You Can Lead a Horse to Water…
Anhydrosis in Horses
By Leesa Nacht of J&M
Consulting
Providing Information
for the Equine Industry
It's a familiar sight in
the summer months, especially around the racetrack: a horse that
is dripping with sweat and blowing hard, working at cooling off
his body after the workout. But thermoregulation involves more
than just sweating. The body also systematically moves blood
from the inner (hot) core to the outer (cooler) surfaces of the
skin. Blood flow, panting, sweating; all of these mechanisms
work together to cool the horse, but sweating is the predominant
method. The horse is the only mammal, other than humans, that
cools himself by sweating. As the body heats up from exercise,
sweat is produced, which evaporates from the skin resulting in
a lower body temperature. Even short term, submaximal exercise
can cause an increase in body temperatures. Once internal body
temperatures rise to above 105o F, heat stroke and other thermal-related
injuries become an issue. In hot weather, always ensure that
there is an ample supply of water, to allow conversion of moisture
within the body to perspiration.
But what about the horse
that doesn't have this efficient cooling system? The inability
to sweat in response to work and/or body temperature increases
is called anhydrosis. Horses with this affliction are also referred
to as dry coated, puffers, or non-sweaters. Anhydrosis can develop
suddenly or slowly over a period of time, and is usually, but
not always, seen in athletic horses. This is likely due to the
greater levels of exertion; therefore an increased occasion for
expected profuse sweating. Generally, it is found in horses that
are raised in cooler climatic areas and then shipped south to
regions that have high temperatures and high humidity. It is
estimated that 20% - 30% of horses in these southern regions
are anhydrotic to some degree.
Anhydrosis was first documented
in the 1920's, when the British began to send their horses to
Indonesia, Malaysia, India, and other tropical and sub-tropical
countries. This problem is most common in Thoroughbreds, but
this may be related to the type of intense exercise (short duration,
high intensity) that most Thoroughbreds are engaged in. Recently,
there has been an increase seen in the incidence of anhydrosis
in Quarter Horses, but it is being surmised that this may be
due to the increase of Thoroughbred blood in the Quarter Horse
bloodlines.
A horse can also be known
as a shy sweater, or one that sweats lightly compared to what
a trainer may expect for the level of exercise. The amount that
they sweat is not enough to properly cool the body. Usually the
first sign of a problem with a shy sweater is a decrease in the
level of performance, possibly with some panting. In extremely
hot, and humid conditions, you can help to cool a horse, anhydrotic
or not, by using a misting machine. These devices can lower the
ambient temperature by as much as 15o F, making recovery easier,
but still not addressing the underlying condition.
A study done by the University
of Florida revealed some interesting data. A large group of horses
from the Central Florida area were surveyed. Apparently, not
only working horses can be affected. The results showed a percentage
of each various category of Thoroughbreds suffered from anhydrosis.
The incidence was greatest in training horses, then non-pregnant
mares, pregnant mares, stallions, and the lowest level of occurrence
showing up in youngsters. Though no explanation was given for
the variation, this work may lead to future keys to solving the
problem.
There are a variety of signs
to look for in an anhydrotic horse. Other than the obvious inability
to sweat, they will show an increased heart rate, increased respiratory
rate, higher body temperature (as high as 108oF) and a decreased
tolerance for exercise. Panting is also a common sign, with as
high as 150 breaths per minute for as long as 2-3 hours post-exercise.
Horses that suffer will exhibit hair loss, especially around
their face, and a dull, rough hair coat. The skin becomes dry,
flaky, and itchy because body oils are not brought to the surface
of the skin through the usual sweating mechanisms.
There is a test that can
sometimes help in the diagnosis of a non-sweater. Epinephrine
is injected into the horse's skin. This mild dose of adrenal
gland hormone will cause a normal, healthy horse to break into
a sweat in the injection area. An anhydrotic horse will show
no response. The problem with this test is that there is a wide
range of results between individuals and these results could
also be influenced by the ambient weather conditions.
Ralph Beadle, DVM, PhD.,
from Louisiana State University has spent many years working
on the possible cause(s) of anhydrosis. It has been suggested
that epinephrine could be related. First, one needs to understand
how the sweat glands work inside the body. Each sweat gland has
beta-2-receptors. These receptors cause the gland to react when
stimulated and the result is that the horse perspires. Epinephrine
is one stimulus to this system, and another possibility is through
nerves. Dr. Beadle believes it is a stress-related issue.
Increased stress in the body
can cause a down-regulation or desensitization of the beta-2-receptors
in the sweat glands. This is basically a shutting-off of the
normal process of balance or homeostasis in the body. Some big
stressors would include heat and humidity. It is possible, that
for some reason, the body releases too much epinephrine. With
the continually high levels, the beta-2-receptors will stop responding
for a while. Once in cooler weather, the receptors will up-regulate
and the horse will once again sweat normally.
There are some nutritional
factors that seem to apply to these horses as well. They sometimes
exhibit an electrolyte imbalance. Low amounts of sodium, chloride,
and potassium are found in the diets, and this can be easily
remedied. It is common to find that anhidrotic horses suffer
from hyponatremia (low sodium levels in the blood) and hypochloremia
(low chloride levels in the blood).
Unfortunately, electrolyte
supplementation does not help all of these horses, and prophylactic
use doesn't seem to make a difference. There has also been some
relief seen with the administration of thyroxine and casein,
but again, with no consistency. To date, the best and most consistent
results are achieved by sending these horses into cooler climates.
To continue to exercise them as non-sweaters is taking a huge
risk, inviting heat stroke and death. Until a definite cause
and cure can be found, careful management is the only thing that
can be done.
Republished
with permission from Canadian Thoroughbred
Other articles: Mating
Strategies |Ergogenic
Aids
