No Sweat: Anhidrosis in Horses

Anhidrosis, the inability to sweat correctly, can spell trouble for your horse’s performance and health.
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Anhidrosis, the inability to sweat correctly, can spell trouble for your horse’s performance and health.

It’s another scorcher, with humidity to match the temperature. Twenty minutes into your ride, you’re dripping sweat and your horse is moving like a banana slug. But his coat is perfectly dry—and that dry coat should make you worry. Sweating is your horse’s main way of shedding excess body heat. If he can’t sweat, he’s in trouble.

Sweating allows your horse to regulate his body temperature by shedding excess body heat. If he can’t sweat, he’s in danger. | © Paula da Silva/

Sweating allows your horse to regulate his body temperature by shedding excess body heat. If he can’t sweat, he’s in danger. | © Paula da Silva/

The veterinary term for this condition is anhidrosis, and horses that suffer from it are sometimes called nonsweaters. Anhidrosis can come on gradually or appear all at once, and it can be mild or severe. “There is still very little understanding of the cause,” says Robert J. MacKay, BVSc, PhD, DACVIM, a professor at the University of Florida College of Veterinary Medicine. Here, we’ll explain the dangers and tell you how to help your horse if he’s a nonsweater.

Sweat: The Details

Sweating helps your horse regulate his body temperature, keeping it in a (resting) range of 98 to 101.5 F. He sweats to cool himself in hot weather, during exercise and in periods of stress such as a trailer ride. Here’s how the cooling system works:

• As the horse works, his muscles burn energy and generate heat. Circulating blood absorbs heat from the muscles and carries it to the lungs (where some dissipates when the horse exhales) and to the skin (where some radiates from the body).

• If the muscles generate more heat than the horse can unload through breathing and radiant cooling, the excess builds up. The horse’s core temperature begins to rise.

• The hypothalamus, a part of the brain that acts as the horse’s central thermostat, senses the increase and sends signals to the adrenal glands. The glands release the hormone epinephrine into the bloodstream, and it circulates to sweat glands located in the skin over most of the horse’s body.

• The sweat glands spring into action, pumping out sweat. It’s the evaporation of sweat from the skin, not the act of sweating, that cools the horse. The sweat carries heat from the skin surface as it evaporates into the air.

Horse sweat is mostly water, with small amounts of dissolved minerals (electrolytes). Uniquely, it also contains latherin, a protein that acts as a surfactant—it helps the sweat spread through the coat, so it evaporates rapidly. Horses can sweat at double the rate that people can, and for good reason: A horse has much greater body mass than a person does, and he needs to sweat more to cool that mass.

The harder your horse works, the more heat he generates and the more he needs to sweat to cool himself. A number of factors affect how well this system works.

Physical condition: Horses become better at regulating body temperature during exercise as they become more fit. Their muscles use energy more efficiently and produce less heat. They sweat more readily, and they lose fewer electrolytes through sweat.

Weather: Warm, humid air is saturated with moisture, so sweat may not evaporate quickly enough to cool the horse. He keeps sweating, but heat continues to build up in his body. (In contrast, on very dry days, sweat can evaporate almost as fast as it forms. The horse can lose much more fluid than you would think from the amount of sweat you see.)

Acclimation: Horses are better able to regulate their body temperature in hot climates when they have a chance to get used to the conditions. Research suggests that a ten- to fourteen-day acclimation period helps the cooling system adapt.

Insulation: A heavy haircoat insulates the skin and slows evaporation. This makes the cooling system less efficient on warm days in spring (before the winter coat has fully shed out) or fall (when the new winter coat is coming in) than in summer.

A hardworking horse can lose significant amounts of body fluid through sweat—as much as 12 liters (3 gallons) an hour—along with electrolytes that are essential for muscle and other body functions. His temperature may near 104 F during exercise, but it should start to drop within minutes after he stops. If it doesn’t, he’s at risk. A temperature above 104 F may lead to heat stroke, in which the horse’s cooling system shuts off and his temperature can rise quickly to 106 F or higher. The result can be multiple organ failure and even death.

Hot and Dry

A nonsweater faces much greater risk of heat stroke than a horse that sweats freely because he has no effective way to get rid of heat that builds up in his body. Panting is just not enough—without the cooling effect of evaporating sweat, his core temperature can easily reach dangerous levels. While a horse that doesn’t sweat at all faces the biggest health risks, even a mild case can hurt performance, especially in summer.

The mechanism that triggers anhidrosis isn’t well understood.  “It is thought that the response to the circulating ‘fight or flight’ hormone epinephrine is switched off in the sweat glands, although how that happens is a mystery,” says Dr. MacKay. The condition shows up most often in warm climates, and that’s not only because heat and humidity make it more apparent.

“A predisposed horse will rarely become anhidrotic in a temperate or cold climate. It takes exposure to heat and humidity for some period of time—the actual length of time is not clear—before the condition develops,” he says. “The thought is that the sweat glands get overstimulated in those conditions and shut down after constant bombardment by epinephrine in the blood.” The link to climate showed up clearly in a 2010 survey of 500 farms and nearly 5,000 horses in Florida, carried out by Dr. MacKay and his colleagues. Overall more than 11 percent of the horses were anhidrotic, but farms in hotter southern Florida were five times more likely to have at least one anhidrotic horse than farms in the cooler north.

Anhidrosis can affect any horse, but it seems to occur more often in some breeds. “In our study there was an indication that Thoroughbreds and warmblood breeds may be more susceptible than others. The interesting exception is the Arabian: In our study there was not one anhidrotic Arabian horse,” Dr. MacKay says. The research also suggests that a tendency to develop anhidrosis may be heritable, he adds. “This won’t be a simple type of inheritance, and it needs to be confirmed by more comprehensive studies, but it seems very likely that there is a genetic component to the condition.”

Trouble Signs

If your horse develops anhidrosis, here’s what you can expect to see.

Dry coat:“The condition is usually easy to diagnose clinically—horses overheat during exercise and/or hot weather but don’t develop much sweat,” Dr. MacKay says. The horse’s coat may be completely dry or damp in patches—under the tack, between the hind legs—and dry elsewhere. The dry areas feel warm to the touch.

Panting: The horse breathes harder during exercise than you would expect for the amount of work he does, and on hot days he may huff and puff even at rest, panting (taking rapid, shallow breaths) to lose body heat. Rapid, shallow breathing is common after work in warm weather even for horses that sweat, but the horse’s breathing should quickly return to his normal resting rate (in most cases below 20 breaths per minute). If it doesn’t, he could be overheated.

Subpar performance: He may seem lethargic and reluctant to work, and he tires quickly.

Thinning hair: Some nonsweaters lose hair on the face or neck and develop dry, flaky skin.

In a mild case the signs may be subtle enough to overlook—unless the horse is in a demanding training program. Event rider Boyd Martin has had a number of horses that developed the problem, among them Remington XXV, who he competed at CCI**** level for several years, and Trading Aces, a fairly new addition to his team. “Both these horses are not full Thoroughbreds, so they start at a disadvantage in the sport,” Boyd says. “They have to be performing at top levels to be competitive.” (Remington is a Hanoverian; Trading Aces (Oscar) is an Irish Sport Horse.)

To interpret subtle signs of anhidrosis, it helps to have a frame of reference, Boyd says. “Day to day, these horses sweat less and pant more than others. Because we have a number of horses in training here, it’s easy to make the comparison. Oscar always sweated, but at a point where other horses would be lathered he would be damp. He wasn’t sweating enough for the amount of work he did,” he says.

Dr. MacKay notes that if the signs aren’t clear, sophisticated tests can measure the horse’s sweating response. “Various concentrations of epinephrine or similar drugs are injected into the skin to trigger sweating, and the amount of sweat occurring over each injection site is measured by eye or by collecting it onto pads,” he says. “These responses are compared with the known responses of normal horses.”

Managing Anhidrosis

It’s easier to diagnose anhidrosis than to correct it. Many popular remedies have been reported to help, including acupuncture, nutritional supplements and a daily can of beer in the horse’s morning feed. Boyd credits daily beer and a nutritional supplement along with a new fitness program for getting Oscar on track to win at the 2014 Red Hills International CIC*** and hit the optimum time at the Rolex Kentucky Three-Day Event this spring. “His performance improved significantly,” Boyd says.

But anhidrosis is a strange condition, and it’s often as hard to know why it disappears as it is to know why it develops. “Because there is a tendency among affected horses for sudden spontaneous remissions, the drugs being used during those remissions are usually touted as miracle cures,” Dr. MacKay says. “The fact they don’t work on the next set of horses is conveniently overlooked.”

No medication or supplement has been shown to work in a scientific study, Dr. MacKay says. A recent study at the University of Florida found little benefit from acupuncture, for example. “Although there was a trend toward increased sweating, this trend was not scientifically significant nor was it persistent,” he says. User testimonials vouch for various blends of proteins, minerals and vitamins marketed as supplements for nonsweaters. But the success is anecdotal, Dr. MacKay says, and so far there’s no conclusive research to back it up. Beer? “I generally recommend that people give the beer to me. It will work just as well,” he says.

These treatments aren’t likely to harm your horse, so there’s no reason not to try them if you’re so inclined. But to really help your horse, look to changes in management. First, take steps to keep his temperature from reaching dangerous levels:

• Change his work schedule. In warm weather ride early in the morning or in the evening, when temperatures are cooler than they are in the middle of the day. Skip work on days when heat and humidity are brutal.

• Take frequent breaks in your rides. Let him walk and note how hard he’s breathing. Resume work when his breathing returns to normal, not before. Don’t let him overheat.

• Change his turnout. Turn him out at night in warm weather, if you can. Let him spend the hot part of the day in a well-ventilated barn, with fans to keep the air moving. If he must be out during the day, make sure he has shade.

• Watch his diet. “Although we don’t have proof, it is widely held that high carbohydrate concentrates and hays are a risk factor, so grass hays and low carb/high fat concentrates are favored, especially as preventative,” Dr. MacKay says. “An electrolyte supplement may be helpful.”

After work, cool him out aggressively:

• Move him into the shade.

��� Use cold water to spray or sponge him off. Cold water on the skin can reproduce the cooling effect of sweat—if it evaporates. That may happen quickly in hot, dry conditions; but in humid air the water will just sit on his skin, actually slowing heat loss. So scrape it off, spray or sponge with more cold water and repeat until the water no longer heats up on his skin.

• Keep the air moving. Moving air will help carry heat away from his skin, so set up a fan if there’s no breeze. Better yet is a misting fan, which adds water vapor to the air to lower its temperature.

• Offer water to drink. A horse with anhidrosis clearly doesn’t face a risk of dehydration risk from excessive sweating, but he still loses fluid and needs to replace it. (Contrary to old beliefs, drinking after exercise isn’t harmful. Let him drink a gallon or so, walk him for 15 minutes and offer water again.)

• Know his normal vital signs and monitor them closely during cool out. If his temperature and breathing and heart rates are falling and he’s alert and interested in grass and water, he’s recovering. If they’re still high half an hour after exercise or he seems dull or disoriented, he may be at risk for heat stroke. Put in a call to your veterinarian, who can assess the situation.

If these precautions don’t keep your horse comfortable, consider a last resort: moving him to a cooler climate. Just as a susceptible horse can develop anhidrosis after moving from a cool climate to a hot and humid zone, relocating to a less steamy environment often seems to restore a horse’s ability to sweat.

“The most important message is that this condition is better prevented than treated,” Dr. MacKay says. “Don’t overheat horses during hot humid weather. Provide shade, water, salt and ventilation. Cool horses properly after exercise with water and a scraper. Don’t feed high carbohydrate diets, and realize that any horse that has an anhidrotic relative is more likely to become anhidrotic itself.” 

This article originally appeared in the August 2014 issue of Practical Horseman.