Anhidrosis Dealing with the Inability to Sweat

Sweating is an important function of the horse’s body during hot weather and/or exertion to keep him from becoming too hot. A small amount of body heat is removed through air exchange via the respiratory system, 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 (anhidrosis) and are at risk for heat stress and heat stroke.

This problem was first noticed many years ago in British Thoroughbreds taken to tropical countries for cavalry use, racing and polo. A significant percentage of horses in hot, humid regions suffer from some degree of anhidrosis. This dysfunction is most common in horses in tropical regions and some areas of the U.S., especially the Gulf Coast states. Anhidrosis, which literally means without sweat, is most common in hot, humid regions but can occur as far north as Minnesota and Michigan, and in dry climates like Arizona and California.

Matt Randall, DVM of Collier Equine in Waller, Texas, sees cases of anhidrosis every year in his part of southeast Texas. “Here in our climate, horses are sweating year-round. One of the problems we run into with constant heat and high humidity in summer is horses that stop sweating. I grew up in southern Montana and we never saw any horses with anhidrosis,” he says. An arid climate cools off at night. Constant heat and humidity put a horse more at risk.

Why And How Horses Sweat

Martha Mallicote, DVM, DACVIM, Clinical Assistant Professor, University of Florida Large Animal Hospital, says horses are very efficient sweaters, though slightly less so in a humid environment. “It is very important for performance horses to be able to sweat adequately to dissipate heat from exertion,” she says.

The horse can quickly pull fluid from the bloodstream and put it onto the skin through sweat glands, to evaporate and help cool the horse. “In the horse, the sweat glands are already primed--all the time--with electrolytes, proteins, and lipids, and all they have to do is draw water from the blood. It’s ready to go. When they get the stimulus (when the body determines it is too hot and needs cooling), this triggers the glands to draw in water from the blood and then contract and expel it out as sweat,” says Mallicote.

“This is a very efficient mechanism, and it doesn’t take long for sweat glands to recover and be prepared to produce more sweat.” Horses that work hard in hot weather, especially if they are fit and in good condition, sweat efficiently and don’t lose as much fluid and electrolytes as an unconditioned overweight horse. Fit horses can keep going for miles or exert strenuously for a race and keep within a normal range of body temperature.

Which Horses Are At Risk?

There are very few risk factors regarding which horses are more susceptible—in terms of breed, color, gender, age or where they are born. It doesn’t seem to make a difference whether a horse is born into a hot climate or not. “The only association that’s been found is a genetic component. This is based on early research done here in Florida, published in 2011. We have now done additional investigation, collaborating with a researcher (Dr. Samantha Brooks) at the University of Florida,” says Mallicote.

“Beyond individual genetic connections, not much else has been identified as risk factors. There are, however, no reports of anhidrosis in Arabian horses. There may be a few that have experienced this problem, but it’s not been reported in the literature.” This may be due to their evolution in a hot country; they may have some natural adaptation to heat and more resistance to this problem than other horses.

“Prevalence is hard to accurately identify because of the effect of climate. But here in Florida, we find somewhere between 2 and 6% of horses affected. Reports from Pakistan state that as many as 13% of horses are affected. It’s difficult to put together the regional aspects and come up with a meaningful figure, but it’s a common problem in this type of climate,” she says.

Small horses and young horses may not be as adversely affected. Their smaller bodies are more efficient at getting rid of body heat and they are usually not exerting as much in hot weather like an adult horse in training or one with an athletic performance career. A larger animal has a larger volume-to-surface area and holds more heat; a large, heavy horse will overheat more quickly than a small one.

“The less severe cases, if they are out at pasture and not working, and are able to maintain a safe body temperature in their environment, might not be identified. We do see some severe cases in which the horses can’t tolerate being outdoors in the heat. They will stand in a shady place in the pasture and still can’t cool themselves enough. The less-severely affected animals may be able to tolerate the heat, unless they are being asked to work,” says Mallicote.

A horse genetically prone to anhidrosis may never have problems in a cool climate, unless you move him to a hot, humid climate. Some horses seem more prone to develop 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 a number of endurance horses but never had to treat an Arab for this; it’s mostly been Quarter Horses and a few Friesians.” The Friesians originated in a northern, cooler climate and have a lot of body mass, which makes it harder to dissipate heat. 

“Dark colored horses seem more at risk. Any horses with a lot of muscle mass, like Quarter Horses, are affected severely if they develop anhidrosis,” says Randall.

“There is also an association between use of antihistamines and anhidrosis, but we don’t know if the antihistamine is a direct cause of non-sweating or because whatever mechanism is causing these horses to not sweat is somehow associated with the allergic problem and the need for antihistamines,” says Randall.

There is also an association with certain antimicrobial drugs that are sometimes used in foals with Rhodococcus equi pneumonia. “Foals tend to have a problem in hot weather anyway, because they don’t sweat as efficiently as older horses. Young foals, especially, may be adversely affected by hot weather because they don’t thermoregulate as well as older horses,” he says. 

Amy Stieler Stewart, DVM, DACVIM, North Carolina State University, did a lot of research with anhidrosis during her residency at the University of Florida, looking at medication-induced anhidrosis, specifically the effects of macrolide antimicrobials (erythromycin, clarithromycin, azithromycin) that are often used to treat foals with R. equi pneumonia. There had been reports of adverse effects during treatment of foals for R. equi which included overheating, and in severe cases, death of those foals. “Our group at the University of Florida found that once foals began this medication, their ability to sweat normally and dissipate heat was severely diminished. This was a serious situation, since they are often treated in warm summer months and thus at severe risk for overheating,” she says.

“In our original study we found that erythromycin disrupts foals’ ability to sweat within 48 hours after starting treatment, essentially making the foals non-sweaters. Interestingly this effect persists after the treatment has stopped; foals can remain at risk of overheating for up to two weeks after stopping the medication,” says Stewart.

“We also tested the more commonly used macrolides—clarithromycin and azithromycin—and these also decreased normal sweat responses but not as severely as erythromycin. Our studies showed that all macrolides commonly used for treating or preventing R. equi pneumonia in foals suppress normal sweat responses to varying degrees. Foals that are treated with these antimicrobials should be considered at risk for hyperthermia during and after treatment. It is recommended to keep treated foals in a cool environment and out of direct sunlight while being treated with a macrolide antibiotic (erythromycin, clarithromycin, azithromycin) and that these precautions be continued for at least 2 weeks after discontinuing treatment,” she says. 

Horsemen should also keep in mind that the thermoregulatory center in newborns is not fully developed. They lack the ability to stay warm in cold weather or cool in hot weather. “Their normal body temperature tends to run a degree or two warmer than an adult horse,” says Randall.  

“Temperature of 102 degrees is not something we’d worry about in a young foal, but if it gets up to 106 or 107 degrees for very long it affects their brain; some of them are a little dumb for the rest of their life. My family had a foal like that when I was a kid; he had a bad case of Rhodococcus pneumonia with 106-degree temperature for a couple of days. His registered name was ‘106 In The Shade’ and he never was a very smart horse!”

Other Species Have Different Cooling Methods

People sweat to cool off, though they don’t have as many sweat glands as horses. Some humans also develop anhidrosis and have a hard time staying cool. “In our climate it would probably happen more if people weren’t in air-conditioned buildings all the time,” says Randall. Humans must sweat to stay cool, but horses are the most efficient sweaters as a species.

“Dogs and cats don’t sweat; dogs cool themselves by breathing faster and panting (air exchange in the lungs). Cows don’t sweat much (fewer sweat glands); they may get damp and have sweat on their nose, but mainly breathe faster to cool down,” he says. 

There are various ways to cool the blood before it gets to the brain. “One of these mechanisms is called the rete mirabile, a Latin name meaning ‘wonderful net’ to describe the vascular complex that helps cool the blood going to the brain. This is a complex of arteries and veins lying close to each other, where the blood spreads out into smaller vessels as it comes to the brain. It does this in an area close to a lot of airflow to help cool the blood. Cattle have a well-developed rete mirabile, but horses do not,” he says. 

“The only thing horses have in the head region is the guttural pouch which provides some air exchange, and a little area where the carotid artery is exposed inside the guttural pouch. Perhaps one reason it’s there may be to help cool the blood a little as it goes up to the brain. If the horse is breathing rapidly and drawing in a lot of air, this can move the air around a little. The pouch is a dead-end, however, with only one opening (air comes in and swirls around and has to go out the same opening), so it’s less efficient for cooling. Horses depend more on sweating to cool the body. Each species has its own way to regulate body temperature,” says Randall. 

Signs Of Trouble

Any horse that just sweats under the mane and saddle pad should raise suspicion, because this isn’t enough to cool the body. If a horse at pasture is staying in the shade during the day, or playing in the water all the time, you need to look more closely to make sure he is not developing a problem. “These are signs that horses are trying to do whatever they can to cool themselves and stay out of the heat,” says Randall. Some of those horses’ feet become soft and unhealthy because they are standing in mud and water all the time trying to stay cooler.

Anhidrosis usually develops during the hottest part of the year and may appear quickly or gradually. In a mild case the horse may still sweat, but not enough to cool himself, and may have exercise limitations. The rider or trainer may notice performance slipping as weather gets hotter and more humid. If the horse must sweat continually to cool himself, as when working hard or living in a hot stall, sweat glands work overtime and eventually shut down, and the horse becomes dry-skinned. He may have fast respiration and flared nostrils, trying to create more air exchange in the lungs, and an elevated temperature. 

“Sometimes the first thing that’s noticed is that the body temperature is higher than normal because the horse can’t cool himself,” says Mallicote. “Even if a person doesn’t notice that the horse isn’t sweating as much as the other horses in the barn, if it’s a barn where they are checking temperatures daily, they might identify a horse with a fever. Then when we look closer at that horse, with a good history, we usually realize this horse is anhidrotic. First, however, we look for other possibilities and make sure the horse does not have something else, like an infection, that could cause the fever,” she says.

The horse’s sweat patterns may change. This can be subtle and sometimes people don’t realize that the horse is not sweating enough. The horse may sweat a little, with a few patches of sweat behind his ears, under his mane or saddle pad, at the elbows and flanks, but no moisture over his body.  

Other signs might be dry, flaky skin and hair falling out, especially around the eyes and on the forehead. “As a horse becomes chronically affected, hair quality changes,” she says. Oils from the sebaceous glands are no longer taken out onto the skin by sweat. The dry skin may itch, and there is often hair loss on the forehead.

“This is the one place that they all seem to get scurfy skin and hair loss. It’s a fairly reliable sign; I look at horses that have patches of hair loss on the forehead and when I ask the owner or trainer if this horse sweats, they usually say, ‘Not really.’”

Generally, the most noticeable sign is heat stress; the horse can’t handle the heat and has poor performance, and it takes a long time to cool out after exercise. “In severe cases the horse is just standing there panting with respiration rate of 100 or higher. These horses can also look like they have heaves because they are breathing so hard, but not sweating. You take the temperature and it’s high, but the first thing you see is increased respiratory rate,” Mallicote says. 

There are diagnostic tests that can be done, but these are generally not needed for clinical diagnosis because the signs (particularly lack of sweating) are obvious. 

Anhidrosis may be acute or chronic. “Many of these horses just quit sweating, and then a month later the sweating turns back on--an immediate reversal. It doesn’t always affect the horse for the rest of its life, especially if the owner/trainer can make changes in the management and environment to give the horse a break from the heat.”

In acute cases, the horse may have a problem for a summer or two, but then can revert to normal. “But once a horse becomes chronic, which we find in cases that continue for the third summer and beyond, we see changes in structure of the sweat glands. They not only suffer from whatever the original problem might be (causing the inability to sweat) but also start to atrophy and can’t work properly. Once a horse slips into that chronic situation, it’s much harder for them to ever switch back to normal,” she says.

“Even severely anhidrotic horses may often revert during winter, however. They improve in cooler weather and may be able to sweat a little, but then the anhidrosis returns the next summer.”

Treatment

“There are multiple ways to treat these horses, and I tell people that if there are lots of ways to treat a certain 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. There are some horses that respond to beer, and it helps them, but there’s no consensus on what kind of beer! If a horse owner wants to go that route, it’s worth a try to see if it helps, and if it doesn’t, try another kind!”

There are also some products on the market aimed at trying to manage the non-sweating horse. One of the classic treatments that’s been in use a long time is One AC, a powdered supplement containing L-Tyrosine, Choline Bitartrate, Niacin, Pyrodoxine Hcl, and d-Calcium Pantothenate, to be added to the horse’s feed. “For some non-sweating horses this product seems to work, while for others it doesn’t make any difference. Adding regular electrolytes to the feed doesn’t work very well, either, but adding potassium chloride (Lite salt) does seem to help,” says Randall.

“Another thing I’ve had a fair amount of luck with is acupuncture. It definitely helps in early cases of anhidrosis. In longstanding cases and horses that have completely shut down (not sweating at all), it doesn’t work as well. I haven’t had any luck treating those horses. It works best when the horse has just started having a problem, especially if you are very aggressive with the acupuncture in the beginning. I use this method two or three days in a row and then once a week for a while,” he says.

It also helps if you diligently try to keep these horses cool so they don’t have to sweat when you first notice the problem, giving the sweat glands a break. “If ambient temperature cools off at night, this also helps. Continuously elevated temperature and humidity is the big problem for these horses,” says Randall.

Using fans and misters can help. “With my own horses in the summer, I may keep them indoors with fans during the day (out of the hot sun) and turn them out at night. My barn is pretty open, so they still get some sunlight, but they are not baking in the sun, or getting really fat being out at pasture 24/7,” he says.

“For treatment of an anhidrotic horse through summer we generally try One AC and potassium chloride. That helps a fair number, and if that doesn’t work, I’ve used a product called Cholodin and it seems to help some of them—though it’s very expensive. It’s always trial and error trying to find something that helps a certain horse,” he says.

“In the late spring and early summer, if a horse stops sweating, we generally start them on the One AC and potassium chloride along with acupuncture a couple of times. If we start soon enough, a lot of those horses will be okay for the rest of the summer, though there are some we don’t seem to be able to help,” Randall says.

Prolonged inability to sweat puts the horse at risk for heat stroke and other problems, but if discovered early, this condition can often be reversed by giving his cooling system a break. The best way is to remove the horse from the stressful environment, taking him to a cooler climate or trying to change the conditions he’s living in. This may mean using deep shade, misting fans, and other ways to cool the horse.

“Management changes are what we recommend,” says Mallicote. If the horse is overheated, he should be cooled with water or sprayed with a fine mist (fans with misters on them work well), to bring his temperature down to normal, and then his temperature should be kept low, so he doesn’t have to sweat. Keep him out of the sun, but not in a hot stall. A portable fan may help keep him cool until his sweating reflex recovers. If corrected as soon as it occurs, anhidrosis may resolve within a few days. If a problem is longstanding, however, the horse may need help for several weeks. The horse should only be exercised during the coolest times of the day.

“If the horse is at pasture, you could use a sprinkler that’s on a timer so it will turn on every so often,” says Mallicote. The horse will take advantage of the spray of water, which will cool him with subsequent evaporation, like the effects of sweat. She also suggests putting him in a pasture with a pond so he can go in the water.

“Some people with valuable performance horses build air-conditioned stalls. That’s a big investment but it can really help. With some severely affected horses, the owners send the horse to a cooler climate. For some horses anhidrosis can be life-threatening, and they have no quality of life in a hot climate. Change of environment may be necessary, just as with a horse that has really bad heaves,” she says.