Research at the University of Florida has shown the value of One AC, a feed supplement consisting of L-tyrosine, cobalt, niacin, and vitamin C developed by Raymond LeRoy, a biochemist from Phoenix. LeRoy theorizes that anhydrosis is caused by a depletion of dopamine in the brain.

"The available dopamine is used first by the brain, second by the cardiovascular system, and third by the sweating system," LeRoy explained. "If the horse is not producing enough dopamine to satisfy the three systems, then sweating is compromised in favor of the brain and cardiovascular system. One AC provides the necessary raw materials to compensate for the depletion, and the animal returns to the sweating function."

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WHEN THE weather is particularly hot and muggy, horses often are drenched with sweat even if they are doing nothing more strenuous than grazing. They might appear to be uncomfortable, but they are actually much cooler than the horse that is standing in the shade and panting because he is not sweating.

Anhydrosis-the partial or total inability to sweat-has been recognized in horses since the early 1920s. No definite reason has been discovered as to why some horses develop the problem, which can affect all ages and breeds. Anhydrosis is most commonly seen in horses living in America's Gulf Coast states, including Florida and Louisiana, and in South Texas.

The combination of heat and humidity appears to cause the problem; heat alone is not to blame. The condition can occur just as easily in imported horses as in horses that are native to the area.

The danger in hot, humid climates is that horses that do not receive a break from the heat can become heat stressed (compromised sweating) or stop sweating completely.

"It's important to get horses out of the heat for at least a few hours every day," noted Ralph Beadle, D.V.M., Ph.D., and professor emeritus at Louisiana State University School of Veterinary Medicine. Beadle taught at LSU's Veterinary Clinical Sciences Department of Equine Medicine for 25 years before he retired in 1999.

"If you continuously stimulate the sweat-gland receptors, eventually they quit responding," Beadle noted. "We see the problem here in Louisiana when the temperature doesn't go below 70 degrees at night, plus we have the high humidity to go with it. A nonsweater shuts down altogether, while heat-stressed horses still sweat some, but not as much as we'd like to see. If temperatures drop down into the low sixties at night for several consecutive days to weeks, then we'll see them start sweating again."

Understanding Anhydrosis

Horses are designed to eliminate excess body heat and cool themselves through the evaporation of sweat. When a horse stops sweating, he tries to cool himself through a panting form of breathing. This forces cool air to move across his respiratory tract; when water evaporates from this surface, the horse is helped somewhat, but it is not an efficient method of cooling. Even at rest, the nonsweater's body temperature may rise to between 102 degrees and 103 degrees. Add exercise to this dangerous equation and the animal's temperature can reach 105 degrees to 108 degrees. Brain damage is possible when the body temperature exceeds 106 degrees.

Signs of anhydrosis include panting, rapid breathing with noticeable flaring of the nostrils; decreased energy; and sparse, dry hair coat, sometimes with scaliness or thinned hair on the face, neck, and shoulders. A veterinarian can perform a "sweat test" to see how severely a horse is affected and to determine if the horse is actually a nonsweater or is heat-stressed.

Normally, a horse will cool out after exercise within 30 minutes. If the horse's rectal temperature remains elevated for longer than this, anhydrosis should be considered.

A combination of stressors can make a horse stop sweating. "We sweat-tested a group of horses at a racetrack and examined one that was sweating fine on Friday," recalled Beadle. "He raced over the weekend and chipped an ankle. By Monday, this horse was not sweating; the combination of heat, stress, and pain threw him into it. We don't know if horses are genetically predisposed [to become nonsweaters], but we do know that environment plays a big part in it."

A horse might sweat normally, even excessively for a few days, then shut down altogether. "Some horses will stop sweating overnight," noted Gary Norwood, D.V.M., of Backstretch Surgery and Medicine in New Orleans. A former president of the American Association of Equine Practitioners, Norwood has been involved with anhydrotic horses since the mid-1970s. "It's a very puzzling problem for those of us who work in the South."

Managing anhydrosis

Because there is no cure for this frustrating and potentially dangerous condition, managing the anhydrotic horse and its environment are the only options. The obvious, though often least practical, solution is to ship the nonsweater to a cooler, less-humid climate. When this is not possible, the key is careful, observant management, particularly before the animal stops sweating completely. Therefore:

  • Restrict physical activity of affected horses during hot, humid hours; and
  • Exercise horses only in the early morning or late evening.

Anhydrotic horses will attempt to find ways to cool off by seeking shade, lying in damp areas, splashing water on themselves from the trough or bucket, or standing in any available water. Some horses will take advantage of sprinkler systems set up in the pasture, but to monitor the horse better, the animal should be in a shaded area with moving air, such as in a stall equipped with a fan or, even better, a misting fan. Use of misting fans or simply running cool water on the barn roof can lower temperatures in the barn by as much as 10 degrees.

Norwood notes there are numerous anecdotal remedies that horsemen have employed to manage anhydrosis. Adding coffee grounds to the feed is one old Cajun trick that some claim is effective. Adding beer to the feed is another method that has proven helpful in some cases. "Lite" salt (a combination of regular salt and potassium chloride) also can be added to the horse's daily grain ration to help promote sweating.

When used to treat respiratory conditions, sweating is often a side effect of the drug clenbuterol. Because of this, some people have used clenbuterol to manage anhydrosis, but Norwood cautions against this. "If sweat receptors are over stimulated enough, they will actually become desensitized," Norwood said.

Research at the University of Florida has shown the value of One AC, a feed supplement consisting of L-tyrosine, cobalt, niacin, and vitamin C developed by Raymond LeRoy, a biochemist from Phoenix. LeRoy theorizes that anhydrosis is caused by a depletion of dopamine in the brain.

"The available dopamine is used first by the brain, second by the cardiovascular system, and third by the sweating system," LeRoy explained. "If the horse is not producing enough dopamine to satisfy the three systems, then sweating is compromised in favor of the brain and cardiovascular system. One AC provides the necessary raw materials to compensate for the depletion, and the animal returns to the sweating function."

Horses generally start sweating again within ten to 14 days of starting on One AC, noted LeRoy. He stresses that horses must be taken out of training for about three weeks when initially started on the supplement to allow the system to respond properly. Reintroduced too soon, hard cardiovascular work will deplete the amount of dopamine necessary for stimulating the sweat glands. Horses can usually be put back into training after three weeks without disrupting the sweating process. Some owners keep their anhydrotic horses on One AC year-round and simply lower the dose during the cool months.

Norwood also reports that some veterinarians have successfully used methyldopa on resistant cases when they have not had luck with other treatments.

Attentive management

Norwood has found One AC to be about 30% to 70% successful in his practice. It is important to catch the horse early and start using the supplement before he totally stops sweating. In the case of a horse that has been anhydrotic in the past, you should assume he will have this problem again and put him on the supplement before the arrival of hot, humid weather.

Going into the summer months, make sure horses are at optimum health and up to date on deworming and vaccinations. Try to have horses as fit as possible coming into the hot, humid season so they will not have to train as hard during this potentially stressful time of year.

Keep horses eating and well hydrated. Oral electrolytes often are recommended and can be added to the horse's water, grain, or both. Beadle suggests giving the horse two sources of water. This can be as simple as hanging two buckets-one with plain water and the other with water and electrolytes. Hanging two buckets in the stall will easily show you if the horse is drinking the electrolyte water. In some cases, your veterinarian may recommend administering electrolytes via nasogastric tube.