As our understanding of the equine endocrine system advances, so does the demand for research on diagnosis, discovery, and efficacy of treatment for the most common endocrine disorders equine practitioners and owners deal with: equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID, or equine Cushing’s disease).

Horse owner and practitioner education about these diseases, however, is lagging behind. There continues to be a great amount of confusion in the horse world surrounding these two endocrine disorders. It is critical to understand each of them in order to properly treat and manage affected horses. In this article we will discuss the differences between EMS and PPID as well as relevant studies under way at the University of Kentucky Gluck Equine Research Center.

Pituitary Pars Intermedia Dysfunction (PPID)

Cushing’s disease is the most common equine endocrine disorder. The veterinary community prefers to call it by the acronym PPID because it provides a more accurate name for the disorder, as the cells of pars intermedia of the pituitary gland are dysfunctional. Typically, this disorder occurs in older horses, with the average age of onset being 19 years and the frequency of diagnosis generally increasing with age. Although veterinarians rarely see PPID in horses younger than 10, it has been reported anecdotally. To understand the condition, you first have to understand how the endocrine system works.

The endocrine system is comprised of glands that communicate with other glands, which then communicate with organs–all through hormone signals sent through the bloodstream. In PPID-affected horses, the system of pituitary-adrenal gland communication functions abnormally. These horses have a loss of dompaminergic inhibition which leads to an overgrowth (hyperplasia) of cells in the pars intermedia region of pituitary gland. This lack of communication in the pituitary-adrenal gland axis contributes to the production of abnormally high levels of many pituitary hormones, including adrenocorticotropic hormone (ACTH), melanocyte stimulating hormone (alpha-MSH), and other products of a large precursor hormone called proopiomelanocortin, or POMC. Excess ACTH likely interferes with a horse’s ability to regulate cortisol synthesis by the adrenal glands. Although the exact pathophysiology remains to be determined, these processes lead to the long list of outward problems seen in affected animals. A horse diagnosed with PPID can exhibit some of these common clinical