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.”

Cooling System Overload

Anhidrosis can develop at any point in a horse’s life, regardless of breed, age or sex, and repeat episodes are likely. Anhidrosis is potentially lethal since 65% of a horse’s excess body heat is released by sweating. Horses living in hot, humid regions are at greater risk than horses living in moderate climates. An abrupt move from a cool climate to a hot, tropical climate may trigger anhidrosis. Keep your horse fit, since excess weight and strenuous exercise may be factors. Your horse may need hormone therapy since there is a link between hypothyroidism and anhidrosis. If your horse stops sweating in extreme heat, keep him cool with fans, mist, shade and shelter and call your vet.

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.

When the temperatures soar, yet your horse’s coat remains dry, you may want to check for secondary signs of overheating, such as red membranes in the mouth.

Symptoms and Care
Horse owners need to be proactive in identifying anhidrosis in their horses, which means they should pay attention to their horses and know what is normal for them. Most often, the symptoms of anhidrosis occur when the horse is being worked and usually a veterinarian is not present, Gravlee notes. Some symptoms that may indicate anhidrosis include:

Lack of proper or adequate sweating.

High respiratory rate

Short, choppy, labored breathing that does not subside after adequate rest

Elevated body temperature

Hair loss around the face, neck and shoulders

Exercise intolerance

Gravlee says that a horse with labored breathing, obviously hot body temperature, and red membranes in the mouth is trying to lose excess heat. “If the horse is overheating easier than he should, that’s the symptom and it’s time to call the vet and try to help the horse.”

Potential Causes of Anhidrosis
While researchers and veterinarians are not sure what causes anhidrosis in horses, they have several ideas on what may affect the condition’s onset. Some factors that may play a role in anhidrosis development include: Hypothyroidism Atrophied sweat glands Organic iodine deficiency Nutritional deficiencies Electrolyte deficiencies

Horses with anhidrosis may take up to twice as long to cool after exercise than normal horses, Gravlee notes. Some horsemen call these horses “blowers” because of their exaggerated respiratory rate and dilated nostrils when trying to cool themselves during and after workouts.

Essentially, anhidrosis results in mild to severe heat stroke in the horse, and it becomes very important to reduce the animal’s temperature quickly to prevent permanent damage. “In an acute case, they need to put ice on the horse,” Gravlee says. Fluids may also need to be administered by a veterinarian to help keep the horse’s electrolyte levels balanced.

If you’ve observed some symptoms of anhidrosis in your horse and think he may have the condition, you should take your horse to the vet for further diagnosis. One way veterinarians can determine if a horse is suffering from anhidrosis is with skin tests.

In some cases, a cooled box stall can help anhidrotic horses begin to sweat again. “The most economical and best treatment is to put them in a box stall with plastic sheets on the walls and a window air-conditioning unit,” Gravlee suggests.

Veterinary Diagnosis of Anhidrosis

In skin tests, the veterinarian injects increasing concentrations of epinephrine into the horse’s skin. Your veterinarian will be able to determine if your horse is anhidrotic and the severity of the condition by observing  how your horse sweats in response to the various dosages. In a normal horse, a low concentration of epinephrine should induce sweating. Anhidrotic horses may only sweat after a dose of highly concentrated epinephrine or may not sweat at all. Source: “No Sweat!” by Shawn Clark, www. vet.purdue.edu/horses/anhidrosis.html.

After a horse has suffered from anhidrosis, it may be necessary to pursue long-term treatment and prevention methods in order to keep the horse healthy. “Once they’ve been affected, they’re more likely to develop it again,” Gravlee says.

According to the article by Breuhaus, some long-term treatments include using fans or mist on the horse, providing access to shade, or permanently moving the horse to a cooler climate. An article written by Shawn Clark, a veterinary student at Purdue University, suggests that supplementing anhidrotic horses with iodine casein, thyroid hormones, or vitamin E and selenium may help treat anhidrosis as well. Exercise should also be limited to cooler parts of the day, Gravlee adds.

In some horses, nutritional support may help horses with anhidrosis. Hall discovered this somewhat by accident. The Texas veterinarian was treating several barrel racing horses that had stopped sweating. “I was treating them for some other issues, and I had them on organic iodine and on the [Life Data Labs] Work + Farrier Formula.”

When his clients saw that their horses were beginning to sweat again, they asked Hall if they could increase the dosage of the Work + Farrier Formula and he agreed. “We intensified it just a little bit, and the horses got even more normal and started sweating again.”

Hall contacted Gravlee to report this unexpected turn of events and get his take on it. Gravlee was less surprised by the result, explaining the reason that the Life Data Lab product may have helped treat the anhidrotic barrel horses is because it contains tyrosine, an amino acid building block. Gravlee says tyrosine helps balance and build thyroid and melatonin hormones which, in turn, help control the endocrine system.

Hall says organic iodine and tyrosine are important factors in horses’ sweating processes. “There’s no one single thing [that controls sweating], but if there’s one important thing missing, then they can stop sweating.”

Paying attention to your horse is one of the most important things you can do to prevent anhidrosis, Hall notes. His clients’ “acute watchfulness” helped them identify the problem in their horses.

“They know their horses like mothers know their babies,” Hall said. “If their horses’ ears twitch the wrong way, they notice it and it means something. They pick up on those things.”

The earlier owners observe symptoms of anhidrosis, the easier it is to treat.

Grass grows abundant in hot, humid climates, and so do the horses who graze on it. Unfortunately, fat horses are at greater risk of anhidrosis.

“Horse owners need to be the first to look for the symptoms,” Gravlee confirms.  “Observe the horse for overheating. If they get overheated, cool them off. If you can’t get them cooled off, call the vet. The sooner you can get them to start sweating again, the better they are and the less likely they are to have the condition reoccur.”

Anhidrosis is a dangerous condition that can affect any horse. By staying vigilant and getting expert advice when you notice a change in your horses, you will be able to help identify, treat and perhaps prevent anhidrosis from harming your herd.

A Quick Check For Sweat
Dr. Frank Gravlee and Dr. James Hall agree that it is important for owners to observe their horses’ physical signs to detect the onset of anhidrosis. One easy way you can check your horse’s body heat and sweating processes is by running your hand under the horse’s mane.

This area gives you a great indication of how hot your horse is and allows you to quickly check if your horse is sweating without having to remove your saddle. “The first place to sweat with a horse is under the mane, and it’s also the last place to quit sweating,” Gravlee explains. If your horse is working hard in hot, humid conditions and is not sweating, it could be suffering from anhidrosis and may need medical attention.