Anhidrosis is a common condition that frequently presents during the long, hot days of summer and is the inability of the horse to produce an adequate amount of sweat. The decreased ability to maintain proper body temperature limits athletic potential, increases risk for heat stroke, and may compound other disease processes.
The cause of anhidrosis is not well defined, but it is believed to involve over-stimulation of the horse’s sweat glands by stress hormones, typically occurring in the summer months. A horse may only have minor decreases in sweat production, resulting in subtle clinical signs, or a total loss of sweat production and severe signs of hyperthermia. Most commonly diagnosed in performance horses, it also affects non-performance horses and seems to be more prevalent in dark-colored animals.
Anhidrosis can be especially problematic in animals with coexisting medical diseases (e.g. metabolic or respiratory conditions) by increasing cortisol levels and respiratory demands.
The most common form seen is incomplete or partial anhidrosis and should be considered if a decline in performance with an increase in ambient temperature during summer months occurs. A complete history and clinical examination following exercise may be adequate to arrive at a diagnosis. Clinical signs of partial anhidrosis include an elevated respiratory rate and increased rectal temperature that requires an extended period of time (greater than 30 minutes) to return to the normal range upon cessation of exercise.
Chronic cases of anhidrosis typically present with a poor, dry-looking hair coat accompanied by a history of lethargy during the hotter times of the year. It should be noted that any horse may suffer from anhidrosis in the summer months, including brood mares and retired performance horses with more sedentary lifestyles. Affected horses will have an elevated respiratory rate at rest and will tend to seek shade if available.
A definitive diagnosis of anhidrosis can be made by injecting a series of dilutions of terbutaline. This may be useful in determining severity of the condition and evaluating response to treatment. Blood work, including an electrolyte analysis, may also be helpful in formulating a treatment plan. Lastly, a skin biopsy may be performed in severe cases to evaluate the histologic structure of the sweat glands, however, this is rarely necessary to confirm a diagnosis.
There are some treatment options for anhidrosis, and what works on one horse might not work on another. Until the horse with anhidrosis is successfully medicated, it should have limited intense exercise during the heat of the day and should be accommodated in facilities that minimize an increase in body temperature by providing shade, movement of air, misters, or even cold-water hosing. The simplest of treatments is supplementation with electrolytes based on abnormalities identified by the blood chemistry combined with environmental management. As a last resort for cases that do not respond to conventional therapies, moving the horse to a geographically less hot and humid climate may eventually restore its ability to sweat.
This article was written by Dr. Joan Gariboldi. It was first published in the Equine Disease Quarterly, published by the University of Kentucky College of Agriculture, Food and Environment Department of Veterinary Science and sponsored by Lloyd’s of London and its Kentucky agents. You may subscribe to this publication for free.