Horses with Anhidrosis Require Careful Management

A horse that lacks the ability to produce sweat in normal quantities has a condition known as anhidrosis. Such an animal is sometimes called a nonsweater or a drycoated horse. Horses that sweat lightly or only in patches such as under the mane, in the saddle area, and on the chest are known as shy sweaters. The condition seems to show up most often in Thoroughbreds, but Quarter Horses and horses of other breeds may also be affected. A veterinarian can make a definite diagnosis by injecting epinephrine under the skin. In normal horses, the area around the injection site will rapidly produce sweat. Anhidrotic horses exhibit a slow, weak, or nonexistent sweating response.

Some degree of anhidrosis may affect up to about 25% of horses in hot climates. Among mature horses, there seems to be no correlation with sex or coat color. Often the problem is not noticed until a horse that has been conditioned in a cool climate is moved to a region with hot, humid weather. However, anhidrosis can also develop suddenly in horses that have been trained and shown exclusively in warm climates. Typically, such a horse might perform on an extremely hot day where he sweats excessively. Several days later, his sweat production is greatly reduced or absent. This sign is followed by other characteristics of the syndrome: reduced appetite, sluggish performance, dull coat, and hair loss on the face and croup. Core temperature rises quickly with exercise, and pulse and respiration stay greatly elevated after work as the body attempts to lose heat from the lungs. Anhidrotic horses are sometimes referred to as puffers because of this prolonged period of rapid breathing after exercise.

An inactive horse in a cool climate might not be bothered by diminished sweating. A more serious situation is encountered when a nonsweating horse is asked to perform hard work in a hot, humid climate. The conversion of stored energy to fuel for exercising muscles is not an efficient process, with more than three-fourths of the energy being given off as heat. A horse performing treadmill exercise under hot, humid conditions has shown a rise in body temperature as rapid as 0.2° C (.36° F) every 10 seconds. It is estimated that a horse on the cross-country phase of a three-day event can produce enough heat to raise its body temperature more than 15° C (27° F). In a normal horse, about 65% of body cooling comes from evaporation of sweat, with another 25% attributed to respiration. A horse that is unable to sweat can rapidly accumulate more heat than he can easily eliminate. The horse’s brain is very sensitive to heat stress, and high body temperature can lead to incoordination, unwillingness to continue exercise, collapse, convulsions, and death. Horses that survive may have permanent brain damage.

The cause of anhidrosis is not known. Electrolyte supplementation seems to trigger a return to normal sweat patterns in a number of cases, and nonsweaters often resume sweating when they are moved to a cooler climate or when temperatures moderate in the fall. Acupuncture has been effective for some horses. One study has found evidence that a hormonal or metabolic imbalance may cause the horse to produce high levels of epinephrine. It is thought that the sweat glands become habituated to the abnormal stimulus, stopping the standard sweat-producing response. Examination of the skin of some anhidrotic horses has found a high percentage of sweat glands that are abnormal, atrophied, or plugged.

A presentation at the 14th Equine Nutrition and Physiology Symposium reported on the use of a supplement containing cobalt proteinate, L-tyrosine, and vitamins B1 and C. Claimed to be helpful to 90% of anhidrotic horses, the supplement may work by providing amino acids that help to restore a normal reaction of sweat glands to adrenaline.

Some studies have tied anhidrosis to low thyroid function. Hypothyroid horses may resume sweating when they are treated for this condition, but indiscriminate thyroid supplementation to a horse with normal function can cause dangerous increases in heart and metabolic rates as well as weight loss and excitability. Thyroid supplementation should not be done without the guidance of a veterinarian.

Ideally, cool-region horses that will be asked to perform intense exercise in hot, humid conditions should be moved well in advance of the competition to allow them to adapt slowly to the climate. Up to three months of increasingly demanding exercise may be necessary before the horse gains maximum thermoregulation efficiency. Among the physiological changes are an expansion in plasma volume, increasingly stable cardiovascular function, and alterations in the sweating pattern. A horse that is asked for a high level of performance without a suitable conditioning period is at an increased risk for heat-related problems.

Because fat metabolism releases less heat than starch metabolism, replacing part of the horse’s grain ration with a fat product may help anhidrotic horses stay more comfortable in hot weather. These animals should be given free access to fresh water, and electrolyte supplementation should be provided.