Cushings (Pituitary Pars Intermedia Dysfunction)
Cushings in horses is a disease of the pituitary gland, specifically the pars intermedia. This is unlike Cushings disease in other species in which either the pituitary gland or the adrenal gland may be affected. Due to this species difference, experts prefer to call this disease Pituitary Pars Intermedia Dysfunction (PPID). This area of the pituitary becomes hyperplastic (enlarged; often on a microscopic level) and causes the body to secrete excess of certain hormones. This leads to a variety of symptoms, such as a long hair coat, failure to shed out in the spring, a cresty neck, abnormal fat deposits, wasting of the topline, chronic infection, and laminitis.
Testing for PPID usually involves a blood test, either an endogenous ACTH test or a low-dose dexamethasone suppression test. Both tests should be done in a low-stress situation as stress can artificially raise the levels and give a false positive result to the test. When testing from August – November a higher reference range for these tests must be used as all horses have natural increase in these levels during the fall months
For most horses PPID is very manageable. The drug Pergolide is the mainstay of treatment, but sometimes the drug Cyproheptadine is also used. Cyproheptadine is generally not as effective in managing PPID when used alone but when used is often in conjunction with pergolide.
Though PPID is treatable it is not curable and these horses need daily treatment to manage the disease. Horses with PPID will often then need years of treatment which can be a financial strain. The disease in some horses will become resistant to treatment, requiring an increased dose of pergolide or pergolide + cyproheptadine to control symptoms, which then further increases the cost of treatment.
The worst consequence of PPID (particularly in uncontrolled PPID) is laminitis. Laminitis is a disease involving severe inflammation within the feet. It can range in severity and each case is different, but it is the number one reason horses with PPID are euthanized.
Equine Metabolic Syndrome
Equine Metabolic Syndrome (EMS) is a condition in horses involving obesity and insulin resistance. These horses are the classic “easy keeper”, maintaining or even gaining weight with fairly little food. Insulin resistance is a condition in which the horses’ insulin receptors are no longer sensitized to insulin and the horse therefore must make more and more insulin in order to get the same response by cells. Symptoms include obesity, abnormal fat deposits (termed by specialists as “regional adiposity”) and laminitis, which can range in severity.
Testing includes a fasting insulin blood test. For this test the horse is fasted for 10-12 hours prior to blood collection. Insulin levels can be altered by feed, in particular grains, which is why most clinicians prefer fasting prior to the test. Since the symptoms of EMS can be similar to those of PPID, often testing for both is done at the same time.
This condition is treatable with diet and exercise and occasionally medications, such as thyroid medication. While true hypothyroidism is rare in horses, thyroid medications can improve glucose utilization and insulin sensitivity and can encourage weight loss, particularly those that are not able to be exercised.
Getting these horses to lose a significant amount of weight can be very frustrating and takes perseverance. Most of these horse require a strict low-carbohydrate/low sugar diet, which usually means little to no grass. These horses often need to be on this strict diet for life, as they gain weight easily and are at an increased risk for laminitis. A consistent exercise program is also essential in maintaining these horses. Exercise is a key factor in achieving weight loss, and it increases the sensitivity of the insulin receptors as well.
Like PPID, this disease can lead to laminitis. EMS horses with laminitis are in the proverbial “between a rock and a hard place” – they need to lose weight to improve their insulin sensitivity, but they are too lame to get the needed exercise to help their condition.