Anhidrosis in Horses

After a hard ride, have you ever noticed your horse not sweating enough? It is 90+ degrees outside and humid; why is he not drenched!? You only found sweat under the saddle pad while his neck lacked any sweat or lather. Over the past couple of weeks, you’ve noticed that your horse is sweating less and less, but the outside temperature has climbed higher and higher. This is abnormal behavior for him. You’re panicked for a moment, wondering if your horse is sick. Then a friend mentions your horse may have developed anhidrosis, also known as a horse that is a “non-sweater.” In part one of our Excel Supplements blog on anhidrosis, we will explore this condition and what causes the different forms of anhidrosis.

What is Anhidrosis?

A horse’s main way of regulating their body temperature is through sweating. An anhidrotic horse lacks the ability to sweat in order to cool himself down. This is believed to caused from a disconnect in the signaling mechanism in the sweat glands. Research is ongoing on the direct causes of anhidrosis in horses. The condition is also known as “the absence of an adequate amount of sweat which will result in several clinical signs” (Hagyard). Typically, horses lose “65–70% of body heat via sweating, so the inability to sweat can have a tremendous effect on these regulatory mechanisms” (AAEP). Horses who lack the ability to sweat properly are at risk for developing heat stroke, hypothermia (severe overheating), and organ and muscle damage, which can lead to death. Horses with anhidrosis will struggle to regulate their internal temperature with the changing climate.

Humidity in the air can also be a factor — the more humid it is, the less sweat will evaporate off a horse. This leads to difficulty lowering the horse’s internal body temperature. Anhidrosis can be more discernable in performance horses, but it does not plague a competition horse more than a non-competing horse. Both performance horses and sedentary horses (like retirees, broodmares, etc.) are equally as susceptible to developing the condition. It can also make a horse’s performance and day-to-day functions quite difficult if left undiagnosed and untreated. Veterinarians can have difficulty diagnosing anhidrosis because every horse exhibits a differing degree of sweating.

Hosing a Horse

Causes of Anhidrosis

Anhidrosis typically occurs during the hot summer months. Though, scientists struggle to determine the exact cause. It is thought that it involves an overproduction of stress hormones, which in turn overstimulate a horse’s sweat glands. The severity varies depending on each horse. “A horse may only have minor decreases in sweat production, resulting in subtle clinical signs, or the horse may have a total loss of sweat production and severe signs of hypothermia” (Hagyard). More often than not, anhidrosis is commonly found in the Southeast and Gulf states in the United States. “In a recent study of non-racetrack Florida farms, 1.8% of horses were anhidrotic and 11.2% of farms reported at least one case of anhidrosis” (UFL Vet). However, the condition can pop up in cooler states as the warm weather
lingers on.

Several studies show that the development of anhidrosis does not depend on age, sex, breed, or color of the horse. “The prevalence of anhidrosis in horses is estimated to be between 2–6% of horses” (Life Data Labs). Contrary to popular belief, horses that are born in hot climates do not have a lower risk for developing anhidrosis. In addition, horses imported from other countries are just as susceptible to developing anhidrosis as locally-bred horses.

What is the Most Common Form of Anhidrosis?

The most common form of anhidrosis is incomplete or partial anhidrosis. Horses that experience incomplete anhidrosis may not perform as well or may be a bit lethargic. This decline in performance will correlate with a rise in temperature during the summer. The horse also will not sweat as much as normal. Some of the signs for this type of anhidrosis include “an elevated respiratory rate and an elevated rectal temperature that requires an extended period of time (more than 30 minutes) to return to the normal range after exercise ceases” (Hagyard).

A horse’s normal body temperature ranges anywhere between 99.5 to 101 degrees F. The normal respiration rate for a horse at rest is 8–14 beats per minute. Your horse should be able to return to normal levels within 30 minutes of exercise. If he does not, it is possible that he is struggling to regain his internal body temperature. A horse that experiences chronic anhidrosis will be extremely lethargic in the warm summer months. Researchers discovered that there are “additional irreversible changes to the sweat glands in chronically anhidrotic horses” (AAEP). This means the sweat glands become very weak and can atrophy.

Horses with chronic anhidrosis may seek the shade more than their herdmates. They also may not be able to tolerate daytime turnout at all. Chronic anhidrotic horses will exhibit a dry or “dingy”-looking coat with loose flaky skin or hair loss. This flaky skin is most often located on the forehead area. In addition, chronic anhidrotic horses exhibit a decreased interest in food or water. This can lead to dehydration and weight loss. Before you decide that your horse is anhidrotic, it’s important to consult your veterinarian first.

In Part 2 of our Anhidrosis blog, we will discuss the diagnosis process, several
treatments available, and proper management of horses affected by anhidrosis. Keep your eyes peeled for the next blog — stay tuned!


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