Equine Cushing’s disease is a common affliction that affects horses over the age of 18. A typical situation might be an owner who notices her 25-year-old horse did not shed out his winter coat very well. Then over the summer, the horse seemed to have lost some weight, despite keeping his “hay belly.”

Equine Cushing’s disease, otherwise known as pituitary pars intermedia dysfunction (PPID), is a condition that results in increased levels of circulating cortisol, an important stress hormone. The disease begins when a benign tumor of the pituitary gland develops, which causes over-stimulation of the adrenal gland. This in turn leads to increased cortisol production. The problem can occur in all horse breeds, although pony breeds and Morgans may be more predisposed. Older horses, primarily in their late teens or twenties, are most commonly affected. It has been reported, however, in horses as young as seven years of age.

The most classic sign associated with PPID is delayed shedding of the winter haircoat or a persistently long haircoat, termed hirsutism. Affected horses may also lose weight and muscle condition, developing a “pot-bellied” appearance. They will often have abnormal areas of fat accumulation, even though overall they are underweight. This most commonly occurs in the neck, resulting in what is called a “cresty neck.” These fat deposits may also occur over the eyes, at the base of the tail, or in random areas over the body.

Horses with PPID are especially prone to developing laminitis, and may be more susceptible to infections or have slower healing from wounds or surgery. This susceptibility also increases a horse’s risk of parasite infections.

“The presence of a persistently long abnormal haircoat is in and of itself diagnostic for PPID since there is no other disease that results in this condition,” said Dr. Siddra Hines, a Washington State University equine medicine resident with experience dealing with this problem. “Several diagnostic tests are also available for PPID. Most commonly performed is the dexamethasone suppression test, which can be done by your veterinarian in the field. There are some limitations to this test, however. One of the most important things to keep in mind is that many false positive results occur in the fall, especially in the month of September. A false positive occurs when the test comes back positive, when in fact the horse is negative for the disease.

“The domperidone response test is a newer test that may be less prone to this problem, but it is more difficult to perform in the field,” she said. “It is not commonly used yet by most veterinarians based on practicality and the inability to get quick results. Both tests are offered at WSU, but the results of the dexamethasone suppression test can be available within 24 hours, whereas the domperidone response test requires several days.”

PPID is most often confused with Equine Metabolic Syndrome, because horses often present with nearly identical symptoms. There is no diagnostic test for Equine Metabolic Syndrome; therefore diagnosis of the condition is often contingent on a negative test result for PPID. Another disease that PPID is commonly mistaken for is hypothyroidism. Most horses that have historically been diagnosed with hypothyroidism have actually been affected by PPID.

Treatment

Once a diagnosis is made, horses are most commonly treated with a drug called pergolide, which is generally considered very effective. Response to the drug may take time, however, because the signs of PPID are somewhat non-specific or not quickly reversible. Over time, owners may notice improved shedding or loss of an abnormal haircoat, as well as improved weight gain. The dosage can be increased or decreased based on response to therapy.

Unfortunately, there is no surgery to remove the tumor that causes PPID. The condition is gradually progressive, and signs worsen over time if it is not treated. This can lead to debilitating laminitis or significant weight loss and infections.

Other treatments that may be necessary include management of laminitis with pain medications such as non-steroidal anti-inflammatory drugs (NSAIDs), and potentially special trimming and/or shoeing. PPID horses often need to be restricted from pasture, especially during the summer when the grass is lush and green, as grazing pasture can spark a flare-up of laminitis.

Horses that are losing weight will also require additional supplementation to their diet. “It is important to provide enough calories without adding high levels of sugars,” Dr. Hines said.
“This can be accomplished by utilizing complete pelleted senior diets in addition to good quality grass hay. Owners ideally should get their hay tested for levels of ‘non-structural carbohydrates,’ which should be low in feeds provided for horses with PPID or even just laminitis.”

Fat supplements such as corn oil, other commercial horse fat supplements, or high-fat feeds can also be used to increase the caloric content of a horse’s diet. Horses with a persistently long haircoat may require clipping during the summer in warm areas to prevent overheating and excessive sweating. In addition, horses with PPID must be monitored closely for injuries or infections since their immune system is impaired.

“PPID is a condition that owners should be familiar with, especially if they own an older horse,” Dr. Hines said. “Early diagnosis and treatment can often slow the progression of the disease. Horses that are treated can often go on to live relatively normal, productive lives.” One of the main limiting factors is the severity of lameness associated with laminitis, which results in a poor quality of life for a severely affected horse.

For more information about PPID, contact the WSU Veterinary Teaching Hospital at 509-335-0711. More information about the facts and myths of equine hypothyroidism will be featured in our spring 2010 issue.

The above article was reprinted with permission from the Equine News published four times a year by Washington State University's College of Veterinary Medicine. For more information contact Emmy Widman at esunleaf@vetmed.wsu.edu.