Learn more about anhidrosis, a potentially career-ending disease.
By Sarah Wynne Jackson in The American Quarter Horse Journal | August 9, 2011
As athletes, American Quarter Horses perform amazing feats of strength, agility and grace. To accomplish that, every body system must function at its very best. But some horses have developed a condition that makes exercise difficult if not downright dangerous: anhidrosis, the inability to produce sweat.
Some degree of anhidrosis is thought to afflict as many as 30 percent of horses in the hot, humid areas of the United States, but the disease can appear anywhere. Performance horses seem to suffer most often, but even nonexercising horses can be affected.
Dr. Robert MacKay, a professor of large-animal medicine and large-animal clinical sciences at the University of Florida College of Veterinary Medicine in Gainesville, Florida, has been seeing anhidrosis cases since 1978.
Watching this devastating disease end the performance careers of talented horses spurred Dr. MacKay to begin a serious search for answers to the questions that surround the condition.
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The Danger of Anhidrosis
Heat builds rapidly in the body of an exercising horse, especially on a warm day. If the horse doesn’t sweat sufficiently for dissipating that heat, thermal injury and heat stress become a major concern. A body temperature of 105 degrees or higher is a red flag. If forced to continue working, such a horse can suffer heat stroke, seizures and death.
Horses can remove as much as 25 percent of their internal heat with an increased respiratory rate, but that method is inefficient and far less effective than sweating, which contributes about 65 percent to the cooling process.
Although the cause of anhidrosis remains unknown, it appears that the hormone epinephrine (also known as adrenaline) is involved. The body releases epinephrine during times of stress to prepare itself for action with responses such as an increased heart rate, dilated air passages and sweating.
The current theory maintains that the hormone receptors on the sweat glands stop responding appropriately to epinephrine. This effect could be due to over-stimulation during times of constant stress or some other cause of degradation. Stresses such as hot and humid weather, hard training, respiratory infections and injuries may contribute.
“Horses aren’t born with anhidrosis,” Dr. MacKay says. “It is acquired as early as age 2, but most often in middle age. There is probably a genetic susceptibility to it because one of the risk factors is the horse having a relative with anhidrosis.”
Although anhidrosis occurs most often in warmbloods and Thoroughbreds, there has been an increase of incidences in Quarter Horses through the years. Some surmise that this might be due to the addition of Thoroughbred blood to Quarter Horse bloodlines. Arabians appear almost immune to it.
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Horses that are moved from a cool, dry location to a hot, humid place are at particular risk. In fact, anhidrosis was first documented in the 1920s when the British took their sport horses to Indonesia, Malaysia and India, and found many of them ceased sweating in the hotter climate.
“If there’s no effort to acclimatize the horse to the new environment, that increases the risk of anhidrosis even more. They need to be gradually put back into work over at least a couple of weeks,” Dr. MacKay cautions.
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