Equine Cushing’s Disease - Pars Intermedia Pituitary Dysfunction, PPID
Uncontrolled activity and enlargement of certain cells in the middle portion of the pituitary gland in the brain of middle aged and older horses can lead to PPID. These abnormal cells overproduce the hormone, ACTH, which stimulates the adrenal glands to overproduce cortisol. Excess cortisol causes several metabolic changes including insulin resistance and higher blood sugar, and a tendency to develop laminitis (founder). In addition, horses with PPID syndrome have a reduced immune response to disease.

Clinical signs are variable, but can include a heavy hair coat that may fail to shed in the spring, abnormal fat accumulations in the neck, tail head, sheath in geldings, and above the eyes, weight loss or a pendulous abdomen, increased water drinking and urinating, sweating, lethargy, irregular heat cycles, increasing incidence and severity of infections and parasitic infestations, and an increased incidence of laminitis (founder).

Diagnosis of PPID in horses with clinical signs requires blood testing. An equine panel is done to assess blood cells and serum enzymes and electrolytes. A fasting blood sugar and insulin test may be done. Pituitary gland function can be tested by taking blood samples on two consecutive days and measuring plasma cortisol. First a baseline cortisol is taken at 4-5 PM followed by an injection of dexamethasone (a type of cortisone). This should suppress production of cortisol in the normal horse. A second sample is taken at 10-12 noon the following day. Most Cushing's horses will not show normal cortisol suppression. An alternative test is a measurement of endogenous ACTH. All tests should be done between January and August as there is a natural increase in test values during fall and early winter.

Untreated, horses with PPID have gradually reducing quality of life and may be euthanized with a year or two. But treatment and management can maintain many of these horses for years.

Treatment of Equine Cushing's Syndrome is specific and for life. Treatment includes medication, a low starch diet, weight control, management of laminitis, if present, and possibly immune stimulation. The medication, Pergolide, is the most effective daily medication. The estimated cost of generic compounded Pergolide is approximately $1.00 per day. Recent studies have shown that the plant extract, Hormonize, does not have a beneficial effect on Equine Cushing's Syndrome horses. There is ongoing research on additional therapies.

Horses with PPID should be fed primarily grass hay at 1.5% of their ideal body weight. Alfalfa hay should not be more than 50% of the forage. Underweight geriatric horses may need a more customized diet with senior supplements containing high fiber and fat, and low carbohydrate. Pasture access should be limited to nighttime or early morning, or eliminated if laminitis is an issue. A balanced vitamin mineral supplement must be provided (Ranch-O-Min 1011, Purina 12-12+, or Grostrong) at 1 ounce twice daily and horses should have free access to salt and water. The concentrate Triple Crown Lite or Low Starch, SafeChoice or Sound Starce and other feeds were developed to have a very low carbohydrate source so that excess glucose is not produced. Vitamin C at 10,000 mg per day, and Vitamin E at 5000 IU have been shown to provide antioxidant protection and improved immune function in stressed horses.

More than 50% of PPID horses develop chronic laminitis. Treatment of laminitis includes specific therapeutic shoeing, anti-inflammatory medications like phenylbutazone, and controlled exercise. (See article on Laminitis).

Owners should be aware of the immune suppression effects of cortisone in Equine Cushing's Syndrome horses, and be alert to signs of infections like fever, coughing, dental problems and sole abscesses.

Horses maintained on pergolide should be retested several months after beginning medication, then once to twice yearly or after dose adjustments.

As better care increases the geriatric population of horses, Equine Cushing’s Disease, PPID, is a frequent issue in the management of older horses.
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