Anhidrosis in Horses

The condition known as anhidrosis ‒ the complete absence of sweating ‒ can pose serious problems for your horse in the hot weather. If it isn’t recognized and treated quickly, potentially fatal heat stroke can result.

Katharina Lohmann, associate professor of large animal medicine at the University of Saskatchewan’s Western College of Veterinary Medicine, explains what anhidrosis is and how it can manifest in horses. “Anhidrosis is an inability to sweat properly. In horses it’s a big problem with performance ability, because a lot of the thermo-regulation, or the ability to regulate body temperature during exercise, is through sweating. Not being able to sweat leads to overheating and therefore an inability to perform.

“It’s a disease that happens primarily in very hot and humid climates,” Lohmann continues. “How it works, as far as we know, is basically the sweat glands get over-stimulated and at some point they shut down entirely.”

The Importance of Evaporative Cooling

Lohmann reports that while breathing accounts for approximately 25 percent of a horse’s ability to control their internal temperature, sweating is their primary mechanism for cooling and can account for up to 70-75 percent of a horse’s ability to cool itself off. Looking at thermo-regulation in general, sweating is the most efficient way a horse can moderate its body temperature.

During periods of intense activity, heat production can increase by as much as 50 times. Lohmann points out that although an anhidrotic horse may be breathing labouriously, attempting to cool itself off, its body temperature will continue to rise because it can’t produce sweat.

How Anhidrosis Develops

Under ordinary circumstances, sweat glands produce perspiration in horses when triggered by hormones after they are prompted by the body’s adrenal glands. One reason horses may suffer anhidrosis is an exhaustion of the sweat glands caused by over-stimulation of beta-adrenergic receptors due to excessive production of stress hormones.

According to Lohmann, the general consensus is that anhidrosis can affect horses regardless of their breed, gender, age, or birthplace. Studies are continually underway to determine if there is a strong genetic pre-disposition.

“From the studies that have been done, my understanding is that there’s no relationship to age, breed, or sex of horse. One study from Florida that looked at epidemiology found that horses from families with a history of anhidrosis were more likely to have anhidrosis,” Lohmann says. “Whether that speaks to a genetic component, I don’t think we could say for sure at this point.”

Lohmann also notes that if anhidrotic horses are treated early, it may prevent long-term structural damage of the sweat glands. If it’s not treated in a timely matter, permanent cellular changes may result.

Signs and Symptoms

Fortunately, by observing a horse’s physical and physiological signs, anhidrosis can be spotted, especially during limited periods of activity. Visual signs include a slow gait, parched skin that flakes, and visible bald patches. Clinical signs of anhidrosis may present as a faster pulse, higher body temperature, and a higher respiratory rate.

Lohmann says, “Normally, if you see a horse that’s exercising hard, you’ll see a horse that’s all lathered up in sweat. With these [anhidrotic] horses, they often only have certain areas where they’ll sweat – under the jaw, along the neck, at the base of the ears, between the hind legs, and under the saddle, but it’s not the typical sweating patterns. Some will show some sweating behaviours; it’s just not enough to regulate the body temperature.

“In extreme cases, the horses will altogether lose their ability to sweat; that’s why they also call the condition dry-coat, or dry horses. They never sweat at all.”

If your horse’s performance is not up to par or you suspect something is off, it’s important to have your horse seen by your veterinarian as soon as possible to determine if a diagnosis of anhidrosis is the correct one. “Most of the times when these horses are presented to a vet, it is because they’re not performing well. Depending on what [a horse does], they just can’t get to the level of performance that the owner is expecting, or is used to,” Lohmann explains, adding that other common presenting complaints are that the horse is breathing hard and its temperature is high.

Lohmann warns that this non-sweating problem can mimic other conditions. “Equine anhidrosis could be confused with a horse having an infection as well, because it would be combined with lethargy, breathing hard, and a fever. One could certainly draw the wrong conclusion from those presenting complaints.”

Treatment Options

Lohmann explains that there are a lot of anecdotal treatments used to treat equine anhidrosis, including vitamin E, iodine, and sodium chloride. Everything from beer (containing yeast extracts and B-vitamins that are helpful in sweat gland function) to clenbuterol (which must be administered by a vet) have had some success. The supplement One AC by Miracle Powder Company (nonsweater.com) contains a combination of vitamins, amino acids and minerals and has proved helpful for some anhidrotic horses. Supplementing with electrolytes is also suggested as a treatment, as it’s known to support general equine health.

Treating anhidrotic horses by placing them in a cooling environment as soon as possible for between 10 and 30 days is still recognized as the best treatment option for mild to moderate cases. Lohmann says, “The only real treatment to my knowledge is to move these horses to a different climate, as this is what really helps the horse. One thing that can be done short of moving them to a different climate is just trying to keep them as cool as possible – an air conditioned barn, using a misting fan, making sure they have enough shade, making sure they don’t get exercised during the hottest part of the day.”

While there is no proven cure for anhidrosis, most horses can live normal lives with sensible management. And occasionally the condition is transient and these more fortunate horses resume sweating normally once the weather cools.

Why Sweating Occurs

• Muscles produce heat during exercise due to energy metabolism (generating energy from nutrients).
• The bloodstream absorbs the heat and transports it to the lungs, where exhalation dissipates some of it, while the skin radiates away some of the heat as well.
• If the horse can’t dissipate or radiate all the excess heat away, his body temperature begins to rise (greater than 37-380C).
• The hypothalamus in the brain detects the temperature increase and signals the sweat glands to produce sweat (a combination of water and electrolytes).
• The sweat on the skin evaporates, carrying away more heat and lowering the body temperature.
• An anhidrotic horse’s temperature can reach critical levels (41-430C) after exercise, leaving him open to life-threatening heat stroke.

Signs of Anhidrosis

• Coat remains totally dry and hot to the touch after exercise in very warm weather; can also be slightly damp, mainly under the saddle and between the hind legs;
• Laboured breathing during and long after exercise;
• Lethargy and exhaustion;
• Loss of appetite;
• Reduction of water intake;
• Thinning, patchy hair coat, facial hair loss;
• If you suspect anhidrosis, your vet can do a intradermal terbutaline sweat test, injecting a small amount of the drug under the skin of the horse’s neck to encourage local sweating, which will not occur in an anhidrotic horse.