Anhidrosis in Horses

A healthy horse releases 65 percent of his body heat by sweating. If his breathing is fast and labored, yet he hasn't broken a sweat like this horse, he may be in trouble.

Many horse owners have wished that their horses would sweat less. It would certainly help keep our mounts and tack cleaner. However, sweat plays a vital role in our horses' lives, and a lack of sweating in your horse may signal a potentially deadly disorder: anhidrosis.

Anhidrosis, or dry coat, is the inability of a horse to sweat normally. Dr. Frank Gravlee, a veterinarian and owner of Life Data Labs, says the severity of anhidrosis varies by horse, ranging from a mild or unrecognizable lack of sweating to an absolute inability to sweat. In the most severe cases, horses that quit sweating risk brain damage or even death from a lethal increase in body temperature.

Why Anhidrosis Occurs
The cause of anhidrosis is unknown, as little research has been completed on this disorder. However, anhidrosis is a serious problem, since as much as 65 percent of a horse's body heat is released by sweating, according to a North Carolina Horse News article by Dr. Betta Breuhaus, associate professor of equine medicine at North Carolina State University. In her article, Breuhaus says many anhidrotic horses suffer from hypothyroidism-the insufficient production of hormones by the thyroid gland.

Researchers at North Carolina State University conducted a study that compared the thyroid functions of anhidrotic horses to those of normal horses. According to the university's 2004 Annual Report and Research Overview, the baseline thyroid hormone concentrations and the result of stimulation by thyrotropin releasing hormone (TRH) were comparable between anhidrotic and normal-sweating horses. However, the report noted that "serum baseline concentrations of thyrotropin (TRH) and responses to TRH were increased compared to TSH concentrations and responses to TRH in horses that could sweat normally."

Gravlee believes there may be a nutritional component to anhidrosis, and Life Data Labs is in the process of developing a testing laboratory that is expected to shed some light on the subject. The laboratory will study the nutritional and hormonal relationships of several equine metabolic conditions, including anhidrosis.

Any horse can develop anhidrosis, and it seems that the disorder does not affect any particular breed, sex or age of horse more than any other. Horses living in hot, humid climates are more at risk, as this type of weather, coupled with strenuous exercise, seems to overburden the cooling system of some animals, interfering with their ability to sweat normally.

Sometimes the shock of moving from a cool climate to a hot climate can trigger the onset of anhidrosis. "Usually, you have a problem when they're transported from areas with cool temperatures and cool nights to an area with a tropical climate," Gravlee notes. While temperature and climate changes can affect the onset of anhidrosis, horses that are native to hot, humid climates can also develop the condition.

Other factors that may contribute to the development of anhidrosis are the strenuous training and competition schedules of some horses. "Equine athletes in tropical climates are more at risk," Gravlee observes. "Usually, it affects equine athletes because they're moved around the world and moved to different climates."

Dr. James Hall, a veterinarian in Dayton, Texas, says a horse's weight may affect its susceptibility. "The fatter a horse is, the more probable it is that one will go into anhidrosis," he says. Fatter horses have more insulating tissue on their bodies, creating a higher body temperature and greater need to sweat.

While no official record is kept on the number of horses affected by anhidrosis, people have told Gravlee that more than half of the Thoroughbred racehorses shipped to Trinidad, an island in the Caribbean, develop anhidrosis. An article on Petcaretips.net estimates that 20 percent of horses in Miami, Florida, suffer from anhidrosis.

Anhidrosis can develop at any point in a horse's life, regardless of their training and traveling schedules. "Many times they appear normal for one to two months after arriving at a new location," Gravlee says. As a horse's training, transportation and body condition changes, its risk for developing anhidrosis may vary as well.