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Equine Cushing’s disease remains a diagnostic challenge for veterinarians and a management puzzle for many horse owners. Although commonly referred to as equine Cushing’s disease, the more correct term, pituitary pars intermedia dysfunction (PPID), better describes the condition.

“The pituitary gland drives the production of hormones in the body to maintain homeostasis, or a state of balance in the body. PPID results from the abnormal, but benign, growth of a specific region of the pituitary gland called the pars intermedia,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research (KER). “Growth of the pars intermedia occurs secondary to the degeneration of the hypothalamus, another region of the brain. The hypothalamus links the endocrine and nervous systems, playing key roles in controlling thirst, hunger, sleep, and mood.”

As the pars intermedia grows, it produces excessive amounts of various hormones, most importantly adrenocorticotropin hormone (ACTH). While ACTH performs many important tasks, among the most renowned is its ability to affect cortisol levels. Cortisol, frequently referred to as the stress hormone, controls several body functions, including maintenance of blood sugar levels when a horse is fasting, generation of energy during exercise, activation of the “fight or flight” response, and important roles in detoxification, antioxidation, and infection-fighting pathways.

Excessive levels of cortisol, however, wreak havoc on essentially every body system. Consistently elevated cortisol causes muscle-wasting, increases susceptibility to infection, and contributes to insulin resistance and laminitis, to name only a few.

Recognizing PPID

All horses are at risk for PPID. Current estimates suggest that between 20 and 33% of all horses develop PPID by the age of 20. This makes PPID the most common endocrine condition of horses.

In days gone by, veterinarians and owners relied heavily on the classic signs of PPID to make a diagnosis. Those included excessive hair growth or lack of seasonal shedding, recurrent laminitis, muscle-wasting and pendulous abdomen, recurrent infections (e.g., sole abscess, skin infections), abnormal sweating patterns; excessive thirst and urination, and behavioral changes, primarily dullness or depression. Most of these clinical signs result from excess cortisol levels circulating in the body.

With an improved understanding of PPID, owners and veterinarians now recognize early signs of PPID such as:

  • Lethargy, decreased athleticism, or poor performance;
  • Decreased muscling along the back;
  • Delayed shedding or excessive hair growth;
  • Regional deposition of abnormal fat pockets; and
  • Laminitis.

“An earlier diagnosis allows horse owners to begin appropriate management strategies to control the progression of the disease,” Crandell advised.

In cases of suspected PPID, the best test to perform is a TRH test. This test involves obtaining a blood sample, administering thyrotropin-releasing hormone (TRH), and collecting a second blood sample exactly 10 minutes later. If ACTH levels in the blood samples increase excessively in response to TRH, the test is considered positive for PPID. Alternatively, advanced cases of PPID can still be diagnosed by identifying elevated ACTH levels in circulation.

Management Recommendations

A review of nutritional strategies for horses with PPID can be found in Nutritional Management of Metabolic Diseases. In addition to diet changes, horses can be treated with pergolide, the only FDA-approved medication for PPID.

“Together with medication, dietary changes, and exercise, horses respond well to nutritional supplements designed to support the coat and hoof,” advised Crandell.

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