Anhidrosis: Should I Sweat It?

Anhidrosis, or nonsweating, is a poorly understood condition affecting thousands of horses worldwide. Horses are most commonly affected in areas with hot, humid climates, such as in the southeastern United States. Let's look at the causes of and treatments for this condition.

What Causes Anhidrosis?

Veterinarians and researchers are still trying to deduce the exact cause of this condition. Originally, it was thought that nonsweating was due to problems with acclimatization to hot weather; however, a survey of horses in Florida found that more native horses were affected than imported animals.

In normal horses sweating is initiated by stimulation of sweat glands, which are activated by nerve signals (neurotransmitters) called catecholamines. Recent studies have shown anhidrotic horses have higher levels of catecholamines than normal horses. It is possible that overstimulation of receptors by high catecholamine levels leads to desensitization. In addition, microscopic examinations of sweat glands in anhidrotic horses show abnormal changes in their cellular structure. This research suggests the condition is more likely a problem with the sweat glands and their response to nervous signals, rather than a lack of nervous signal production. Alternatively, many veterinarians suspect alterations in endocrine (hormone) stimulation of sweat glands.

Does My Horse Have Anhidrosis?

Horses that do not sweat are easy to spot, as they are usually more lethargic in hot weather and have a dry coat even after strenuous exercise. Often these horses have increased breathing (respiration) rates, higher heart rates, and elevated body temperatures, even after short periods of work. Animals affected with this condition for long periods might have dry, flaky skin with areas of hair loss.

But what if your horse can sweat, yet still has some of these clinical signs?

Many horses are considered to have hypohidrosis, or the reduced ability to sweat. Practitioners can perform a simple test to determine if a horse has this condition.

Why is Sweating Important?

Horses, like people, need to sweat for thermoregulation (cooling) during periods of hot weather, high humidity, and intense exercise. Heat production can increase by more than fiftyfold during strenuous work. Evaporation of sweat dissipates as much as 65% of the heat produced during muscular activity. It is not surprising that anhidrotic horses often have poor performance in hot weather.

What Can be Done?

Vets have used dozens of treatments to help anhidrotic horses, including sodium chloride, iodine, potassium, vitamin E, and even beer. These treatments were largely unsuccessful.

Clenbuterol has been used to treat these horses, but it is not recommended due to the potential for prolonged desensitization of sweat gland receptors. Vets have reported anecdotal success using acupuncture to promote sweating, but the duration of the effect is unknown. Anecdotal reports also have shown that a feed supplement called One AC can increase horses' sweat production.

There seems to be more questions than answers about anhidrosis. We do know that maintaining proper hydration and electrolyte balance is very important to overall health. The most effective strategy for anhidrotic horses is making environmental/management changes. Training and acclimatization benefit a horse with reduced sweating ability. Train/ride during the coolest time of the day, and provide access to shade/water at all times.

Moving an affected horse away from a hot, humid climate can sometimes relieve the signs in 10-30 days, especially if the signs are recognized early. Misting fans and other barn-cooling methods can help. In severe cases these approaches might be ineffective; your horse might need a career change or a move to a cooler climate.

Work with your vet to develop strategies that fit your situation best. With early recognition and proper management, even a poor-sweating horse can be a successful equine athlete.

About the Author

Jeremy Frederick, DVM

Jeremy Frederick, DVM, is a resident in large animal medicine at the University of Florida. His main areas of interest include equine neurologic diseases and critical care.

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