Equine anhidrosis, also known as dry coat, is a disease in which
a horse is unable to sweat in response to temperature extremes.
The disease occurs in areas where hot and humid conditions
predominate. In North America, the disease is most common in the
Gulf Coast states. It has been estimated that 20 percent of
horses in Miami, Florida are affected. Horses involved in
training are more susceptible to the disease. There is no
coat-color, age, gender, or breed predilection.

The cause of anhidrosis is unknown. One theory suggests a
malfunction in the horse’s sweat gland receptors. Low levels of
chloride and other electrolytes may also play a role in
anhidrosis. Anhidrosis has been associated with inefficient
production of thyroid hormones (hypothyroidism).

Abnormalities found in horses with anhidrosis include failure to
sweat, increased respiratory rates at rest, exercise intolerance,
elevated rectal temperature, and hair loss around the face, neck
and shoulders. The skin may also be dry and flaky in these areas.
Other signs may include lethargy, and decreased feed and water
intake. Horses can suffer from different degrees of anhidrosis,
ranging from partial sweating to complete absence of sweating.
Some affected horses may continue to sweat under the mane,
underneath the saddle, and on the abdomen.

The diagnosis of anhidrosis is based primarily on recognition of
the above abnormalities. Skin testing, using different
concentrations of epinephrine injected directly into the skin
(intradermal) can also be used. Normal horses will sweat at all
epinephrine concentrations within 30 minutes of the injection.
Horses with partial anhidrosis will sweat at only the highest
epinephrine concentrations, while the completely anhidrotic horse
will not sweat at any concentration.

The most reliable treatment for horses with anhidrosis is
environmental control. This may include providing a cooler
environment for the horse or moving the horse to a cooler region
of the country. Fans in the horse’s stall and misting the horse
with water may aid in keeping the horse cool. Some suggest that
adding electrolyte preparations containing sodium and potassium
chloride to the water or feed may be beneficial in treating
anhidrosis. Others suggest supplementation with vitamin E and
selenium, iodinated casein, or thyroid hormone.