Don’t Sweat It: Managing the Anhidrotic Horse

As summer temperatures rise, it is vital to keep a close eye on the sweating patterns of horses. Take note of those that sweat lightly or not at all, a condition called anhidrosis.

In horses with anhidrosis, the sweat glands do not function properly, greatly reducing a horse’s thermoregulation capacity. Horses release heat through sweating, exhaling hot air, and vasodilation (expansion of blood vessels). Sweating removes 65-70% of excess heat from the body. Without this essential cooling mechanism, anhidrotic horses are at a significantly higher risk for heat stress and heat stroke.

While true anhidrosis is marked by the complete absence of sweat, even after strenuous exercise, horses may also show reduced sweating and be considered anhidrotic. All nonsweaters are at risk for overheating. This is especially true in hot and humid environments in which the outside temperature does not allow for heat transfer out of the body. In humid climates where the temperature doesn’t go below 80° F (27° C) at night, predisposed horses are at higher risk of developing the signs of anhidrosis.

Recent research from the University of Florida has confirmed that genetics play a large role in a horse’s susceptibility to anhidrosis. According to researchers, anhidrosis has a heritability of nearly 76%, making it a highly heritable trait.^* Be cautious of a horse’s family history of anhidrosis when making decisions about purchasing or competing.

Signs of anhidrosis include inadequate sweat or no sweat at all when working. Anhidrosis often comes on gradually, so assessing sweat production is essential in predisposed horses. Other signs include puffing or panting during exercise, increased body temperature, elevated heart rate, and overall lethargy and unwillingness to work. Even at rest, anhidrotic horses may exhibit nostril dilation. Because of these physical manifestations, anhidrosis causes significant stress in affected horses.

If anhidrosis is suspected, a veterinarian familiar with the condition should be consulted. Aside from clinical presentation, partial and incomplete anhidrosis can be diagnosed by assessing the sweating response after injection of β2 antagonists.

While there is no cure or proven treatment for anhidrosis, many management strategies can be used to improve comfort of the horse and reduce the risk of heat stroke. The most widely recommended management strategy is to move the horse to a cooler climate, if possible. Many horses may regain their ability to sweat in cooler environments, especially if the condition is of recent onset.

In terms of nutritional management, anhidrotic horses should always be provided with plenty of fresh cool water as well as research-proven electrolytes. Certain diets may help in managing anhidrosis by producing less heat during their metabolism. For instance, fat metabolism produces less heat in the body than carbohydrate or protein metabolism, so using fat-rich energy sources, like stabilized rice bran or vegetable oil, may help anhidrotic horses. Many performance feeds are high in fat.

Ensure horses receive optimal nutrition, double-checking that all macro- and micronutrients are offered in proper amounts. Consultation with a professional equine nutritionist might be warranted.

While researchers are unsure if anhidrosis is preventable, especially given the genetic component, certain management approaches may help keep horses from developing the condition:

• Avoid overheating horses in warm climates.
• Allow 10-14 days for horses to become familiar with a warmer climate before beginning work.
• Provide plenty of cool water and supplement electrolytes.
• Ensure horses are properly cooled and rinsed after exercise.
• Exercise in the cooler times of the day, such as early morning.

Without a known cure for anhidrosis, it is important to note symptoms early and change management strategies in order to keep the horse comfortable, avoid heat stress, and prevent the condition from worsening.

*Patterson-Rosa, L., N. Walker, M. Mallicote, R. MacKay, and S. Brooks. 2019. Genetics of chronic idiopathic anhidrosis in the American Quarter Horse. Journal of Equine Veterinary Science 76:61–62. doi:10.1016/j.jevs.2019.03.064

 This article was written by Alex Trauner, a student at the University of Florida and a summer intern at Kentucky Equine Research. Learn more about internship opportunities at Kentucky Equine Research.