Understanding Pituitary Pars Intermedia Dysfunction (a.k.a. Cushing's Disease)

Pituitary pars intermedia dysfunction--PPID, or Cushing's disease--is the most common disease of horses and ponies 15 years of age or older. Although it's not fully understood yet, researchers are learning more about how to treat and prevent it.

In a presentation at the 2006 AAEP Convention, Dianne McFarlane, DVM, PhD, Dipl. ACVIM, assistant professor of physiological sciences at Oklahoma State University's Center for Veterinary Health Sciences, discussed normal and abnormal function of the pituitary pars intermedia lobe of the pituitary gland.

"The horse has three distinct lobes of his pituitary gland--the pars distalis, pars intermedia, and pars nervosa," she began. "Each produces different hormones."

The pars intermedia produces a protein called pro-opiomelanocortin (POMC) that is converted into adrenocorticotropin (ACTH). This, in turn, is processed into several different hormones:

  • Alpha-melanocyte stimulating hormone (alpha-MSH), a potent anti-inflammatory hormone that plays a role in skin coloring, appetite/satiety balance, and fat metabolism.
  • Beta-endorphin, an endogenous (originating within the body) opioid that provides analgesia and behavioral modification and suppresses immune responsiveness and vascular tone (the degree of blood vessel constriction).
  • Corticotrophin-like intermediate lobe peptide (CLIP), which stimulates insulin release.

Seasonality of Hormones Season has been recently found to play a big role in secretion of some pars intermedia hormones in horses; this was already known in many other species (humans, hamsters, sheep, and weasels). Alpha-MSH levels are highest in the fall, coinciding with peak body weight, appetite, and body condition in sheep. This seasonal increase might occur in horses and ponies as well, "to metabolically prepare them for a decrease in accessible food observed in the wild in winter," explained McFarlane. "If so, dysregulation of this pathway might be associated with abnormalities in body weight and fat storage."

This might also explain the heavy haircoat of horses with PPID--it's literally a winter coat gone wild.

"Ponies show a much greater response to seasonal hormone changes than horses," she added.

Why is seasonality relevant? Given the increased activity of pars intermedia hormones in the fall, you're more likely to see clinical signs, false positive tests, and PPID-associated laminitis in fall, said McFarlane. This might have implications for treatment as well.

"It's possible that we might be able to treat affected horses (medically) in summer and fall when their hormones are highest, and wean them off medications in winter and spring," she theorized. "This is untested, but it's something to think about for mild cases."

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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