Sweating is important during hot weather and/or exertion to keep the body from overheating. Air exchange in the lungs removes some heat, but more than 70% of excess body heat is dissipated by sweat evaporating from the skin. Some horses in hot climates lose their ability to sweat—a condition called anhidrosis—putting them at risk for heat stroke.

This dysfunction is most common in hot, humid regions but can occur anywhere. Matt Randall, DVM, of Collier Equine, in Waller, Texas, sees cases of anhidrosis every year in Southeast Texas. “In our climate horses are sweating year-round,” he says. “One problem with constant heat and high humidity is some horses stop sweating. I grew up in Montana, and we never saw horses with anhidrosis,” he says. In arid climates temperatures cool at night, whereas regions with constant heat and humidity create more risk.

What Horses Are At Risk?

The climate in which a horse was born doesn’t seem to affect his chances of becoming anhidrotic. According to researchers in Florida, there is a genetic component to anhidrosis. “It seems to occur in all breeds, but I’ve never seen an Arab with this problem,” says Randall. “I’ve seen anhidrosis in endurance horses but never had to treat an Arab.” This may be due to their evolution in a hot country.

In Florida about 2 to 6% of horses are affected. Small horses and young horses might not be as adversely affected, because smaller bodies are more efficient at getting rid of body heat and young horses don’t usually exert themselves as much in hot weather as adults in training or performance careers, notes Randall.

In severe cases affected horses can’t tolerate being outdoors in the heat. They stand in a shady place and still can’t cool themselves enough. Less-severely affected animals might be able to tolerate the heat unless they are exerting themselves.

Horses genetically prone to anhidrosis might not have problems in cool climates, unless they relocate to hot, humid climates. “Dark-colored horses seem more at risk,” says Randall. “Any horses with a lot of muscle mass, like Quarter Horses, are affected severely if they develop anhidrosis.”

Signs of Trouble

If a horse sweats only under the mane and saddle pad, it’s not enough to cool the body. If a horse at pasture stays in the shade all day or plays in water all the time, make sure he’s not developing a problem. “These horses are trying to cool themselves,” says Randall.

Anhidrosis can appear quickly or gradually. In a mild case the horse might sweat but not sufficiently. The rider or trainer might notice performance slipping as weather gets hotter and more humid. If the horse must sweat continually to cool himself (when working hard or in a hot stall), sweat glands work overtime and eventually shut down; the skin remains dry. He might have fast respiration, trying to create more air exchange in the lungs, and an elevated temperature. He might have patches of sweat behind his ears, under his mane, or at the elbows and flanks but no moisture over his body. He has poor performance, and it takes a long time to cool out after exercise.  

Anhidrosis can be acute or chronic. A horse might quit sweating suddenly, then a month later starts sweating again. It doesn’t always affect a horse for the rest of his life, especially if the owner/trainer can make changes in management and environment to give him a break from the heat.

Affected horses might have problems for a summer or two, but then revert back to normal. Once a horse becomes chronic for several summers, however, changes occur in the sweat glands. They start to atrophy, and it’s much harder for them to switch back to normal.

Treatment

“There are multiple ways to treat these horses, and I tell people that if there are many ways to treat a condition, this means none of them work reliably,” Randall explains. A certain treatment might help one horse but not another.

“People have tried everything from vitamins to beer. Some horses actually respond to beer, and it helps them,” he says.

A classic treatment is to add One AC, a powdered supplement containing L-tyrosine, choline bitartrate, niacin, pyridoxine HCl, and d-calcium pantothenate, to the horse’s feed. “For some horses this product seems to work, while for others it doesn’t make any difference,” says Randall. “Adding regular electrolytes to the feed doesn’t work very well, either, but potassium chloride (lite salt) does seem to help.

“I’ve had a fair amount of luck with acupuncture,” he adds. “It definitely helps in early cases. In long-standing cases and horses that have completely shut down (not sweating at all), it doesn’t work as well.”

When you first notice a problem, be diligent about keeping these horses cool so they don’t have to sweat, giving their sweat glands a break. “If the temperature cools off at night, this helps,” says Randall. “Continuously elevated temperature and humidity is a big problem.

Using fans and misters can also help. “With my own horses in summer, I keep them indoors with fans during the day (out of the hot sun) and turn them out at night,” he says. “My barn is pretty open so they still get sunlight, but they are not baking in the sun.”

If the horse is at pasture, you could use a sprinkler on a periodic timer. A horse will appreciate the spray, which cools him with evaporation.

Take-Home Message

Horses with anhidrosis are at a greater risk of overheating, and often struggle with poor performance. It is important to monitor these horses, especially during hot and humid months, and intervene at the first sign of a problem.