One of the problems many horses face during the summer is the inability to sweat, a condition called anhidrosis. It is believed to be related to horses' lack of acclimation to hot and humid weather, but Dr. Ben Buchanan indicates in the Remuda that the condition develops in horses native to Texas just as often as those imported from cooler areas. Anhidrosis can affect a lot of horses as it is reported to occur to some degree to in up to 20 percent of horses in the southern United States, and is more common in younger than older horses. Sweating is the manner horses cool themselves, and while it is under hormonal and nervous system control, little is known about why this happens to some horses and not to others. It is thought excessive stimulation down-regulates the receptors and just basically wears them out so that no sweat is produced. Some mildly affected horses will just exhibit poor performance while many seriously affected horses have a high temperature and are breathing rapidly to attempt to cool themselves.
A sweat test is used to diagnose the condition; a small amount of a chemical known to cause sweating is injected into skin on the side of the neck, and the volume of sweating is measured. If your horse is diagnosed with anhidrosis, treatment is not easy as there are many supplements that claim to be effective for it, but unfortunately none have been scientifically proven to be effective. The only treatment that is effective is moving the horse to a cooler climate. Mildly affected horses can be worked in cooler parts of the day and keeping them under fans and misters can be helpful. So if you have a horse with a high temperature and labored breathing in the summer, it may be anhidrosis and not an infection.
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