Sweating is a vitally important way to reduce body temperature during and after exercise, and especially in hot conditions. Anhidrosis is a dysfunction of the sweat gland function that renders horses unable to sweat properly. It affects horses of all breeds and ages, it can range from mild (less sweating) to severe (no sweating).

The condition is poorly understood, but has been thought by some to be caused by chronic stress, along with exercise in warm climates. This combination can result in changes to the sweat gland - an inability of the glands to respond to usual stimuli of heat and epinephrine. It usually appears to develop gradually, but sometimes can appear to come on suddenly.

The signs of a horse with anhidrosis are far more likely to be seen in a horse living in a hot, humid climate. In the US, this is most frequently the Gulf Coast States.

Horses with anhidrosis have a dry coat, even when they should be sweating, during hot weather and exercise. Another common sign is heavy, rapid breathing and nostril flaring, especially after exercise or during hot, humid conditions, again without what should be moist, sweaty skin. Anhidrotic horses also do not perform as well as they should, as their body temperatures skyrocket when they are exercised. Hyperthermia can potentially be life-threatening. The condition has earned the name “puff disease”, because horses may continue to breathe heavily long after exercise. The normal methods of body cooling have been lost and the deficit must be made up through heat loss through respiration, essentially panting like a dog to reduce heat.

Diagnosis is presumptive, based on history and clinical signs. There is at least one laboratory test used to confirm the disease in more subtle cases, and it involves the use of drugs with actions similar to epinephrine (adrenaline).

Currently, there is no known cure for anhidrosis. As the weather cools, horses' signs improve. Affected horses may need to be moved to a cooler climate, or expectations for performance may need to be reduced. Horses with this condition must be exercised in cooler times of the day.