tract. But if the horse hasn’t had enough to water to drink, cell water is not replaced, and the functions begin to suffer.
As the horse’s temperature climbs, overheated blood goes to the small capillaries at the skin surface, where the sweat glands open up and release fluid from the blood. Water can leave the blood swiftly as sweat, but is replaced more slowly. Heavy sweating uses water faster then drinking replaces it.
You usually can’t tell a horse is dehydrated until he has lost at least four gallons of fluid (or about 32 pounds of body weight.) By then his performance is affected. He needs to slow down. The dehydrated horse must have his water replaced (he needs to drink), needs to stop exercising and stay in a cool place or have his body sponged with cool water until he recovers his internal water balance and his temperature returns to normal.
SIGNS TO LOOK FOR
There are several ways to tell if a horse is dehydrated. If you pull out a pinch of skin on the horse’s neck or shoulder, and it springs right back into place, he’s not very dehydrated. If he is moderately dehydrated, the skin will stay elevated a few seconds after you pull it out.
Some people estimate that fluid loss is about one percent of body weight for each second it takes the skin to return to normal position. Since about five percent dehydration occurs before any change is noticeable, if the skin takes on e second to spring back, the horse has a fluid loss of about six percent of his body weight. The more dehydrated, the longer the skin will stay elevated.
Another sign of dehydration is dry, red mucous membranes inside the nose and mouth, and dry gums. Color of the membrane under the horse’s eyelid is normally pinkish to pink-yellow, but becomes more red with stress. Brick red is serious. If stressed even more, the horse’s membrane color turns blue from lack of oxygen.
Capillary refill time is also a good clue. If you press your finger into the horse’s gum just above his front teeth, that spot will turn white since you have pressed out the blood. In a normal horse, as soon as you remove your finger the blood will come back. Normal color will return within one to four seconds. But in a stressed and dehydrated horse, the spot will stay pale and bloodless longer. The more dehydrated the horse, the slower the capillary refill time.
The eyes of a dehydrated horse will seem shrunken and dull, with eyelids wrinkled. The cornea is normally moist and clear, but in the dehydrated horse it may look glazed because it is drying out.
As the horse dehydrates, his sweat becomes thicker and more lathered, like that of a soft horse. Temperature rises, heart rates increases and these rates don’t return to normal quickly when the horse stops exerting. They stay elevated or rise even more. A big respiratory rate is one way the horse cools off, through the lungs, but if it does not drop soon it may mean the horse is dehydrated.
Other signs of dehydration are shallow panting, muscle tremors and weakness, weak pulse, depressed attitude. The horse’s brain is sensitive to fluid and electrolyte imbalances, and when dehydrated he may experience mental changes. Dehydration can also trigger muscle cramps (typing up or colic) since fluid and electrolytes necessary for proper muscle function are depleted.
The stressed and dehydrated horse may not feel like eating. If he has some appetite, it’s a good sign. He’ll be more interested in green grass than in hay or grain. Grass may be 50 to 90 percent water, and it is the best possible feed for a dehydrated horse. Dry hay is only five to eight percent water and can be pretty hard to eat. Grain is more apt to mess up the digestive system of a stressed horse than hay or grass.
Research with endurance horses has shown they do best with all the good quality roughage they can eat, and not much grain. The roughage in the digestive tract helps it act as a big vat to hold moisture, and the horse doesn’t dehydrate as quickly on a long ride.
To combat dehydration, condition slowly and gradually, and carefully prepare for the work the horse will be asked to do. Electrolyte replacement is sometimes beneficial during or after a strenuous long ride, but is of little value if given before the ride (and can actually be harmful). The horse can’t store extra minerals; he must flush them out of his system and uses up extra water in doing it.
But when he is stressed and dehydrated, replacement of these crucial minerals can be helpful. Remember that you don’t have to replace everything he loses during a hard ride and that salt and electrolytes should never be given without adequate fluid at the same time.
Severe and dangerous dehydration can best be reversed by giving electrolyte fluid intravenously. Fluid by stomach tube or into the rectum (where it is readily absorbed) can be helpful, but a severely dehydrated horse will need lots of intravenous fluid, and veterinary attention. Moderate dehydration can be reversed by giving the horse all the water he will drink, and allowing him some free-choice salt or electrolyte powder with his feed or dissolved in his water.
Give the horse a chance to fill up on water before a long ride, and let him drink at every opportunity. Some horsemen are afraid of watering a hot horse, but most horses are better off if allowed to drink whenever they wish. A horse won’t have trouble handling water, if he is going to continue exercising after drinking. The danger in watering a hot horse is when he is allowed to tank up on very cold water and then must stand idle. In this situation he may colic or get muscle cramps as blood leaves his tired muscles to rush to the stomach to warm the cold water.
If the day is hot and humid, periodically sponge the horse with water to help cool him and conserve his own body fluids. He won’t have to sweat so much. If he’s sweating and over-heated, keep him wet. If the day is hot and his temperature is over 103 degrees, use cold water on head, neck, and insides of his legs, to bring down his temperature. But under most conditions, cool or lukewarm water is better. Don’t wet his whole body. Applying cold water all over him (especially over heavy muscles of hindquarters, shoulders and back) can cause muscle cramps. Cold water also chills the skin and may stop his sweating reflex, interfering with the body’s own cooling system.
If the air is very dry or windy, causing water to evaporate quickly, the horse will cool out rapidly on his own with just a normal rubdown; don’t use cold water. On cool or windy days you may even need to walk or blanket him after strenuous exercise so he won’t cool too fast and chill.
Anyone who competes in strenuous athletic events or uses a horse hard should be familiar with his horse’s abilities, recovery rates, conditioning and body needs, and aware of the signs and effects of dehydration – and how to prevent excessive fluid loss. Some degree of dehydration is inevitable with strenuous performance, learning to cope with it will keep your horse healthy.