Anhidrosis is a condition defined by the decreased ability to sweat in response to an increased body temperature. It’s often thought to be intricately tied to high heat, but its occurrence is more selective: high temperatures must be in conjunction with high humidity. This condition is cause for concern because between 65 and 70 percent of a horse’s body heat dissipates from sweating, so the inability to cool down can cause a significant impact on health.

Signs of Anhidrosis in Horses

Though horses with anhidrosis are often called “non-sweaters,” this term is misleading: The majority of horses with the condition suffer from incomplete or partial anhidrosis, meaning they don’t sweat as much as they should, or they sweat in abnormal patterns—not that they don’t sweat at all.

This condition is most often diagnosed in performance horses that are in work, but it can affect any horse, including those that are retired and not ridden. Studies have shown that darker-colored horses are often more affected by anhidrosis than their lighter-colored counterparts¹.

This condition doesn’t appear on most horse owners’ radar until their horse has a fairly serious health episode that is linked back to environmental conditions. As temperatures rise, horses suffering from anhidrosis may show a decrease in exercise tolerance and a higher-than-expected respiratory rate for the amount of exercise that was undertaken.

These horses may also have dull, dry haircoats or be seen leaving the herd to stand in the shade alone. Lack of sweating is the most serious manifestation of the disease and often occurs after other clues are present.

As with many equine diseases, it’s better to catch the condition early, when intervention has the greatest chance for impact. Martha Rodgers, VMD, owner of Shephard Hill Equine in Lexington, Ky., recommends learning your horse’s sweat patterns in different temperatures, especially if he’s located in an area with high heat and humidity.

“With a noticeable reduction in sweat capability or pattern, diagnosing the condition earlier has typically made it easier to stimulate increased sweating with the available treatments,” Rodgers says. She notes that if the horse has a complete inability to sweat, the treatment is often less effective overall or can take much longer to see any effects.

What is the Cause of Anhidrosis in Horses?

The cause of anhidrosis isn’t clear, but it’s believed to be linked to overstimulation of the horse’s sweat glands by stress hormones, often in the heat of the summer. Anhidrosis can be brought on by a plethora of triggers, including electrolyte imbalance and overtraining².

Antihistamines and macrolide antibiotic (broad-spectrum antibiotics used against many gram-positive bacteria) use can lead to temporary anhidrosis, but the condition typically abates once the drug ceases to be administered.

The form of anhidrosis most horses experience is called chronic idiopathic anhidrosis (CIA). CIA is frustrating for horse owners as it is the most difficult form to control. A treatment for complete resolution is not yet available, but steps can be taken to make the horse more comfortable.

Researchers at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) have identified a genomic marker that contributes to CIA. The scientists discovered a genetic marker pointing to a defective potassium transporter channel³. The research team hypothesizes that sweat glands trying to use these damaged channels eventually lose their ability to sweat.

Diagnosis and Management of Anhidrosis

A horse is often diagnosed with anhidrosis by clinical observation and examination, including a medical history. However, a “sweat test” can be done to quantitatively assess how well the horse can sweat.

A sweat test involves a series of intradermal terbutaline injections, which should increasingly stimulate the horse’s sweat glands as medication concentration rises. The vet will likely also run a blood panel that includes an electrolyte analysis; this may assist in the formation of a treatment plan. A skin biopsy can also be performed, but it’s rarely necessary to look at the skin microscopically to make a definitive diagnosis.

Once a horse is diagnosed with anhidrosis, he must be given supportive care to assist with thermoregulation. Management changes are often necessary to ensure the horse’s safety, as well. This doesn’t often mean moving your horse to a cooler climate, although in extreme cases, relocation may be considered.

Simple management modifications can include limiting sun exposure and minimizing turnout during the hottest time of day. Reducing training and showing schedules is often necessary, but making sure the horse isn’t ridden during the hottest part of the day may suffice and allow the horse to remain in work.

Supplements and Acupuncture

Many owners find themselves eager to try anything that could help their anhidrotic horse. You may have heard about adding a daily dark beer to his grain ration; there is no harm in this, though it hasn’t been shown to do much good.

“There are a few supplements that have shown some anecdotal success over the first few days [of use], so response to treatment can be assessed quickly,” Rodgers says. “There are other [supplements] that are typically given over a month or more, and response to treatment with those products may be harder to evaluate, as environmental conditions may change over that length of time.”

Rodgers notes that a 2010 study of Xiang Ru San, a Chinese herbal medicine, used in conjunction with acupuncture showed statistically significant benefit to all 18 horses with anhidrosis that were studied⁴. However, this study relied on owner reporting and didn’t have a control group or double-blind methodology.

A double-blind study of 44 horses conducted in 2012 by the University of Florida also found significantly improved sweat-test results from acupuncture and herbal medicine, and did have a control group. However, these results returned to baseline four weeks after stopping treatment⁵.

“Consistent use of Xiang Ru San while the horse still has some ability to sweat, albeit reduced, has been helpful in my practice,” says Rodgers. “I’ve had the most beneficial, longer-lasting results with consistent management changes and Neuro-Vet paste use soon after diagnosis.”

Unfortunately, it’s important to note that no anhidrosis treatment is one and done.

“Most of the treatments have to be ongoing until consistent increases in sweating ability are seen [which can take a month or more], or until the combined ambient heat/humidity index has declined,” says Rodgers.

Once a horse has had anhidrosis, he should be monitored whenever environmental conditions might trigger it.

Managing anhidrosis can be difficult and a drain on an owner in terms of time and finances. However, with diligent care and a dedicated veterinarian, a horse suffering from anhidrosis can often lead a safe and healthy life—no relocation necessary.

Sources

1. Hagyard Equine Medical Institute
2. University of Florida IFAS Extension
3. University of Florida IFAS Extension
4. American Journal of Traditional Chinese Veterinary Medicine
5. University of Florida

This article about anhidrosis in horses appeared in the June 2022 issue of Horse Illustrated magazine. Click here to subscribe!

The post Anhidrosis in Horses appeared first on Horse Illustrated Magazine.

When I bought my pony, Ferrous, the former owner disclosed that he had trouble sweating, but a supplement was all he needed to be healthy in the summer. In truth, I quickly realized how anhidrosis, the inability to sweat properly, affected my horse year-round despite supplements. I took him out of the show ring, kept exercise to a minimum, and largely took the summers off to keep him comfortable and healthy.

What is Anhidrosis?

Anhidrosis is a condition characterized by lack of perspiration or sweat. This is a potentially dangerous condition that, while rare, is seen often in hot or humid climates regardless of breed or gender. Working horses must sweat to regulate their body temperature adequately, so the inability to cool themselves down by sweating can greatly affect their health, resulting in heat stroke, dehydration and in some cases, death.

Signs of Anhidrosis

Generally, anhidrosis is diagnosed through clinical signs, although there are tests your veterinarian can perform to make an official diagnosis. Owners and trainers are the line of first defense as they are the ones untacking after exercise. While most horses may be quite lathered after work, a horse with anhidrosis or “no sweat” may have limited sweat marks under saddle that gradually decrease over time. For example, in spring my pony often shows sweat marks under the girth but not under the saddle pad. As the weather continues to warm and especially during heat waves, I begin to see zero sweat patterns at all. Additional clinical signs include panting or heaving, increased heart rate, dehydration of the skin, or increased body temperature over 104 degrees.

How to Care for a Horse with No Sweat

The reality of anhidrosis is complicated but should not necessarily be something to prevent you from purchasing a horse. While many animals do well on a daily supplement, often they do not do enough to achieve the perspiration that is needed to properly regulate body temperature. Horses with mild signs may take the supplement during warmer months while animals that do not sweat well regardless of temperature should include this in their feed year-round. In addition to diet and nutrition, there are a few other things horse owners can do to keep a horse comfortable.

• Stall your horse during the day, away from the heat and sun.
• Provide access to fans or misters to keep him cool.
• Hose down your horse during the hottest part of the day and allow him to naturally evaporate the water in his stall (don’t remove excess water!).
• Feed electrolytes or salt mixtures.
• Give him a lot of fresh water. I’ve recently found Hydration Hay by Forage Solutions, which is put in a bucket of water and expands to encourage them to eat and drink.
• Avoid exercise during hot, summer months. If you intend to compete during that time, an anhidrotic horse may not be the choice for you.

Dr. Ian Baines, DVM, who is on the academic staff at the School of Veterinary Medicine, University of Namibia, and is a FEI 4* endurance vet, has dealt with two cases during his career, and both showed exercise intolerance and overheating after only moderate exercise.

“Namibia has a relatively hot and dry climate (low humidity), so symptoms in anhidrotic horses are easily exacerbated,” he says. “No treatment was attempted. Literature tells us most medical treatment is experimental at best and gives poor results. Managing these cases by exercising early in the morning, using cold water sprayers and regular hosing down proved futile and both cases retired from competition. I have reports of some pretty positive outcomes from regular acupuncture, which may be something worth investigating.”

While cases are relatively rare, the eastern United States and the southeastern United States are high in humidity and cases tend to be higher. If you see signs of limited sweating, please consult your veterinarian for a diagnosis.

I constantly worry about my pony’s comfort and health. While my friends go on hunter paces and long trail rides, we cannot join because it’s dangerous for us even during the fall because of his severe signs. We have limitations that other horses don’t, but I wouldn’t trade him for the world. I’m grateful that I can give Ferrous what he needs to live his most comfortable life. I haven’t had to move us to the Arctic yet, but I certainly won’t rule out the possibility.

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