our expertise: VETERINARIAN ADVICE
Tom Hutchins, DVM, DABVP
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hat time of year has come…the grass is turning green, the
trees are blooming and the heat is on full force! It is also
that time when some horses may show signs of a condition
known by many as “nonsweaters” or “dry puffers”. This is
actually known as anhidrosis.
Anhidrosis is the inability to sweat effectively in response to
appropriate stimuli. The current theory is that overstimulation of
the sweat gland β 2 -receptors causes diminished function or a
period of unresponsiveness of the receptors.
There are no predilections for a specific coat color, age,
sex, or breed of horse. Up to 20 percent of horses may be
affected when exercising in hot, humid climates. Welcome to
Texas if you aren’t from there and for those of you who are, no
big surprise.
Some of the clinical signs that may be evident include
increased, rapid breathing rate after exercise, later combined
with a lack or reduction of sweating. Horses recently introduced
into a hot and humid climate may sweat excessively before
showing signs of anhidrosis. With acute onset of the condition,
horses may demonstrate partial or complete absence of
sweating when exposed to the appropriate stimuli. Horses
with long-standing anhidrosis may exhibit dry and flaky skin
with alopecia or hair loss. They may also be lethargic and
decreased water intake may be noted. Body areas that may
retain the ability to sweat include under the mane, saddle and
halter regions, and the inguinal, perineal, and axillary (where
the front leg connects to the chest) regions.
Heat-stressed horses may have higher-than-normal levels
of circulating catecholamines in their system. Anhidrotic horses
have significantly higher levels of epinephrine at rest as well.
These catecholamines act as β 2 -agonists (stimulators) and
may overstimulate the sweat gland receptors, which results in
either desensitization of the receptor or down-regulation of the
receptor. Horses maintained in hot, humid climates are at risk,
and exercise magnifies the risk.
It is important to get an accurate diagnosis because
respiratory diseases that cause an increase in respiratory
rate can exhibit similar clinical signs. Examples would include
chronic obstructive pulmonary disease (COPD) or infectious
causes such as pneumonia or lung abscesses. Complete blood
counts and serum chemistries may indicate dehydration and
urinalysis may show significant increases in concentration.
The good news is that there is a diagnostic procedure
available to aid in the diagnosis of the disease. Intradermal
injections in the neck area below the mane, of a specific β 2 -
agonist (terbutaline sulfate or salbutamol sulfate) in different
dilutions and a control injection of sterile saline are performed.
After 30 minutes has elapsed, the response to the injections
is interpreted. Normal horses sweat in response to all of
the dilutions, whereas anhidrotic horses show a diminished
response to some or all of the dilutions. There is no response
to the saline control injection in any horses.
Sound environmental management is an important part
and one of the truly reliable treatment options. Horses with
acute anhidrosis who exhibit signs of heat stress should be
immediately taken to a cooler environment, and attempts
to reduce the body temperature should be made. Restrict
Dr. Tom Hutchins
the horse to a stall with adequate air movement (i.e., a fan)
during the hot periods of the day. If exercise is necessary, do
so during the cooler periods of the day. After exercise, make
sure the horse is “cooled off” adequately by hosing it down
with water. Grain rations should be fed in reduced amounts
especially if high protein, carbohydrate diets are implemented.
Allow access to cool, fresh water as well as water with
electrolyte supplementation. It should be noted that horses
with anhidrosis will be prone to poor performance and will only
improve once the capability to sweat effectively has returned.
It may not occur again in a horse’s lifetime but is usually a
lifelong problem. If other β 2 -agonists such as clenbuterol
(Ventipulmin™) for concurrent respiratory problems are being
administered, consider this as a probable cause and stop
administration.
Some other treatment options include supplemental
electrolytes, especially potassium salts; can be added to the
feed or water. Amino acid supplements, especially those with
tyrosine, are commercially available. Tyrosine is necessary
for the resensitization of the β 2 -receptors. There are some
anecdotal reports of success with iodinated casein and with
vitamin E administered daily for at least a month.
The take home messages from this are:
• don’t expose anhidrotic horses, especially when
exercising, to extreme ambient temperatures
• exercise during the cooler periods of the day
• stall the horse in a cooler environment during the hotter
periods of the day
• relocate the horse to a more temperate climate
• avoid administration of oral β 2 -agonists such as
clenbuterol
Most horses respond to a change in environment and
begin to sweat normally within a few weeks. Horses that
have previously suffered from the disease will usually, but
not necessarily, become anhidrotic if exposed to hot, humid
conditions again.