Failure to Sweat

A horse in motion is a striking image: Vigorous, powerful, animated. In your mind's eye you can imagine his ears pricked forward, nostrils flared, limbs strumming the ground, streaks of sweat punctuating his rippling muscles. To create locomotion, a horse converts food substrates into energy fuel to drive the muscles, and in the process, he releases heat as a by-product of this metabolic conversion. Energy production is only 20% efficient, with the remaining 80% generated as heat. Without developing effective mechanisms of heat dissipation, a horse could generate enough internal heat to "boil to death," as it were, possibly elevating his internal temperature by one-half a degree Fahrenheit for every minute of aerobic exercise.

A significant way for a horse to relieve heat buildup is through evaporative cooling, which uses sweat to convey heat from the horse's skin to the ambient air. Fitness and conditioning develop a horse's cardiovascular system to maximize movement of heat from deep in the working muscles to the skin, where the heat is evaporated from the sweat glands. Evaporative cooling contributes about 65% to a horse's cooling process, while his lungs might blow off as much as 25% of his internal heat. In hot and humid conditions, you have likely noticed such a cooling mechanism as a hot horse stands blowing or panting.

When moving a horse to a hot, humid climate, or when training a horse hard in those conditions, there is often a period of acclimatization that allows a horse to enhance heat-dissipating mechanisms. During the initial five to 10 days in an adverse climate, some adaptive changes occur, reaching a plateau by three weeks. Such changes include a drop in a horse's internal temperature at rest and decreases in his sweat losses while at work.

Yet, in some cases, a horse might not be able to adapt to heat and humidity because of an inability to sweat. Such a horse can suffer from heat exhaustion because he can no longer sweat to rid his body of heat created by working muscles. This syndrome is called anhidrosis, derived from the Greek word meaning "without sweat."

Clinical Signs of Anhidrosis

Jeremy Hubert, BVSc, MS, is an Equine Surgeon Certified Specialist and an assistant professor of veterinary surgery in the department of veterinary clinical sciences at Louisiana State University, where the hot, humid climate of the Gulf Coast states often presents a challenge to athletic horses. According to Hubert, the first signs noticed in a horse suffering from anhidrosis might be facial alopecia (hair loss) and a changing sweat pattern, with sweat sometimes retained under the mane and saddle and between the legs in the armpits and flanks, but no sweating elsewhere. These localized spots of sweating are insufficient to promote cooling. One significant tip-off to a problem is that the horse's skin is dry and feels hot to the touch when he is worked, when it should be coated with sweat.

Hubert notes that signs of a problem can come on suddenly. He says, "In many anhidrotic cases, the horse will perform poorly, lose his appetite, and drink less than necessary, while also appearing lethargic and tired."

Betta Breuhaus, DVM, PhD, an associate professor of equine medicine at North Carolina State University's veterinary school, has also devoted her time to researching equine anhidrosis. She remarks on obvious signs of heat stress from anhidrosis: "The horse breathes faster and/or harder than he should for the level of exercise. He may be reluctant to work. Once exercise is stopped, the horse will take a long time to cool out and continues to breathe hard for an abnormally long period of time."

Hubert clarifies, "A working horse should be able to revert to normal body temperature in about 30 minutes once exercise has stopped."

Breuhaus' observations are telling: "If an anhidrotic horse is forced to continue to work in a hot, humid climate, he will eventually become overheated and unable to continue. If rectal temperature rises much over 104°F, the horse may collapse."

With no outlet for heat to efficiently escape from the body, anhidrosis creates a dangerous condition for an exercising horse. Not only does this pose a potential health impact on the horse, but it also has financial repercussions due to athletic limitations of a horse that might possess tremendous innate potential, yet is unable to perform.

What Causes Anhidrosis?

What causes these horses to lose the ability to sweat at rest or while exercising? Hubert reports, "There are abnormal cellular changes in the sweat glands of an anhidrotic horse if observed during a period of non-sweating. This is considered to be a result of the syndrome, not a cause. After a period of de-stressing in a cool environment, the histological abnormalities resolve."

Breuhaus considers the physiologic changes related to decreased function of the sweat glands. "Most researchers have been trying to document some sort of change in the beta-adrenergic (beta-2) nerve supply to the sweat glands," she says. "Either something is wrong on the stimulation end (decreased neurotransmitter, which means the nerves don't tell the sweat glands to sweat enough) or something is wrong on the receptor end (decreased numbers of receptors or decreased sensitivity of receptors to stimulation, meaning the sweat glands lack the ability to receive the nerve's message to sweat)."

Hubert says a possible cause might be "over-stimulation of beta-2 receptors that results in a lack of response of the receptor and thus no signal to sweat (the receptors become overloaded and quit responding). What is usually a normal physiological response of cell turnover or a down-regulation of the receptors turns into a long-term response of altered protein synthesis capabilities of these cells."

Research to identify the crux of this perplexing problem is still a work in progress at this time.

The cause of hair loss (alopecia) associated with anhidrosis is also somewhat elusive. Hubert speculates, "Changes in dermal perfusion (skin blood supply) may be a factor, although vascular supply to glands appears OK. Yet histopathology shows significant changes in the gland architecture." He questions, "If the duct isn't functional, it gets blocked up, damaging the normal physiology, and then perhaps the result is hair loss?"

Hubert states that the cooling ability of an affected horse is dependent on the degree of thermal stress incurred. He says, "Most horses, once diagnosed, will repeatedly become anhidrotic when stressed if not managed differently."

No Link to Thyroid Disease

A paper written in the 1950s claimed that anhidrotic racehorses in Hong Kong improved when supplemented with thyroid hormone, so this idea has stimulated further investigation. Breuhaus has been researching the link between anhidrosis and thyroid disease by evaluating thyroid function on 10 pairs of horses. In each pair from the same barn, one was anhidrotic and the other was a horse that could sweat normally.

"I did TRH (thyroid-releasing hormone) stimulation tests in each pair both in the summer when the anhidrotic horses were having difficulty, and again in the winter when the weather was cool enough that they did not need to sweat," Breuhaus says. "I found no differences in resting thyroid hormones or their responses to TRH administration between the anhidrotic horses and the normal horses. I interpret the results of my study to show that anhidrotic horses are not hypothyroid."

She stresses that although there might be a brief period of increased stimulation of sweat glands from thyroid hormone supplementation, there is little value in the long term. Thyroid supplementation to a horse that is not in need can elicit adverse effects, such as an increased metabolic rate, increased heart rate, hyper-excitability, and persistent weight loss. All these issues add to poor performance and increased internal heat generation.

How Common is Anhidrosis?

Other questions one might ask are about the frequency of anhidrosis, and how can one monitor for it in advance? (See "Testing for Anhidrosis" on page 98.) The incidence of anhidrosis in Thoroughbreds in a study in Central Florida was around 6% in training and non-pregnant horses, but the syndrome did not affect adolescent youngsters. Breuhaus observes, "A Florida study noted that there was no breed or sex predilection, which would suggest that it is not heritable. However, it is my impression that it is more common in certain breeds, especially the Thoroughbred. One veterinarian who practices in the Southeast feels it is possible to predict if a horse will have a problem just by looking at its pedigree. That would certainly suggest heritability, although this is not proven."

Management for Anhidrosis

Once a horse is identified as anhidrotic, management plays a key role in ensuring his safety and welfare in athletic pursuits. In any horse, anhidrotic or not, the less fit he is, the more inefficient his metabolism and the less efficient his cooling mechanisms, so he might tend to overheat. Thus, fitness is critical in enabling every horse to perform as efficiently as possible.

Hubert discusses training and competing an anhidrotic horse: "Maintain a schedule that avoids thermally stressful periods. Acclimatization may be possible, along with careful monitoring of fluid intake and recovery times of heart rate and respiration; this can be monitored with careful attention to fluid intake and recovery times of heart rate and respiration. However, horses born in hot, humid conditions can also become anhidrotic, suggesting that acclimatization cannot be guaranteed."

One way to estimate thermal stress is to use the heat index achieved by adding the temperature (degrees Fahrenheit) to the relative humidity percentage. Any value greater than 140 poses an increased danger to any horse's ability to cool in the face of exercise, particularly if he's unable to sweat. Breuhaus suggests, "Most people recommend that if you live in a hot, humid climate, you exercise your horse in the morning or evening. In regard to the heat index, it is usually cooler, but more humid, in the morning than in the afternoon. Although the heat plus humidity index is certainly useful, 60°F with 90% humidity is not exactly equal to 90°F with 60% humidity. It seems to feel better to exercise in the cooler, moister morning. A horse can also tolerate a higher heat index if it is cloudy out or if he's in the shade, without the element of direct radiant heat. In addition, you will start seeing problems with anhidrosis when the nighttime temperatures do not drop below 70°F."

Breuhaus offers this advice for owners of anhidrotic horses: "If there is a really unusually hot and humid spell of weather for the area, don't ride!"

Hubert is keen on human intervention to improve a horse's comfort zone. "I think misting fans are very useful; they will significantly reduce the ambient temperature by as much as 10°F," he says. "Air conditioners make a big difference in resting the sweat glands, thus allowing an appropriate sweat response when required."

Breuhaus adds, "I don't know if using fans or air conditioners can prevent a horse from becoming anhidrotic. They would decrease heat stress, which would be a good thing. It has been observed that (affected) horses stop sweating soon after a bout of excessive sweating. Many people with anhidrotic horses avoid events that make a horse sweat a lot as summer comes on--for example, forgoing administration of sedatives like xylazine for dental procedures or mane pulling, and avoiding hard workouts in hot weather." In addition, she states, "There seems to be a connection between small airway inflammatory disease and anhidrosis in some horses, so respiratory health strategies are important. Stress also seems to be a complicating component in development of anhidrosis, so it is beneficial to keep stress to a minimum. "

Breuhaus stresses a comprehensive management approach. "Try to keep the horses cool (with misting fans, air conditioning, water soaks, etc.); stop working the horse for several weeks; reduce any other stresses; look for and treat any underlying diseases such as small airway inflammatory disease, equine Cushing's disease, or any other illness; and supplement with electrolytes," she recommends. "Body clip in the autumn or if the horse is beginning to grow a longer coat."

Dietary Management

All avenues of management are critical to improving heat dissipation in an exercising horse. Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor in the Department of Animal Sciences with Cook College at Rutgers, The State University of New Jersey, and a faculty member of the Equine Science Center, comments on the role of diet and feeding in managing heat stress, and these tenets especially apply to an anhidrotic horse. "Contrary to popular belief regarding forages, even though there is significant heat of fermentation (during digestion of forages), the actual resultant energy sources (volatile fatty acids) have a very low heat increment," says Ralston. "Therefore, forages should not be considered as 'hot' energy sources. The biggest culprit for metabolic heat generation is protein with its high heat increment. Horses urinate more on alfalfa (which is high in protein) than on grass hay, urea being a by-product of protein metabolism. A combined increase in sweat and urine losses makes horses on high-protein feeds (greater than 16%) 'at risk' for dehydration in hot weather, whether the protein source is alfalfa hay or soybean meal."

Ralston offers constructive feeding advice to minimize heat load in a horse: "Fats have the lowest heat increment. The best way to provide 'cool fuel' for horses engaged in aerobic-type exercise is to feed all the grass hay (protein of 8-12%) they can eat and 5-10% fat concentrates in amounts necessary to maintain body weight. Concentrates should be offered at no more than 0.5% of body weight (less than five pounds of grain or concentrate per 1,000-pound horse) at each meal."

Ralston's motto in feeding is: "Keep it simple!" In addition, she stresses the importance of electrolytes. "It is essential that salt (preferably pure NaCl in granulated form in a separate bucket) and water be provided free choice," she advises. "Salt deficits and marginal dehydration can cause anhidrosis that will quickly resolve once the horse has free access to salt (NaCl) and water! Giving only limited amounts of electrolytes, especially when they're added to the drinking water, is often not sufficient in hot, humid climates."

Hubert concurs about the value of electrolyte supplementation to exercising horses, "Potassium chloride (KCl) is purported to be useful, although horses with anhidrosis usually have normal electrolyte plasma concentrations. But making certain that a horse is well-hydrated is important, and adding Lite salt to the feed ensures that there is no real potassium deficiency and provides a stimulus for increased water intake."

Ralston cautions, though, that, "Dietary potassium deficits are rare (forages and most grain concentrates contain relatively high levels) and supplementing straight Lite Salt (KCl mixtures) can lead to potassium overdoses, leading to cardiac arrythmias." "The emphasis should be on electrolyte balance."

Drug Therapy

Attempts at environmental and dietary management might not help in all cases. One medical strategy uses oral products that contain specific nutrients like tyrosine and cobalt, which are used in the biochemical pathways of sweat. However, Hubert points out, "Anecdotally, these products are useful if administered to a horse with known anhidrosis before he exhibits any signs. Tyrosine is supposed to be one of the active ingredients, but plasma levels of tyrosine in anhidrotic horses are reported to be normal."

Breuhaus approaches these products logically. "I don't see any contraindication to trying a product such as One AC, Pro-Sweat, or Let'em Sweat," she says. "The ingredients in the different products vary. I know of no controlled studies on their efficacy, or lack thereof."

She sums up the conundrum of drug therapy by saying, "I have seen marketed products fail. I have also seen some succeed. I don't know if successes are due to the product(s) or if the horses would have started sweating again anyway, given the other environmental and management changes. I do think that most products are not helpful if the horse has had a problem for a prolonged time, probably because by that point the sweat glands are too atrophied. I have heard people say that acupuncture and/or Chinese herbs helped their horses, but again, I don't have a clear idea how much of that is real versus placebo."

Other medications might have adverse effects on activity of the sweat glands. One example is clenbuterol, used as a bronchodilatory drug for horses with respiratory problems. Hubert notes, "There are many reports of horses--even in the winter months--becoming anhidrotic after administration of clenbuterol; this is not surprising as it is a beta-2 adrenergic agonist (blocks beta-2 adrenergic effects). It will result in a mildly increased sweat response initially, but then the opposite effect will occur as the receptor becomes over-stimulated with time. These horses usually return to sweating once clenbuterol is removed."

When Management Is Ineffective

Despite the best intentions, management or drugs cannot always fix the anhidrotic horse. Hubert says, "If basic management changes cannot be deemed effective or no practical changes can be made, then it becomes necessary to either substantially reduce the level of performance or to relocate the horse."

If relocation is not an option, Hubert proposes, "Some horses will repeatedly become anhidrotic, and proactive management--starting extra electrolytes, cooling the stall, etc., in spring before the weather gets really hot and anhidrotic signs appear--for such an animal may help. However, if there are still signs of obvious anhidrosis, then one may have to desist from performing at that inclement time of year."

Breuhaus discusses the quandary associated with an inability to manage anhidrosis by saying, "I do know of horses that have been sold North to cooler climates. I also know of horses that have stayed in the South; some of those are ridden in the cooler months, some are retired. I know of horses that have had a problem one summer and not another summer, and of others that have become anhidrotic and remain so year after year. It is my impression--not a fact--that if you can catch a horse early the first time he starts to go anhidrotic and stop working him, reduce the stress, and identify and correct any contributing illnesses, there is a better chance he will start sweating again. Then if you are careful in ensuing seasons, you can keep him sweating to some degree. But if you ignore the problem or the problem has been present for several years, he will likely not sweat again."

However, she says, there are always exceptions: "I have heard of one horse who, after not sweating for years, began sweating when he was moved to a new barn, but remained in the same general community and was exposed to the same temperatures. Does that mean the horse was stressed in the first environment? Or was he allergic to something?"

In climates with high heat and humidity and when a horse is showing even subtle signs of not performing well, it is prudent to stop and look for an underlying physical cause, including anhidrosis. Identification of the problem at its inception gives the best chance of reversing the damage before sweat gland dysfunction incapacitates a horse's athletic abilities.

Testing for Anhidrosis

There might be a suspicion that your horse is afflicted with an inability to sweat normally (anhidrosis), in which case his reactions to a simple injection test can identify the current status of his sweat glands. Betta Breuhaus, DVM, PhD, associate professor of equine medicine at North Carolina State University's veterinary school, explains the procedure: "Increasing concentrations of beta-2 (beta-2)-specific agonists, which stimulate beta-adrenergic receptors (that control sweating), are injected intradermally (within the skin)."

Jeremy Hubert, BVSc, MS, Equine Surgeon Certified Specialist and assistant professor of veterinary surgery in the department of
veterinary clinical sciences at Louisiana State University, recommends the most reliable testing products to determine activity of the sweat glands. "As epinephrine is not beta-2-specific, it is preferable to use a beta-2-specific agonist such as terbutaline or salbutamol," he says. All injected areas of the skin are examined 20 minutes later to see what concentrations make the horse sweat in a small patch over the area of injection. Normal horses sweat at all dilutions. Anhidrotic horses do not sweat at any of the dilutions. Partially anhidrotic horses sweat in response to the higher concentrations only. A control spot is created by injecting only sterile water.--Nancy Loving, DVM

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her recent book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care (available at or by calling 800/582-5604). She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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