What is PPID?
Pituitary Pars Intermedia Dysfunction (PPID) is the medical term for a condition more commonly known as Cushing’s disease. The name refers to the location of the abnormality in the body, the pituitary gland, which is a gland located at the base of the brain. The disease process that occurs in the pituitary of horses differs from what we call Cushing’s disease in other animals, which is why PPID is the preferred term when referring to this disease in horses.
A benign tumor of the pituitary gland develops as a result of decreased dopamine release from the hypothalamus. The pituitary enlarges and produces an excessive amount of a hormone called ACTH, which in turn leads to the over-production of a hormone called cortisol. Cortisol is an important hormone in the body that is produced by the adrenal gland in response to ACTH production. Elevated cortisol leads to increases in blood sugar concentration, it suppresses the immune system, and it stimulates the metabolism of fat, protein, and carbohydrates. Understanding what cortisol does helps us understand the clinical signs that we see.
The average age of horses diagnosed with PPID is 20 years of age, with over 85% of horses diagnosed over the age of 15. However, it has been diagnosed in younger horses. It occurs in all breeds but has a higher incidence in ponies and Morgan horses.
A long curly hair coat, which is medically known as hirsutism, is the best indication that a horse may have PPID. Other clinical signs include laminitis, excessive sweating, loss of muscle mass, a pot-belly appearance, lethargy, increased susceptibility to infections such as hoof abscesses or dental disease, excessive drinking and urination, as well as insulin dysregulation. Sometimes these clinical signs can be quite subtle, so there are tests available for accurately diagnosing PPID in horses where PPID is suspected but not obvious.
The most frequently used test to confirm a diagnosis of PPID is the measurement of ACTH concentration. All this test requires is a blood sample, which can easily be taken on farm. High ACTH levels reflects an over-productive pituitary gland. One thing to note about the level of ACTH in the blood stream is that it can increase significantly in the fall. Even normal non-PPID horses have an increase in ACTH in the fall, but horses with PPID have a much greater increase. This means that ACTH testing in the fall is good for diagnosing PPID, as PPID horses will have an extremely high ACTH. However, it is not the ideal time of year for monitoring ACTH levels in horses that have been diagnosed with PPID and are receiving treatment, as their ACTH values at this time will likely be higher than at other times of the year. For monitoring response to treatment and progression of the disease, the ACTH test is best used in the winter, spring, and summer.
Most horses that we suspect have PPID are diagnosed by testing ACTH levels. However, there are some horses who we highly suspect have PPID that come back with normal ACTH levels. To determine if these horses are PPID, there is a second test that we can do to confirm a diagnosis. This test is called a dexamethasone suppression test. Dexamethasone is a steroid. When we administer a steroid to a normal horse, the steroid acts on the pituitary pathway to decrease production of ACTH and cortisol. We collect a blood sample to determine resting cortisol levels, administer the dexamethasone, and then collect another sample to look at cortisol levels. A normal horse should have decreased cortisol in the second sample, whereas a PPID horse will still have elevated cortisol levels.
One of the clinical signs associated with PPID is insulin dysregulation, which along with elevated glucose levels in the blood stream predisposes horses to episodes of laminitis. However, it is important to note that insulin resistance and laminitis can also occur with other diseases such as equine metabolic syndrome and obesity. Insulin is normally released following a meal and stimulates the uptake of glucose into tissues. When a horse develops insulin resistance, the insulin is no longer effective and muscles and other tissues are unable to efficiently use glucose. This may cause muscle loss, accumulation of fat in abnormal places, laminitis, excessive drinking and urination, and an increased susceptibility to infection. As you can see, it is sometimes difficult to differentiate PPID and equine metabolic syndrome, and horses can have both of these conditions at the same time.
The goal of treating PPID is not to cure the disease (as complete remission is rare), but to improve the quality of life for the horse. PPID largely increases the risk of laminitis and infections and may cause muscle mass loss and insulin resistance. Treatment reduces the likelihood of these negative effects occurring. The drug of choice for the treatment of PPID is pergolide. Pergolide is a drug that works by inhibiting the over-production of the pituitary hormone ACTH. It has been shown to have high efficacy, with the majority of horses having improved clinical signs within three months of treatment. Pergolide must be administered by mouth every day and can be added to feed for easy administration.
It is also important to maintain a high standard of health in horses with PPID. This includes staying up to date on preventative measures such as deworming, dental care, and hoof trimming (and shoeing if necessary). PPID horses with a long coat can also be clipped when it is warm, to prevent them from over-heating.
Since many horses with PPID are also insulin resistant, diet may play an important role when managing the disease. Horses that are insulin resistant require a special diet that is low in starch and sugar. It is important to feed hay that is less than 12% in non-structural carbohydrates (NSC). Hay can be tested for NSC or owners can choose to soak the hay for 30 minutes prior to feeding, which reduces the NSC content. Access to lush pasture should be limited and treats such as sugar cubes should not be given to the horse. PPID horses that are thin and require weight gain can be fed a diet that is high in fat. This may include feeding a complete senior feed low in NSC that is supplemented with rice bran or canola oil.
AAEP Equine Cushing’s Disease: Equine Pituitary Pars Intermedia Dysfunction. 2009.
Boehringer Ingelheim. http://www.bi-vetmedica.com/
Klinkhamer, Kim, Paul PCA Menheere, and Johannes H. van der Kolk. "Basal glucose metabolism and peripheral insulin sensitivity in equine pituitary pars intermedia dysfunction." Veterinary Quarterly 31.1 (2011): 19-28.
McGowan, T. W., G. P. Pinchbeck, and C. M. McGowan. "Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses." Equine veterinary journal 45.1 (2013): 74-79.