Geriatric horse care: Pars Pituitary Intermedia Dysfunction (PPID)
- PPID is a disease that affects geriatric horses (10 years and older).
- It can affect all breeds and genders.
- Early signs can be hard to detect, but include:
- increased coat thickness
- failure to shed the winter coat
- increased thirst, urination and sweating
- Effective treatment allows many horses to live normal lives.
What is PPID?
Previously termed “Equine Cushing’s disease,” PPID is a disease of the pituitary gland that may affect more than 20 percent of geriatric horses based on a recent epidemiological survey.
The pituitary gland is a small organ on the underside of the brain that plays an important role in hormonal (endocrine) regulation of the body. One section of the pituitary, the pars intermedia, produces several substances that play key roles in the regulation of the body’s stress hormones through interaction with the adrenal gland. One of the substances produced is adrenocorticotrophin (ACTH), a hormone that signals the adrenal cortex to produce glucocorticoid hormones, such as cortisol.
Overactive hormone production
A horse’s ACTH should increase in the fall in order to boost the body’s stress response. This hormonal signaling helps the horse prepare for the upcoming winter by promoting energy or fat storage. This allows the horse to survive when food may be scarce throughout the winter. The increase in ACTH also promotes the development of a winter hair coat.
In horses with PPID, the pars intermedia develops one or more adenomas, or non-cancerous growths (hyperplasia). It is unknown what causes this growth, but the multiplication of cells increases the size of the gland and causes dysregulation of normal hormone production. Normally, the hormone dopamine helps to regulate the amount of ACTH produced by the pars intermedia. However, this regulation is decreased with PPID, which typically results in an overproduction of ACTH.
The most detrimental outcomes of the disease are a decreased immune system (immunosuppression) and an exaggerated insulin resistance that may result in laminitis.
How do I know if my horse has PPID?
PPID is a disease that usually only affects geriatric horses 10 years of age or older. The disease is progressive due to the continued growth of the pituitary adenoma, so clinical signs may go unnoticed early in the course of disease. It can affect all breeds and genders.
Signs of disease
One of the most reliable signs of PPID is an increase in or delayed shedding of a thick, winter hair coat. Horses with PPID may also show signs of increased urination (polyuria) and thirst (polydipsia). Another sign may be intolerance of heat, which may manifest as an increase in sweating.
Horses with PPID tend to have a loss of muscle mass and may develop a “pot-belly.” Horses with PPID are not typically overweight, though they may have fat deposits over their eyes, tail head, and neck.
Unfortunately, laminitis may be the first sign noted in some horses with PPID. Since PPID also causes immunosuppression, horses may experience recurrent bacterial infections such as dermatitis (“rain rot”), hoof abscesses and tooth root infections.
If you or your veterinarian suspects your horse has PPID, it can be confirmed with additional hormonal testing. The current first test of choice is the endogenous ACTH test, a measurement of ACTH in the blood compared to a normal range. There are different normal ranges if testing in the fall. However, any current illness, such as a laminitis episode, can affect the results of this test. If your veterinarian strongly suspects PPID but the test comes back as normal, they may suggest additional testing such as a TRH-Stimulation Test.
My horse was diagnosed with PPID, now what?
Fortunately, there is an effective treatment for PPID that allows many horses with PPID to live relatively normal lives. Pergolide is a drug originally developed for humans with Parkinson’s Disease that works by helping dopamine control the amount of ACTH released by the pars intermedia. The only FDA-approved product for horses is Prascend®. Horses may experience mild side effects such as decreased appetite when first starting the medication, so your veterinarian may gradually start the medication.
See your vet regularly
Because PPID is an incurable, progressive disease, your horse will need periodic reassessment and measurement of ACTH to ensure the disease is well regulated. Reappearance of clinical signs (e.g. thick hair coat) is an indication that the disease is not well-regulated and needs to be reassessed. Horses with PPID must stay on daily medication for life.
Manage your horse's diet
Another important aspect of treatment is dietary management for insulin resistance (IR). Your veterinarian and/or nutritionist should help you develop a dietary plan to decrease the amount of non-structural carbohydrates in your horse’s diet.
Are PPID and Equine Metabolic Syndrome (EMS) the same thing?
Some horses may have both diseases, but PPID and EMS are not the same condition. However, both diseases can cause IR and laminitis.