Improved preventive care means many horses’ golden years are extending beyond their 20s. Even still, one aging change is all but unavoidable: an enlargement of a part of the pituitary gland, termed pituitary pars intermedia dysfunction (PPID, also known as equine Cushing’s disease), that can wreak havoc in horses.

To bring veterinarians up to speed on the collective understanding of this all-too-common problem, Dianne McFarlane, DVM, PhD, Dipl. ACVIM, ABVP, associate professor in physiological sciences at Oklahoma State University Center for Veterinary Heath Science, reviewed the condition at the 2013 American Association of Equine Practitioners’ Convention, held Dec. 7-11 in Nashville, Tenn.

McFarlane said PPID occurs in 15-30% of horses and ponies more than 20 years old. Veterinarians have determined that certain equid types, such as Morgans and pony breeds, are predisposed to developing the condition, and their risk only increases with age. “PPID is a slowly progressive disease that can take years to show clinical signs,” she said. “Changes may begin in a horse’s midlife or early teens.”

Horses affected by advanced stage PPID display obvious clinical signs that make diagnosis simple, including a shaggy hair coat that doesn’t shed in spring, muscle wasting (especially over the topline), and a pot-bellied appearance. They tend to be docile and lethargic, prone to secondary infections (such as a chronic sinus infection), and are at an increased risk of developing laminitis. Detecting signs of early stage is more difficult because they are much more subtle. But because PPID is more manageable if identified before it reaches the advanced stage, practitioners hope to identify more effective ways to diagnose early stage disease.

When researchers first reported PPID 1932, they noted hyperplasia (abnormal overdevelopment) of the pars intermedia (PI) portion of the pituitary gland. “In a normal horse, there is a single type of endocrine cell, which is under the regulation of (the hormone) dopamine,” said McFarlane. Dopamine suppresses PI activity and inhibits release of other hormones, such as the precursor hormone POMC (pro-opiomelancortin) that cleaves to form a variety of other active hormones. To balance the system, the thyrotropin-releasing hormone (TRH) stimulates PI activity.

Before domestication horses had to cope with climatic stresses and periods of markedly reduced food availability; their hormonal function evolved accordingly. One of the PI’s main functions became is metabolic preparation for winter, McFarlane explained. In accordance with shortening daylight hours and resulting hormonal rhythm changes the PI secretes more alpha-MSH from August through October, imparting feelings of satiety (fullness) and exerting significant anti-inflammatory effects. This causes the horse to eat less and lose weight, and for his metabolism to increase.

With dysfunction related to aging and hypertrophy, the PI enlarges and increases its secretion of other hormones, including ACTH. “One important feature of horses with PPID is that there is a marked increase in oxidative stress and reduced mitochondrial (energy producing part of the cell) function in the nerve cells of the pars intermedia,” McFarlane said. The secretion of the various hormones then goes unchecked and stimulates many of the previously described clinical signs. Veterinarians prescribe pergolide (a drug that behaves like dopamine) to target the horse’s deficiency.

In addition, the enlarged PI compresses other parts of the pituitary gland, causing loss of still other hormones. This adds to the abnormal clinical effects, including systemic inflammatory conditions.

Take-Home Message

Early recognition can allow veterinarians to implement treatment before the disease progresses into the more-difficult-to-treat advanced stage. Thus, it’s advisable to have a veterinarian evaluate senior horses regularly to watch for subtle clinical signs that could signal the onset PPID.