Hot weather has already hit some parts of the country. An athletic horse performs well based on its ability to dissipate heat from the muscles during exercise, particularly when performing athletics involving speed (racing) or duration (endurance). Fitness and conditioning develop a horse’s cardiovascular system to maximize movement of heat from the depths of working muscles to the skin where the heat is then evaporated from sweat glands. 

“Evaporative cooling” uses the liquid in sweat to convey heat from the horse’s skin to the ambient air, accounting for 65 percent of a horse’s cooling process. The lungs might blow off as much as 25 percent of internal heat. Release of the heat load through the respiratory tract is evident as a hot horse has high respiratory rates after exertion.

While conditioning develops a horse’s ability to perform as efficiently as possible, climate has considerable influence on the potential to develop heat stress. Horses exercising in hot and humid climates might exhaust their sweat glands, leading to overheating problems during training and exercise. An inability to sweat is referred to as anhidrosis; a partial decrease in sweating is referred to as hypohydrosis.

A study in Thoroughbreds in central Florida identified a 6% incidence of anhidrosis. For some individuals, it is difficult to prevent. Strategies aimed at prevention include:

• Developing a horse to a suitable “fit” condition prior to the start of the warm weather season.
• Training the at-risk horse early in the morning when it is cooler.
• Cooling the horse off as soon as possible after training, using fans. Misting fans reduce the ambient temperature by as much as 10 degrees Fahrenheit. Air conditioners help to rest a horse’s sweat glands.
• Body clipping the horse to reduce the insulator layer of hair to improve heat dissipation.
• Maintaining a horse’s hydration is essential for efficient sweat production. Offer salt and water free choice. Although horses with anhidrosis usually have normal plasma concentrations of electrolytes, supplementing with potassium chloride (KCl) found in Lite salt is purported as useful to stimulate water intake.

Another notable finding is relevant: There seems to be a connection between small airway inflammatory disease and anhidrosis in some horses, so respiratory health strategies are important:

• Use bronchodilators, such as clenbuterol, for breathing improvement.
• Manage the environment to promote respiratory health—allow plenty of time outdoors, promote excellent stable hygiene, ventilation and fans in the stable.

Stress also contributes to the development of anhidrosis, so keep stress to a minimum.

Probably the most effective treatment for anhidrosis is to move the horse to an environment that is farther north and less tropical or sub-tropical. Once the horse’s sweat glands begin to function normally, he can return to the warmer climate and re-acclimate.