Aug 16 2019

Anhidrosis by Dr. Christy Moore

Anhidrosis is a condition where horses do not sweat or sweat very little in situations where
unaffected horses would normally sweat. The exact cause of anhidrosis has not been
definitively proven, however the most commonly accepted explanation is exhaustion of the
sweat glands after constant stimulation. It has been suggested in acute cases that there is a
loss of response of the sweat gland to stimulation from the adrenals. Chronic cases show
changes in the structure of the sweat glands themselves, so it has been suggested that these
horses may never fully be able to return to normal sweating. Prevalence for anhidrosis has been
suggested to be about 5%, however the condition is more common among horses living in hot,
humid climates such as that of Florida. Being born and raised in a hot, humid climate, however,
does not protect a horse from developing anhidrosis.
Horses with anhidrosis present with exercise intolerance, panting/increased respiratory rate,
high temperatures after exercise, and little to no sweating when other horses are lathered.
Horses rid themselves of a large percentage of excess body heat via sweating, so non-sweaters
will pant to attempt to give off some of their excess heat via the respiratory system (much like
dogs). Horses may occasionally pant in addition to sweating when hot, so this clinical sign is not
enough to diagnose a non-sweater alone. For example, round-barreled, dark-colored horses
give off heat less efficiently than leaner horses (Thoroughbred or Arabian-like build), so will
sometimes pant to aid in this process. If you have concerns about whether your horse is
affected, take a rectal temperature in addition to observing sweating patterns. If your horse has
a normal temperature, then lack of sweat after work may be acceptable. If your horse has a high
temperature and is not sweating, however, consider having him or her evaluated by a
veterinarian. Additionally, horses with anhidrosis may have a poor hair coat that is dry, flaky,
and thin. Diagnosis of anhidrosis is often made solely on the basis of clinical signs. Your
veterinarian may also perform an intradermal injection (into the skin) of a compound called
terbutaline, which causes the sweat glands to produce sweat when the sweat glands and
signaling pathways are functioning normally. Non-sweaters will not produce sweat or will
produce very little sweat compared to normal horses in response to this injection; this is a more
definitive method of diagnosing the condition.
There are many suggested treatments for anhidrosis, however the only proven effective
treatment is moving the affected horse to a cooler, more temperate climate. In acute cases this
will allow the signaling pathway to the sweat glands to recover by way of removing excessive
stimulation. One supplement, One AC, has been found to be helpful in treating the condition in a
prospective research study. Acupuncture therapy has been suggested anecdotally to help and
there is some research to support its use in cases of anhidrosis. Dark beer and a multitude of
other supplements have been suggested anecdotally to help, however, for the most part, there
have not been any prospective research studies to support the use of these.
The take-away message with anhidrosis is that catching it early may keep the horse from
becoming chronically affected. If it is not possible for affected horses to change environments,
environmental modification (fans, riding at cooler times of the day, electrolyte supplementation)
in addition to the use of supplemental therapies such as One AC and acupuncture, is key to
controlling the excessive activation of the sweat glands that we believe leads to the condition.

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