Heat builds up rapidly in the body of an exercising horse and must be quickly dissipated if thermal injury is to be prevented. For the normal horse, this is not all that much of a problem. Like man, the horse cools its body by sweating, and this seems to take care of thermal buildup. Thermoregulation, of course, involves more than just sweating. There must also, for example, be ample blood flow to carry heat from the core of the animal's body to blood vessels in the skin.

For some horses, unfortunately, dissipating heat under certain conditions is a real problem. These horses can't sweat.

The condition is known scientifically as anhidrosis. Horsemen facing the problem in their stables have applied terms like "non-sweaters," "puffers," and "dry" horses to describe the condition.

Although the condition can afflict horses anywhere, it is most commonly found in the Gulf Coast states. It has been estimated that 20-30% of the horses in hot, humid areas suffer from anhidrosis to some degree.

Normally, the problem will surface in late June and continue through the summer until temperatures cool in the fall.

A typical anhidrotic horse will pant vigorously when hot, and its body temperature will rise to 103° or 104° Fahrenheit, and, in some cases, as high as 105°. Despite this rise in temperature, the horse's skin will, generally speaking, remain dry. There might be some patches of sweating, such as beneath the mane, between the legs, and on the neck, but the rest of the body will be dry.

The anhidrotic horse often will seek a cool area in its stall and lie down. If it is in a pasture, it will find whatever shade is available. If the horse is in training, racing, or competing in some other activity, it is in a constant state of danger during exercise sessions.

The critical nature of the problem becomes more evident when we realize that even short-term, submaximal exercise on a treadmill can produce a rapid elevation of temperature in the horse. In one study at Ohio State University, it was found that exercising a horse on the treadmill at 80% of VO2max for 10 minutes would elevate the body temperature to 103°.

When body temperatures reach 105° or more, as can happen with the anhidrotic horse, there is danger of heat stroke or other thermal injury.

The basic principle involved in the cooling process for both man and horse is that heat produces sweat, which evaporates from the skin, resulting in a lowering of body temperature. The horse, by the way, is the only mammal, other than man, to cool itself in this manner.

The important thing for the sweating horse during hot weather is to provide it with a sufficient quantity of water so that there is a constant supply of moisture to be converted into perspiration. And, during hot, humid weather, when sweat does not evaporate, other steps must be taken to cool the overheated horse. Spraying it with cool water is one of those basic steps.

While the cooling system for a normal horse does an efficient job, it is a total failure in the anhidrotic horse. The only way the anhidrotic horse has to dissipate heat is to pant like a dog, and this is ineffective. Just why anhidrosis occurs is a mystery. And, to make matters worse, there is no known cure for the anhidrotic horse other than to move it to a cooler climate.

M. B. Teigland, DVM, a veteran racetrack practitioner in Florida, who is now semi-
retired, had this observation about change of locale and climate: "I've sent (anhidrotic) horses North, and they start perspiring in the van when they get as far north as North Carolina. The only real treatment is to send them North. You can spend all kinds of money trying to treat them, but all you really do is lessen their value. There are a lot of things you can do to give them relief, but when you add it all up, keeping horses medicated to give them relief, but not curing them, doesn't make much sense.

"There have been 101 remedies where people think they have the answer, but the remedy usually works best on Oct. 15. When you get your first northeast winds in the fall of the year (in Southern Florida) and the bluefish start running, the non-sweaters usually get better. I always used to tell the trainers to watch when the bluefish start running because their horses will start sweating again."

The frustrating thing for horsemen and researchers alike is that no one really knows why the cooling system shuts down when hot and humid weather arrives, and seems to start up again when cooler temperatures prevail.

A Long History

The condition was first documented in the 1920s with early research stimulated by problems that arose when the British moved their sport horses to colonies such as Indonesia, Malaysia, and India. They were surprised to find that a number of their runners and polo horses ceased sweating when they were transported to a hot, humid climate.

A great deal of research has been carried out in the intervening years, but it has seemed to produce more questions than answers.

A basic, baffling question is this: Why do some horses suffer from anhidrosis while others, existing in identical circumstances, do not?

Then there is the matter of breed predilection. It appears that more Thoroughbreds are afflicted with the problem than any other breed. But, some wonder, is this because Thoroughbreds undergo a unique form of stress while in training or racing in hot and humid climates?

Even the matter of locale raises some questions. It is well documented that in the United States, the problem is centered in the Gulf Coast states with their hot, humid summers. Yet, cases have been reported as far north as Minnesota and Michigan and in arid climates such as Arizona. Cases also have been reported in California.

Many Thoroughbreds at Panamanian and Puerto Rican tracks suffer from anhidrosis during the hot, humid season, and reports from Japan indicate that a number of Thoroughbreds in training or on tracks in that country are anhidrotic.

There is another twist to the affliction. Even non-exercising horses can be anhidrotic. This fact came to light several years ago when a survey was conducted by the University of Florida to determine the extent of anhidrosis in the area. Involved in the study were 834 horses residing on four different Central Florida Thoroughbred farms. Of that total number, 91 horses were in training. It was found that 23 of those 91 horses (25%) were anhidrotic to some degree.

The next-highest category for the affliction was that designated as non-pregnant mares. This category included mares which had foals at side and those which did not. There were 74 mares in this category, and 11 of them (15%) were anhidrotic.

There were 217 pregnant mares in the study group, and only nine (4%) of them were anhidrotic.

Showing even less of a problem with anhidrosis was the adolescent group, which included 401 animals ranging in age from foals to colts and fillies which had not begun training or had not been bred. Of that group, only six (2%) were anhidrotic.

Eighteen stallions were included in the survey, and two of them (11%) were anhidrotic.

While that study clearly revealed that anhidrosis is a significant problem on Thoroughbred farms, it raised more questions. For example: Do non-pregnant mares have a greater predilection for anhidrosis than their pregnant counterparts?

"You could look at it the other way around," said Ian (Joe) Mayhew, a researcher at the University of Florida at the time and now teaching in Scotland. He published a paper on the survey, and his question was what if the unaffected animals get pregnant? Did that make a difference?

And what about the young horses? Why did so few of them have anhidrosis, compared to the older animals?

Again, Mayhew had a theory: "The smaller animal has more efficiency in losing body heat. Also, the younger animals are not put into sweat-inducing environments as much. You don't take them down to Miami and start running them around the track. They just wander around the paddock and get more shade, and maybe there is less stimulus to sweat. There are many factors; it may not be just the age of the animal."

Diagnosing anhidrosis is relatively simple for the severely afflicted horse, but is more difficult when the problem is less severe.

"Once you learn how they (the severely afflicted horses) behave, it's easy," said Teigland. "First, they start puffing--they really start hyperventilating--and this is the first tipoff because a lot of horses will hyperventilate without running a fever. The next obvious sign is a lack of sweat on the horse's body."

Difficulty arises in attempting to diagnose the horse suffering from a milder form of anhidrosis. This horse might sweat some, but not enough to cool its body. All that a trainer of a racehorse might notice in the mildly afflicted runner is that the horse's performance is deteriorating as the weather becomes hotter and more humid.

For the less severely afflicted horses, there is a test that can be administered, but it is not infallible. The test involves injecting into the horse's skin a solution containing epinephrine, a hormone secreted by the adrenal glands. A mild dose of epinephrine will cause the skin of a normal horse to break into a sweat near the area of the injection. An increasingly stronger dosage is required to get the anhidrotic horse to sweat in the injection area.

The problem with the test is the wide range of responses between individuals in the anhidrotic population. Results also could be influenced by prevailing weather conditions.

Epinephrine produced by the horse's body might be involved in the onset of anhidrosis, according to the theory of Ralph Beadle, DVM, PhD, at Louisiana State University. Beadle has been studying anhidrosis for the past 20 years.

To understand his theory, we must first understand a bit more about the sweating process in horses.

The sweat glands are equipped with beta 2 receptors, and when stimulated, cause the glands to react and the horse to perspire. One stimulus for the sweat glands is epinephrine in the bloodstream. Another form of stimulus is believed to be transmitted via the nerves.

"My main thought," said Beadle, "is that anhidrosis is a stress-related phenomenon. I think what probably is happening is that we are getting down-regulation or de-sensitization of the beta receptors on the sweat glands."

Beadle believes down-regulation, or shutting off of the normal process, is related to a variety of stresses in the horse. The greatest stressors under this scenario would be heat and humidity. Also included are stresses involved in breaking young horses, respiratory infections, and a variety of injuries.

"When all these add up to a certain point," Beadle said, "we get too much hormone--epinephrine--being released. When there is a constant high level of epinephrine in the circulation, we get a down-regulation of the beta 2 receptors on the sweat glands, and they quit responding for a while. When we get into cooler weather, the receptors will up-regulate, and away we go again and the horse will sweat."

In other words, the sweat glands are so overwhelmed with epinephrine that they simply shut down.

Beadle has been doing research work on drugs that might reverse the effect of epinephrine in anhidrotic horses, but he said no controlled studies have yet been carried out.

Some feed supplements are available that appear to have a positive effect on anhidrotic horses, he said. But, again, there have been no controlled studies to prove their efficacy.

Horses with a persistent case of anhidrosis eventually will show external signs. The skin will become dry and flaky and hair will fall out, especially around the eyes. Dry coats and hair loss result when oils produced by the sebaceous glands are not taken to the skin's surface by sweat. As the skin becomes dry, it also might become itchy. The dryness problem can be alleviated to some extent by applying oil rinses after cooling the animal with water.

The degree to which anhidrosis affects an animal's performance seems proportionate to the degree to which the animal is afflicted. The totally "dry" horse simply cannot perform at maximal level in hot weather. Even if a horse so afflicted could, for example, finish a race, it would be in serious danger of suffering a heat stroke.

Some trainers at Gulf Coast tracks seem to have more problems with anhidrosis than others, said Teigland.

"This is just an observation," he said, "but there are some trainers who have less of a problem with it in general than others. Most of the trainers who are through training real early in the morning in the summer months--the ones that are out before the sun comes up--have the least problems. Maybe they train a little lighter, I really don't know."

Other Questions

Still another question that begs an answer: Does heredity play a role?

Several years ago, Diane Kitchen, DVM, PhD, carried out a study on anhidrosis as part of her doctoral work at Texas A&M University. While there is no proof the affliction is hereditary, she was of the opinion that the increase of anhidrosis in the United States seems to parallel the influx of British bloodlines into the American Thoroughbred. Further, she theorized, the increase of anhidrosis in the Quarter Horse might be the result of adding more Thoroughbred blood.

In her own experience, Kitchen noted the passing of the affliction from one generation to another. Her mother owned a Thoroughbred mare which was anhidrotic. An offspring of that mare, a gelding, also was anhidrotic.

Still another question before the house is this: Is anhidrosis on the increase?

There is no definitive answer. As is the case with a number of maladies in the equine population, diagnostic techniques constantly are improving. Thus, what might appear to be an increase in number could just be more accurate diagnoses.

There aren't a whole lot of choices for the person owning an anhidrotic horse. The best short-term relief is to spray the afflicted horse with water. Also helpful are cooling fans in the stable or, ideally, but not necessarily practical, giving the horse an air conditioned stall.

Beadle's treatment of choice is the use of misting fans, such as were used to cool three-day event horses during the Olympics in Georgia.

While high humidity cuts into the evaporation process, the misting fans can lower barn or stall temperature. Beadle said that studies at Louisiana State have revealed that a misting fan can lower ambient temperature by as much as 15 degrees.

Unfortunately, these cooling steps only will result in keeping the non-exercising anhidrotic horse comfortable. If the animal is going to be asked to perform in any kind of competitive or stressful situation, there is only one answer--send it to a cooler climate.

In the meantime, scientists continue to grope with this baffling problem as they first attempt to determine exactly what triggers the affliction, then try to find a preventative therapy or drug.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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