Every summer in parts of the world where the weather is hot and humid, a significant number of horses (5-25%, depending on their location and activity level) demonstrate a reduced or absent rate of sweating. This condition is best known as anhidrosis, but goes by other names, such as “dry coat,” and affected horses are sometimes referred to as “puffers.”
Information on the subject is plentiful, but anhidrosis is nonetheless a sufficiently disconcerting and perplexing problem. Here’s what you should know.
Anhidrosis’ cause is unknown. The most plausible theory relates to a decreased or zero response to sweat gland stimulation by epinephrine (the hormone and neurotransmitter that triggers normal sweat gland activity).
Thoroughbreds, Quarter Horses, and Warmbloods seem to be diagnosed most frequently, while the condition is rarely diagnosed in Arabians. There could be a heritable predisposition to developing the condition but it is not a simple autosomal inheritance pattern. Rather, multiple complex factors probably determine an anhidrotic horse’s offspring’s likelihood of developing the problem.
The most severely affected horses have dry coats and quickly become hyperthermic (develop an increased body temperature) if exercised in high ambient (greater than 85°F or 30°C) temperatures. Less-affected horses might show signs of sweating between the rear legs, under the saddle and mane, and perhaps over the pectorals, however, the volume of sweat is less, and the rectal temperature is higher than that of unaffected horses doing the same amount of work.
About two-thirds of the heat generated during exercise is normally lost by sweat evaporation, and only 15-25% of it is lost via the respiratory system. So, the loss of sweating as a viable means of heat dissipation puts greater thermoregulatory responsibilities on the respiratory system, with the consequence that affected animals often display elevated breathing rates for much longer than normal during and after exercise and, in the worst cases, will pant or puff at very high rates (hence the name “puffers”).
It is important to realize that anhidrosis does not just occur in association with exercise. Some affected sedentary horses will show signs of reduced or absent sweating when living in hot and humid environments.
Veterinarians can make a presumptive diagnosis from the horse’s history and by observing clinical signs. A definitive diagnosis is based on a reduced sweating response to the intradermal (within or between the layers of the skin, but not subcutaneous, or under the skin) injection of small amounts of epinephrine or a related drug (terbutaline).
No specific treatment has been scientifically demonstrated to cure anhidrosis. There are a number of unsubstantiated or anecdotal recommendations, some of which might appear to help one horse but do nothing for another. However, most of these treatments pose no dangers to affected horses, and owners can experiment with them with their veterinarian’s assistance if they’re prepared to spend the money. Of those treatments, acupuncture and regular electrolyte supplementation (even though the horse is not losing them through sweat) seem to have the most anecdotal support.
If possible, send the horse to a cooler climate, as he could start sweating normally again soon after arriving there. If this is not possible, consider the following steps:
- Only exercise at the coolest times of the day. Learn to take the horse’s vital signs, and know what his “normal” is, especially after exercise.
- Frequently apply copious amounts of cold water to cool the horse after exercise. Although evaporation rates decrease in humid conditions, heat will still be transferred from the horse to the water. You must apply fresh water frequently to get the greatest cooling effect from this process.
- Provide fans, misters, or air conditioning in the horse’s stall, and keep him inside during the hottest part of day.
- If you must keep your horse outside, ensure he has shade and a water source (such as a pond or creek) in which to cool himself.
- If a horse is known to be anhidrotic, watch any related horses closely for signs of the disease.