No sweat

Anhidrosis– the inability to sweat–not only makes horses uncomfortable but leaves them vulnerable to heat stroke and other serious complications.

Anhidrosis---the inability to sweat---not only makes horses uncomfortable but leaves them vulnerable to heat stroke and other serious complications.

Summertime, and the livin’ is … hot and sweaty. Your dogs are panting in the shade, and the barn cats are nowhere in sight, but you and your horse are in the ring, working up a serious sweat. After your ride, you grab your own water bottle as you hose down your horse, and when you turn him out, he takes a long drink and visits the salt block before retreating to the shade. After some rest, you’ll both be ready to go again.

This cycle of heating, sweating, cooling and recovery is such an integral part of life that it’s easy to take for granted. But it’s worth remembering how unique your horse really is. Apart from a few primates, humans and horses are just about the only animals who have sweat glands distributed over their entire bodies, and both produce copious amounts of sweat to shed excess body heat.

As with so many other things that we tend to take for granted, the true value of this system becomes evident when it breaks down. Anhidrosis, the partial or complete loss of the ability to sweat, can leave a horse unable to cool himself adequately. In severe cases, anhidrotic horses overheat quickly in hot weather, leaving them at risk for heat stress and potentially fatal heat stroke.

Even when the consequences are less dire, anhidrosis is not pleasant. “I have personally owned horses that don’t sweat at all, and they are

miserable,” says Martha Mallicote, DVM, DACVIM, of the University of Florida. “It’s not just that you can’t ride them; these poor horses have a daily grind of overheating.”

Although it’s not likely that your horse will lose his ability to sweat--only a small percentage of horses are diagnosed with anhidrosis each year--it’s wise to become familiar with the early signs of the condition and to do what you can to make sure that he can maintain a normal body temperature, no matter how hard he works or how hot the weather gets.

COOLING SYSTEM FAILURE

Why some horses develop anhidrosis, while others kept in identical conditions don’t, is not yet understood. When the condition was first identified in the 1950s, the problem was thought to arise in the thyroid gland, a theory that has since been disproved. Other potential hormonal causes have also been ruled out.

“We now think it is not related to alteration in the systemic signaling for sweat,” says Mallicote. “The physiologic response to being hot, and the body’s signaling for sweat via epinephrine and other hormones still occurs in anhidrotic horses; those hormone levels seem to be adequate. The problem seems to be at the sweat glands themselves, but we really don’t know what is actually going on with the sweat gland.”

Anhidrosis can occur in horses of any age, color, gender or breed, but research currently underway at the Brooks Equine Genetics Lab at the University of Florida suggests susceptibility to the condition is inherited. “Currently, our data shows that genetics play a huge role---more than 76 percent ---in the cause for this condition,” says Laura Patterson Rosa, DVM, who is pursuing her PhD working in the Brooks Equine Genetics Lab.

Although environmental conditions ---primarily hot, humid weather---may work as a trigger for anhidrosis, says Rosa, it is less of a factor than previously believed. “Cases often appear in the South, but the disease is not limited solely to those hot, humid areas,” she says. “We had horses enrolled [in our genetics study] from all over the United States, including New York, Washington State and other northern states as well as Canada, Ireland, the United Kingdom and Australia. This makes an even stronger case that anhidrosis has a genetic cause.”

Indeed, she says, it could be that cases of anhidrosis are more likely to go unnoticed in cooler climates where horses tend to be able to keep cool without a lot of sweating. Broodmares on full turnout or horses who aren’t worked hard on hot days may sweat less than others without reaching a stage of serious heat stress. “If your horse lives in Montana, you might not know it has this genetic tendency until you start selling the foals that are then shipped to Florida,” says Rosa. “Then they may show signs of heat stress in that environment because they can’t sweat normally.”

Likewise, a horse’s build---muscular or slender---or even his color can determine how evident his case of anhidrosis may be. Smaller, more lean bodies are more efficient at getting rid of body heat without sweating, while thicker

Smaller, more lean bodies are more efficient at getting rid of body heat without sweating, while thicker bodies tend to retain heat.

bodies tend to retain heat. “Horses with a lot of muscle mass, like Quarter Horses, are affected severely if they develop anhidrosis,” says Matt Randall, DVM, of Collier Equine Vet Services in Waller, Texas. “Dark-colored horses also seem to be more at risk. I’ve seen anhidrosis in endurance horses but have never had to treat an Arabian for this; it’s mostly been Quarter Horses and a few Friesians.”

SPOTTING THE SIGNS

Anhidrosis may be acute or chronic. “In acute cases, the horse may have a problem for a summer or two but then might revert back to normal. Many of these horses will shut down and quit sweating, and then a month later the sweating turns back on---an immediate reversal,” says Mallicote. “Non-sweating doesn’t always affect the horse for the rest of his life, especially if the owner can make changes in the management and environment to give the horse a break from the heat.”

If the anhidrosis reappears for three or more summers, the case is considered chronic. “Once a horse becomes chronic, we see changes in the structure of the sweat glands,” says Mallicote. “They not only suffer from whatever the original problem might have been that caused the inability to sweat, but their sweat glands also start to atrophy and can’t work properly. Once the horse slips into that chronic situation, it’s much harder to switch back to normal.”

Even in chronic cases, the problem is typically seasonal. “Even severely anhidrotic horses may revert in cool weather during winter,” says Mallicote. “They may be able to sweat a little--but then the anhidrosis returns the next summer.”

Anhidrosis usually develops during the hottest part of the year, and the inability to sweat may appear quickly or gradually. It’s normal for a fit horse to sweat less when he exercises, even in hot weather, so the first evidence of anhidrosis can be easy to overlook. Catching subtle signs earlier in the process is best. Here are some indications:

Changes in sweat patterns. A horse developing anhidrosis may continue to sweat in patches---often behind his ears, under his mane, under the saddle pad, and at the elbows and flanks, but there will be little or no moisture over large swaths of his body, and his skin will feel hot and dry.

Fever. Another early sign is an elevated rectal temperature that doesn’t subside within about 30 minutes after exercise stops. “Sometimes the first thing that’s noticed is that the horse has a fever; the body temperature is higher than normal because the horse can’t cool himself,” says Mallicote. “Even if no one notices that the horse isn’t sweating as much as other horses, if it’s a barn where they are checking temperatures daily--which is always a good idea---they would identify a horse with a fever. Then when we investigate further, with a good history, we usually realize this horse is anhidrotic.”

Elevated respiration. You might notice that the horse is breathing quickly, with flared nostrils. “In severe cases you walk out to the pasture and the horse is just standing there almost panting, with respiration of 100 or higher,” says Mallicote. “These horses

may look like they have heaves because they are breathing so hard and not sweating. You take the temperature and it’s high, but the first thing you see is increased respiratory rate.”

Dry, flaky skin and hair loss. One function of sweating is to carry oil from the sebaceous glands to the skin and hair. When sweating diminishes, the skin gets drier and itchier and hair may begin to fall out. “As a horse becomes chronically affected, hair quality gets worse,” especially on the forehead, Mallicote says. “This is very odd, but that’s the one place that they all seem to get scurfy skin and have hair loss. It’s a fairly reliable sign; when I look at horses who have patches of hair loss on the forehead and ask the owner or trainer if this horse sweats, they usually say, ‘Not really.’ This is far from being a diagnostic test, but it seems very common.”

Lethargy. In a mild case, the horse may still sweat all over his skin but not enough to cool himself, so his ability to work hard will be limited. A rider might notice the horse’s performance slipping as the weather gets hotter and more humid. Anhidrotic horses are likely to show other signs, including loss of appetite, reduced water intake and general depression.

TREATMENT OPTIONS: CARE AND COMFORT

Because researchers have not yet identified the specific cause of anhidrosis, treating the condition remains an ongoing challenge. “I wish there was a ‘silver bullet’ treatment that would help, but there isn’t,” says Mallicote.

Nevertheless, some horses are helped by supplements formulated

to address anhidrosis, and trying them out might be a case of “can’t hurt, might help.”

Mallicote suggests checking the labels of supplements and asking your veterinarian about any ingredients you’re not sure about. “Reputable companies market supplements that just have electrolytes or amino acids. They are not going to hurt the horse,” she says. “I have seen some dubious products, however, that say they contain epinephrine or other drugs, and these could possibly be dangerous.”

Randall recommends light salt, a mixture of sodium and potassium chloride. “Adding potassium chloride to the feed does seem to help,” says

Randall. “For treatment of an anhidrotic horse through the summer we generally try a supplement specifically formulated for non-sweating horses and potassium chloride. That helps a fair number of them. It’s always trial and error trying to find something that helps a certain horse.”

Acupuncture is another anhidrosis treatment people have tried, but recent studies at the University of Florida have not supported the practice. “The manuscript detailing that study is currently being edited for publication,” Mallicote says. “Several years ago we thought it might be helping, but in our study we found essentially no response with acupuncture.”

Some horses with anhidrosis are helped by feed supplements formulated to address the condition.

MANAGING TO KEEP COOL

For now, the best way to manage a horse with anhidrosis is to find ways to keep him cool---the idea is to make sure he can maintain a healthy body temperature without the need to sweat. The best strategies for your horse will vary, depending on how severely he is affected as well as your local climate conditions---including daytime and nighttime temperatures as well as humidity.

Provide plenty of shade and air circulation. Make sure your turnout areas offer deep shade so horses can escape the sun. Run-in sheds need to be well ventilated---consider installing vents and/or gable fans to keep the air moving.

Wet your horse down. Periodic hosing or sponging provides a horse with the benefits of evaporative cooling without the sweating. “If horses at pasture don’t have a pond to go into, use a sprinkler on a timer so it will go off every so often,” says Mallicote.

Switch to evening turnout. If your nights are cool, keep the horse in a stall during the days and turn him out at night. “If the ambient temperature cools off at night, this helps,” says Randall. “With my own horses in the summer, I sometimes keep them indoors with fans during the day and turn them out at night. My barn is pretty open so they still get some sunlight but are not baking in the sun.”

Use cooling garments or therapies. Horse blankets or sheets designed to provide evaporative cooling can help keep a horse cool and comfortable. These sheets, which are wetted down just before or just after being put on the horse, are made with high-tech fabrics that amplify the power of evaporative cooling.

Install artificial cooling technology. “Some people with valuable performance horses build an air-conditioned stall,” Mallicote says. “That’s a big investment, but it can really help.” Fans with misting systems that inject water into the airflow are another way to provide evaporative cooling. A number of models are available that can be used both inside the barn and outside in turnouts or riding rings.

“Fans with misters are readily available today,” says Mike Foss, DVM, of the Alpine Veterinary Hospital in Hood River, Oregon. “When we have summer Olympics or international equestrian events in hot, humid climates, these fans are used a lot. They first came into use during the Atlanta Games in 1996; a lot of research on heat stress was done just prior to that event, to make sure we could cool the horses adequately.”

Encourage water consumption. Drinking plenty of water is important for any horse in hot weather but especially one with anhidrosis. “Always have cool, fresh water available,” says Mallicote. “Free choice salt---especially loose salt---or adding an electrolyte mix to grain meals will help maintain adequate concentration of electrolytes in your horse’s body and encourage him to drink. Frequent or continuous access to good-quality hay also encourages horses to drink and stay hydrated.”

Plan any riding for the coolest part

of the coolest day. It’s a good idea to hose the horse down prior to riding to give him a jump start on evaporative cooling, and wet him again periodically as you ride so he doesn’t overheat. If you’re planning a trail ride, choose a path that takes you through shady areas past rivers or natural water sources where you will have the option to cool your horse down, if necessary.

As a last resort, if your horse is constantly miserable despite all your efforts, the best solution may be to move him to a cooler climate. “For some horses anhidrosis can be life-threatening, and they have no quality of life in a hot climate,” Mallicote says. “Change of environment may be necessary, just as with a horse that has really bad heaves.”

In the long, hot days ahead, keep an eye out for subtle signs of sweating problems. Fortunately, if you catch a developing case of anhidrosis early, and take measures to keep the horse cool, chances are good that he will revert to normal relatively quickly. And then you and he will be able to continue enjoying summer weather for years to come.