Anhidrosis

Is my horse overheating? Does my horse sweat enough?

Jeremy D. Hubert, BVSc MRCVS, MS, Dip ACVS
Assistant Professor, Equine Surgery
Equine Health Studies Program
School of Veterinary Medicine
Louisiana State University

You may have noticed some subtle changes last summer; your horse appeared a little lethargic, out of breath after a fairly light work session, some loss of hair around the face and neck. His appetite wasn’t quite normal and he wasn’t drinking as much as you would expect. Nothing serious just some minor things you noted. The fall came and things seemed to improve, the hair grew and the appetite came back and he seemed to have a little more energy. Come to think of it, the summer before wasn’t much different either. Maybe it’s worth investigating.

The term non-sweater or dry coat disease are synonyms for anhidrosis, which is defined as the inability to sweat effectively in response to the appropriate stimuli. Horses that live in hot, humid climates and exercise frequently are especially susceptible to this problem. There is no coat color, breed, age or sex predilection, and horses that are native to hot humid climates are just as prone to the problem as horses imported in from more temperate climates.

One of the early signs that your horse may be heat stressed is horses that appear to breathe more rapidly than normal especially for an extended period after exercise. Most horses should take about 20 to 30 minutes to cool down after exercise and should be able to return their body temperature to the normal range within this time. So if he appears to be panting an hour after working him or he always appears to be breathing faster than the rest of the horses in the barn you can be suspicious that he may be anhidrotic or partially anhidrotic. The next sign that may become apparent is that he doesn’t sweat easily or all over his body when exercised. There may still be sweat under the mane and tail, between the legs and under the saddle area but not much anywhere else. Some horses will stop sweating completely all of a sudden in an acute episode; this is more serious and attempts to keep him in a cool environment immediately are important so as to minimize the effects of overheating on his body. If your horse has recently been introduced to the southern summer climate he may sweat excessively before he stops sweating.

Horses that have had the condition for a longer period of time start showing additional changes such as dry flaky skin and hair loss. This horse probably isn’t drinking or eating well and may appear a little dehydrated.

The exact reason why a horse stops sweating is unknown at present but there are several plausible theories. Horses that are heat stressed will have high concentrations of adrenaline (epinephrine) in their blood. Adrenaline stimulates sweat glands causing them to produce and release sweat. We know that if the sweat glands are over-stimulated they simply stop working until the stimulating factors are removed and then the receptors start functioning again. The precise molecular mechanisms as to why the sweat glands stop working are not understood and therapy is aimed at slowing or reversing the glands’ malfunction.

It is important for your veterinarian to become involved to ensure that the signs of rapid breathing aren’t because of some respiratory disease and to rule out other causes that mimic these changes, especially the skin symptoms which could be mistaken for a lack of efficient thyroid function. The next step to be taken to precisely diagnose anhidrosis is what we term a “sweat test”. A sweat test involves injecting small amounts of the chemicals that stimulate the sweat glands into the skin itself, or intradermally. The chemicals are diluted out and a control injection of saline is also given as a negative control. This allows the veterinarian a semi-quantitative method of evaluating the horses’ ability to sweat. Normal horses should sweat even with the most dilute injection, whereas a non-sweater will not sweat even at the site of the highest concentration. The test results are very precise with localized sweat just in and around the area injected; it’s not a whole body response so that comparisons can be made among the different concentrations and see whether he is a non-sweater or a partial sweater.

If your horse is a non-sweater all is not lost. Some changes in the management of your horse need to be made and perhaps some adjustment in his local environment. The most important thing to do is to try and make his surrounds cooler. This means ensuring there are fans blowing on him all the time, misting fans work even better. Make sure the stall is in deep shade; if not, try cooling the roof during the heat of the day by running a hose on the roof periodically. If the horse has access to a pasture then make sure he has plenty of shade or a shelter to keep out of the sun. It might be preferable to turn him out only in the evening, night or early morning. You may continue to ride and exercise him if it’s acceptable with your veterinarian, but just do it in the early morning or late evening. After a work session help him cool off by hosing him down to promote heat evaporation. It may take a few weeks of a changed environment for things to get back to normal, and even then the problem has to be monitored closely. Throughout each successive summer attempts to manage the problem before it becomes apparent is necessary. Prevention is better than trying to resolve the problem.

The horse’s diet will have to be changed, including reduction of the amount of grain and offer as much fresh water as possible. Oral electrolyte supplements are recommended, such as lite salt in the feed and in separate water bucket are recommended. There are some commercially available supplements (One AC, MP Co, Phoenix, Arizona) that may help some non-sweaters; these contain amino acids that are believed to help the sweat glands return to normal function. There is currently no one medication that will resolve this problem. However, rest, a cool environment and a few routine management changes may make all the difference to your horses’ performance this summer. Remember, horses that are anhidrotic are that much closer to heat stroke especially if they are exercised excessively during the heat of the day. Please consult your veterinarian if you have questions regarding anhidrosis.