Equine Cushing’s Disease
(Pituitary Pars Intermedia Dysfunction)
For many years, a disease called Cushing’s Disease has been recognized in horses and ponies. These horses are generally middle-aged to geriatric (over 20). The first symptom may be a shaggy or curly hair coat that does not shed out in the spring or grows in early in the fall, founder (laminitis), or increased thirst and urination. These symptoms are not specific to Cushing’s Disease. They are, however, a reason for investigation of an underlying disease such as Cushing’s. This disease is now being more commonly referred to as Pituitary Pars Intermedia Dysfunction (PPID). This name tells the source of the disease within the body.
The pituitary gland is located at the base of the brain and is responsible for stimulating secretion of hormones by multiple other glands within the body. The portion of the gland affected by PPID controls the secretion of cortisol by the adrenal glands. In horses with PPID, a benign tumor replaces the normal cells within the pituitary. These cells then produce an overabundance of ACTH. The ACTH causes the adrenal glands to secrete more cortisol than normal. Cortisol is normally secreted in small amounts on a cycle throughout the day. Too much cortisol in the body decreases the ability to fight off infections and affects body water balance. This can lead to excessive thirst and urination, as well as persistent infections that may be difficult to resolve.
Diagnosis for this disease is made through a blood test. The most effective and practical test is called a Dexamethasone Suppression test. This test is done in two phases and is very dependent upon timing. A blood sample is drawn in the afternoon of the first day. The patient is then given a dose of dexamethasone which is a synthetic cortisol. A second blood sample is then taken the next day in the morning between 8am-noon. The samples are then submitted to an outside laboratory and results are typically received in 7-10 days. A normal horse will have a decrease in cortisol after the dexamethasone injection is given. A horse with Cushing’s (PPID) will have little or no response to the injection. Due to the administration of cortic-steriods, this test is not recommended in all cases. Horses with ongoing laminitis or chronic infections may be tested using an ACTH and Insulin test. This test requires a single blood draw and does not involve the administration of cortico-steriods. Timing is very different with this test, as the sample must receive special handling within two(2) hours. This test is also sent to an outside laboratory and results are received within 7-10 days. A horse with PPID will have increased levels of ACTH and Insulin in the blood. The sensitivity of this test is much lower and may lead to a negative result in a horse that does have the disease.
Therapy for affected horses and ponies should be targeted to preventative health care and health maintenance. This includes routine vaccinations, worming and regular dental care. It is also very important that these animals have regular farrier visits. Dry clean bedding and proper nutrition help to prevent stress and infection. A senior feed is often a good choice as it is well balanced and easily digestible for the older animal. Clean water should be made available at all times, including hanging an extra water bucket for those with increased thirst. Clip the long hair coat as needed in the warmer months to prevent heat stress.
Therapies specific to this disease are targeted toward reducing the symptoms by modifying the production of ACTH in the pituitary. Pergolide is the most effective treatment currently available. It comes in liquid and also in treat (biscuit) form for easy administration. Pergolide binds with dopamine receptors in the pituitary to reduce secretion of ACTH leading to a lower blood cortisol level. Any patient may receive smaller or larger doses depending upon individual response to treatment. Another therapy, Cyproheptadine, is less specific in its activity and often less effective. Cyproheptadine acts to decrease binding of serotonin. This leads to a decrease in melanocyte stimulating hormone, which acts with ACTH to increase cortisol secretion. Again, doses vary based upon individual response. Pergolide therapy has been documented to have the highest success rate in improving clinical signs, but may be cost prohibitive for some owners. In these cases, Cyproheptadine should be tried to alleviate or improve symptoms.