that only afflicts older horses, however, it has been known to occur in
horses as young as eight years old.
Symptoms of Cushings Disease
Horses with Cushings Disease can be easily recognized by their coarse, wavy coat
that often fails to shed out in the summer. A gelding at the barn I used to board at
suffered from Cushings Disease, and even in the heat of a Houston summer, he
had a thick coat of wavy hair.
Other symptoms are excessive thirst, combined with excessive urination. A normal
horse will drink in the region of 5 - 8 gallons per day, whereas a horse suffering
from Cushings Disease will drink as much as 20 gallons per day. Affected horses
often have a pot-bellied appearance, combined with a loss of muscle on the
topline. In addition, horses with Cushings Disease are often more susceptible to
other diseases because their immune system has been compromised.
What Causes Cushings Disease?
Cushings Disease is caused by a tumor of the pituitary gland, which is the small
gland at the base of the brain which regulates the rest of the horse's endocrine
systems. As the tumor grows, it puts pressure on the nearby hypothalmus, which is
what regulates the body temperature. This is believed to be the primary cause of
the distinctive coarse, wavy hair coat. As cells in the pituitary gland become
overactive, they excess quantities of a peptide called pro-opiolipomelanocortin
(POLMC, for short) causing the entire endocrine system to go out of balance.
Diagnosing Cushings Disease
Even though the clinical symptoms are often very obvious, a number of tests have
been developed over the years to positively diagnose Cushings Disease in horses.
These include the dexamethasone suppression test (DST) and ACTH
(adrenocorticotropic hormone) stimulation. In addition, a test which combines the
DST with a thyroid stimulating hormone release test, or TRH was developed by a
team at the University of Tennessee to eliminate the overlap of the values of
normal horses with those with pituitary tumors which was occurring in a number of
cases.
Treating Cushings Disease
The good news is that once Cushings Disease has been diagnosed, treatment is
simple, if long term, and in many cases allowing the horse to return to normal
health.
Bromocriptine mesylate, a dopamine agonist, is one of the drugs used to treat
Cushings Disease. It mimics dopamine to inhibit overproduction of activating
peptides, and it has been shown to mildly decrease plasma ACTH and cortisol
levels. There are problems in absorbtion which limit it's practical use, however, and
there are reported to be a number of side effects.
A more successful drug in the treatment of Cushings Disease is cyproheptadine, a
seratonin blocker. This is available in tablet form, which is easily absorbed into the
horse's system, making it a much more practical treatment.
The simplest way to monitor the horse's improvement is to watch the water intake
over a 24 hour period. The drug levels are slowly increased till the water
consumption returns to normal. Once the horse has shown maintained
improvement for a month, the dosage of the drug is decreased until a maintenance
dosage is reached.
It is important to note that while these drugs treat the symptoms, they do not treat
the pituitary tumor itself. Horses with mild Cushings Disease may be returned to
good health for a number of years, but eventually the tumor will compromise the
horse's life and euthanasia becomes the kindest option
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