We should care because pituitary pars intermedia dysfunction (PPID) is an endocrine disorder that affects nearly 20% of equids greater than 20 years of age, and more than 30% of equids greater than 30 years of age.

If you plan to keep your horse until these golden years, there is a fair chance you will be faced with decisions of whether you should pursue diagnosis and possible treatment of PPID in your old timer.

The most obvious sign of PPID is a long shaggy haircoat that fails to shed. This is not a big problem as it can be controlled with good clippers and some elbow grease. However, equids with PPID can have more serious problems including recurrent infections and loss of muscle mass.

Further, about 50% of equids with PPID can develop laminitis (founder). This problem is more likely to develop when affected equids have a second endocrine disorder—insulin dysregulation. Unfortunately, laminitis affects quality of life and often leads to a decision for euthanasia.

What is PPID?

PPID is caused by age-related degeneration (death) of nerves in a part of the brain called the hypothalamus. These nerves produce dopamine. You’ve probably heard of this neurotransmitter that is released each time your phone beeps. Dopamine from these nerves travels to a special part of the pituitary gland—the pars intermedia. The pituitary gland is the master control center for all other endocrine glands in the body and sits just under the hypothalamus.

As the dopamine-producing nerves in the hypothalamus slowly die, the pars intermedia starts to grow out of control (think of Jack and the beanstalk). It produces and releases hormones that affect other endocrine glands. The most notable of these hormones is adrenocorticotropin (ACTH), that travels to the adrenal glands (in front of the kidneys) to cause increased release of cortisol. The increase in cortisol and other hormones lead to the myriad of clinical signs of PPID. How all this works is not yet fully understood.   

ACTH Testing for PPID

If your veterinarian takes a closer look at your older equid during its annual examination and vaccination visit, he or she might recognize clinical signs of PPID. Then the veterinarian might suggest measuring blood ACTH concentration to support this diagnosis.

This is your first decision: Should you pursue testing? If ACTH is elevated, this finding supports PPID. However, if ACTH is not elevated, the results do not exclude PPID.

What? How can that be?

This is where I like to compare PPID to a similar neurodegenerative disorder in people—Parkinson’s Disease. Unfortunately, you might have an older family member or relative that suffered with Parkinson’s. A subtle tremor developed at 50-60 years of age, followed by more severe problems over the next couple of decades. Parkinson’s disease also develops from degeneration of hypothalamic dopamine-producing nerves. However, these nerves are in a different part of the brain than the nerves that die and lead to PPID. Thus, horses do not shake, but they show other clinical signs. Importantly, both PPID and Parkinson’s disease develop slowly over years. Our diagnostic test for PPID (an elevated blood ACTH concentration) is often not positive until PPID is advanced.

Do You Want to Treat PPID?

If your equid has signs of PPID and an elevated blood ACTH concentration, you will be faced with the next decision of whether you want to pursue medical treatment. The medication we use to treat PPID is one of the early medications used to treat PD in people—pergolide. This drug, easily administered once a day orally, partly replaces the loss of dopamine and can improve signs and quality of life in PPID-affected equids.

Pergolide is safe with minimal side effects, although some horses can have a decrease in appetite in the initial week the drug is started. This problem is usually resolved by stopping treatment for a couple of days and starting back with a lower dose and building up to the desired dose.

It is important to understand that this drug does not reverse PPID, it only keeps it in check.

Response to Pergolide

About two-thirds of equids respond well to pergolide and have an improved quality of life for years, while response in other equids is not as favorable.

A positive response to pergolide should be apparent within the first 30-60 days after starting treatment if the dose is adequate.

One of the earliest positive responses is increased alertness and energy. Improvement in haircoat shedding will depend on the time of year treatment is started.

If you see improvement, the next consideration is the cost of lifelong treatment. It can add up over the years.

In contrast, if little response to initial treatment is observed, the dose might need to be increased. Work with your veterinarian to determine this.

Unfortunately, not all PPID-affected equids respond well to pergolide treatment, and their prognosis for happy golden years is less favorable.

It is important to understand that pergolide is not a treatment for laminitis. However, control of PPID might allow better management of laminitis. Finally, pergolide treatment is unlikely to extend the life of your senior equine friend but it certainly can improve quality of life in those final years.

Further Reading

  • Harold Schott, DVM, PhD, DACVIM-LA, has been an equine medicine clinician at Michigan State University since 1995. He has strong clinical interest in urinary tract disorders and endocrinological disorders, with a focus on PPID. He is a member of the Equine Endocrinology Group. In recent years he has been providing care to working equids in Mexico, taking students for a two-week clinical experience working with Mexican veterinarians and vet students to provide care to burros, mules, and horses in rural communities.
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