Cushing’s Disease in Horses
I recently wrote an article about a study that evaluated PPID-Pituitary Pars Intermedia Dysfunction (aka Cushing’s Disease) risk factors in horses and learned more about the condition that is becoming much more recognized among older horses these days.
Writing that article and then discussing PPID with a friend whose horse is showing several symptoms sparked my interest to learn more about this disease. So I decided to participate in Dr. Juliet Getty’s recent teleseminar on Cushings. Her teleseminars on various topics related to equine nutrition are available for download and are very worthwhile, by the way.
So what exactly is PPID? The condition is described as a neuro-degenerative disorder of aged horses (usually 15+), caused by an enlargement in the pars intermedia of the pituitary gland (which is housed in the base of the brain). The pars intermedia is involved with the production of cortisol (stress hormone) and endorphins (pain-relieving hormones).
The pituitary gland, along with the hypothalamus, affects many regulatory functions in the body, such as metabolism, immune response, body temperature, hunger/ thirst, reproduction and growth, cardiovascular function, and stress response. These functions can all be affected in a horse with PPID.
Much is still unknown about Cushing’s disease in horses, but there are several factors thought to cause PPID including:
- Equine Metabolic Syndrome/ Insulin Resistance;
- genetic factors;
- oxidative stress;
- chronic stress; and
- toxins from the environment.
Symptoms of PPID can be subtle at first. A horse may only show one or a few of the following symptoms:
- Hirsutism (failure to shed or delayed shedding)–often the most common symptom;
- Other changes in hair coat such as patchy shedding, changes in coat color, shaggy hair on legs, face, or neck;
- Decreased performance or lethargy;
- Muscle loss along topline and weight loss with ‘pot-belly’ still existing;
- Laminitis, often occurring in the fall;
- Insulin Resistance;
- Regional fat deposits (neck crest, tail head, above eyes, sheath/ mammary area);
- Excessive sweating or anhidrosis (failure to sweat);
- Increased appetite;
- Excessive urination;
- Newly developed allergies/ hypersensitivity to vaccinations, flies, etc.; and
- Changes in behavior (calmer or more excitable).
So as you can see, the symptoms of PPID can be numerous. If you observe one or more of these signs in your horse, you may want to have him tested.
Diagnosis of PPID can be a bit tricky, especially if your horse is in the early stages of the disease. According to some vets, hirsutism alone can be a reliable indicator of PPID. Several diagnostic tests used include:
- ACTH Test: Usually considered the most reliable indicator of PPID, this test measures the levels of adrenocorticotropic hormone in the horse. Can be performed any time throughout the year, but must be adjusted to factor in seasonally high levels of ACTH during the fall.
- Dexamethasone Suppression Test: Measures cortisol levels both before and after administration of dexamethasone, a steroid. (High cortisol levels after dexamethasone administration can indicate PPID.) This test can be dangerous for some horses as it can trigger laminitis. If your horse has a history of laminitis or you believe he may be at risk for developing it, ask your vet about performing a different test. This test may not be reliable in the fall or early winter months.
- Domperidone Response Test: This is a new test for PPID which uses domperidone (commonly used for decreased milk production and fescue toxicosis), a dopamine-inhibiting drug. PPID horses will quickly double their ACTH production after domperidone administration while normal horses will maintain normal ACTH levels. Many vets may not use this test yet, however.
According to some vets and researchers, many older horses have PPID but do not show any adverse effects from the disease. It’s possible that your horse will not need medication for PPID. However, for those horses which are in the advanced stages or are noticeably affected by PPID, the treatment of choice is the drug, pergolide (Prascend). Some horses will need to be kept on the drug year-round, while others may only need it seasonally.
Nutrition plays a very important role in treating PPID horses and can help to slow the progression of the disease. Since these horses are often affected by IR as well, feeding a low-sugar/ starch diet is necessary. Some guidelines for feeding include:
- Feeding a low-sugar/ low-starch hay–you need to get your hay tested to know the sugar/ starch content. Equi-Analytical can do this for you at a very reasonable cost. According to equine nutritionist, Dr. Juliet Getty, WSC + Starch should be less than 12% for PPID horses.
- If your hay is higher than 12% WSC + Starch or you can’t get it tested, soak it in water for an hour or two before feeding it. (I wrote an article on this, too!)
- Feed low-sugar and starch feeds such as beet pulp, hay pellets, or a commercial feed like Triple Crown Lite. Note: Very few commercial feeds are fall below 12% sugar/starch content–do your research!
- Minimize or eliminate grazing on pasture during the spring and fall (when night-time temps fall below 40 degrees F). The safest time to graze is during the late night and early morning hours. Note: Some horses cannot tolerate any grazing during the spring/ fall though, as it will trigger laminitis.
- Use a grazing muzzle to restrict grass intake.
Other nutritional components to consider for PPID horses include:
- Antioxidants such as vitamins E and C; .
- Omega-3’s such as flaxseed or chia;
- Jiaogulan (Chinese herb) which can help to regulate stress; and
- Adequate magnesium (Calcium: Magnesium ratio should be roughly 2:1).
For those of you interested in learning more about PPID, I highly recommend downloading Dr. Getty’s teleseminar on Cushings as well as joining Dr. Kellon’s ECIR discussion group. They both provide some invaluable information on this disease.