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Cushing’s Disease in Horses

Cushing's Disease: Symptoms, causes, diagnosis and treatment of this disease which mostly affects older horses.

Cushing's Disease, or Cushing's Syndrome, is often thought of as a disease that only afflicts older horses, however, it has been known to occur in horses as young as eight years old.

Symptoms of Cushing's Disease
Horses with Cushing's 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 Cushing's 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 Cushing's 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 Cushing's Disease are often more susceptible to other diseases because their immune system has been compromised.

What Causes Cushing's Disease?
Cushing's 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.

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Diagnosing Cushing's Disease
Even though the clinical symptoms are often very obvious, a number of tests have been developed over the years to positively diagnose Cushing's 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 Cushing's 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 Cushing's 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 Cushing's 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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