Description

Equine Cushing's disease is one of the most common diseases in older horses. It is a disorder of the pituitary gland that results in hormonal imbalances causing a variety of clinical signs.

In Cushing's disease in humans, the major hormone produced by the abnormal pituitary gland is ACTH which stimulates the adrenal glands to produce large amounts of cortisol.

In Cushing's disease in horses, the pituitary gland produces excessive amounts of pro-opiomelanocortin (POMC) along with several hormonally-active derivations including ACTH, but the levels of cortisol are usually normal or below normal.

In most cases of the disease in equines, the pituitary gland is enlarged and overactive as a result of faulty regulation by the hypothalamus.

Because of the significant biological differences between human Cushing's and equine Cushing's disease, the name Pituitary Pars Intermedia dysfunction (PPID) is now favored by most veterinarians and veterinary scientists.

The disease tends to occur in middle-aged and geriatric horses, with an average age of approximately 20 years at the time of diagnosis. PPID has been diagnosed in horses as young as seven years and all breeds of horses can develop the disease. Ponies and Morgan horses have a higher incidence of the disease than other equines.

Without treatment, symptoms tend to worsen over time and many horses are euthanized as a consequence of laminitis, recurrent foot abscesses, or complications related to bacterial infections that occur because of Cushing's disease.

Symptoms

• A long, wavy hair coat that fails to shed according to normal seasonal patterns
• Dry, flaky skin
• Excessive sweating
• Lethargy and poor athletic performance
• Weight loss
• Muscle wasting, especially along the topline
• Drinking large volumes of water
• Passage of large amounts of urine
• Chronic recurrent laminitis
• Increased susceptibility to infections
• Delayed wound healing and repeated infections
• Infertility

Causes

In equines, the middle lobe of the pituitary gland, which is located at the base of the brain, becomes enlarged and over produces hormones, chief of which is Pro-opiomelanocortin (POMC) and several hormonally-active derivatives including adrenocorticoptropic hormone (ACTH).

Function of the pars intermedia is normally kept in check by dopamine-secreting nerve cells arising from the hypothalamus, a part of the brain that regulates body function such as thirst, hunger, water balance, blood pressure and body temperature. When the pituitary gland becomes enlarged and overactive as a result of faulty regulation by the hypothalamus, the disease, equine pituitary pars intermedia occurs.

Prevention

Currently, no scientific and veterinary research literature is available that details how to prevent equine pituitary pars intermedia dysfunction.

Common sense equine health management policies may help prevent the disease as horses grow older. Owners and caretakers can reduce the stress on the animals by making the horse's environment comfortable and safe and making any necessary changes in the horse's life gradually.

Older equines should be fed a high quality diet on a consistent schedule that allows for as much foraging activity as possible.

Having the horse's teeth, hooves, and general health checked regularly by a veterinarian and scheduling vaccinations, dewormings, and hoof care on a regular basis so as to catch any problems before they become serious will help reduce problems that may lead to PPID.

Treatment

At the first indication that a horse might be suffering from equine Cushing's disease, a veterinarian should be called and the horse tested to confirm the disease. Although no single test is 100 percent accurate, the dexamethasone suppression test and the measurement of resting plasma ACTH concentration are often used. Often the dexamethasone test is given first, followed by the ACTH concentration test to confirm the disease.

Optimal management of Cushing's disease involves a combination of medication to normalize the functioning of the pituitary gland plus supportive care to address and prevent complications associated with the disease. In both cases management will most likely be life-long because to date, treatment seldom achieves complete resolution or remission of the disease.

In the past, pergolide has been the drug of choice for treatment of equine Cushing's disease. At times the drug has been banned and at other times, the only way to get the drug has been through a compounding pharmacy, often without consistency of formulation.

With FDA approval in the latter part of 2011 of Prascend (pergolide mesylate) manufactured by Boehringer Ingelheim Vetmedica, Inc., veterinarians and horse owners now have a medication formulated and tested for use in equines with equine Cushing's disease.

Horses with PPID are often insulin resistant, so diets including sweet feed and other feedstuffs high in soluble carbohydrates should be avoided and diets that emphasize fiber and fat should be fed.

In both mildly and severely affected horses, early diagnosis and aggressive treatment of bacterial infections is imperative. Special attention to the horse's feet may help prevent development of laminitis.

While Cushing's disease is not curable by treatments that are currently available, the horse's quality of life can be improved with careful management. Shaving the horse's thick coat when the weather is warm, using effective fly prevention, and keeping the horse groomed can help make the horse comfortable in spite of the disease.