His name was Lucky. And lucky he was. Although the gelding was diagnosed with equine Cushing’s disease in his teenage years, he had an owner who truly loved him and did everything possible to manage his disease until he was pushing 30. He continued to compete for several years after his diagnosis, was actively ridden into his 20s, and was happy until the end.
In recent years, there’s been a lot of interest in Cushing’s disease, which can’t be cured. We’re learning more every day about how to diagnose and manage this hormonal disorder that’s one of the most common diseases found in horses over 15. If you haven’t encountered it yet, it’s likely that you will. An estimated 10 percent of horses over 15 have Cushing’s, and with all the improvements in horse health care, horses are living longer and longer. That means there’s a good chance you’ll experience this disease sometime in your horse life.
Just 20 years ago, your Cushing’s horse would’ve been retired in the pasture, but no longer. I’m going to show you how you and your horse can live comfortably with Cushing’s disease, just like Lucky did.
The Early Years
Lucky was a successful show horse, and spent his early days on the road, traveling from show to show—working hard and eating well. When he turned 12, his owner noticed that he was slow to shed his winter coat. He seemed to lack energy, and he was getting fat. She started worrying about Cushing’s disease.
Although Lucky was young, relatively speaking, his owner was right: He was showing some early signs of the disease. And she was smart to start asking questions before he developed any more serious symptoms, such as laminitis. The most common signs are a long hair coat that’s slow to shed, lethargy, and weight loss or weight redistribution. The average onset of Cushing’s disease is 19 years of age.
Cushing’s disease originates within the brain. In the normal horse brain, the hypothalamus portion releases a neurotransmitter called dopamine that helps to regulate release of a variety of hormones from the pituitary gland that sits at the base of the brain. One of these hormones, ACTH (adrenocorticotropic hormone), stimulates release of cortisol (the body’s stress hormone) from the adrenal glands.
Equine Cushing’s disease (more correctly called pituitary pars intermedia dysfunction, or PPID) is due to hyperplasia (enlargement due to an increased number of cells) of the “intermedia” portion of the pituitary gland. The pituitary gland in a horse with PPID can be enlarged up to five times normal size. Historically it was believed that when the pituitary gland is enlarged, it puts pressure on the hypothalamus and causes a reduction in the amount of dopamine released.
Current thought is that the condition is primarily a problem of the hypothalamus. Damage to the hypothalamus results in a reduction of dopamine secretion, which then causes enlargement of the pituitary gland. In either case, dopamine no longer inhibits ACTH release like it should—meaning ACTH levels increase, resulting in increased cortisol in the blood. The signs of Cushing’s disease are attributed to increased cortisol levels.
Testing for Cushing's
Lucky’s vet suggested that he be tested for Cushing’s disease. The first test (ACTH) came back negative, but Lucky’s blood insulin levels were high, suggesting that he was insulin-resistant. Although insulin resistance (IR) doesn’t always correlate with Cushing’s disease (see the sidebar on page 50 for more on this topic), it is considered a risk factor—horses with IR are more likely to develop Cushing’s as they age.
A number of different tests are available to diagnose Cushing’s disease, but none are very sensitive when the disease is just developing. Nevertheless, researchers now believe that early detection and initiation of treatment may be important for slowing progression of the disease-possibly avoiding a devastating laminitis episode. If your horse begins to show symptoms but early tests are negative, consider retesting annually, at least until more sensitive tests become available. If you discover that your horse has insulin resistance, his risk for developing Cushing’s down the road is even higher, so you’ll want to pay even more attention.
The most popular test for Cushing’s currently is resting ACTH. This test requires a single blood sample that can be drawn at any time of day. Blood samples must be handled carefully, as the hormone isn’t very stable in whole blood, but most veterinarians are familiar with the handling requirements and can easily perform the test at your farm.
Testing for Cushing’s in the fall months (August through October) is generally discouraged, because normal hormonal fluctuations during this time of year can make it difficult to interpret results. However, one advantage of the ACTH test is that it often can still be interpreted even during this challenging time of year.
The gold standard for Cushing’s diagnosis is the low-dose dexamethasone suppression (LDD) test. This an overnight test that requires testing a baseline blood sample for cortisol, administering a dose of dexamethasone, and testing an additional blood sample 18 to 20 hours later. In a normal horse, cortisol levels will decrease following the dexamethasone; if your horse has Cushing’s, cortisol levels will remain the same. Because there’s some risk of laminitis after dexamethasone administration, this test should not be used on a horse that has laminitis issues. It also shouldn’t be used in the fall.
Finally, because insulin resistance is common in Cushing’s horses, testing blood insulin/glucose levels is often recommended in conjunction with the Cushing’s test. Insulin is a sensitive hormone, and can elevate significantly with stress, disease, or a high carbohydrate meal. For best results, blood should be drawn first thing in the morning before anything but hay is fed. It shouldn’t be performed if your horse is sick or suffering from a laminitis episode.
No treatment was recommended for Lucky after this first test came back negative, but his vet and owner came up with a plan to manage his insulin resistance. He was put on a low-carbohydrate diet, and to keep weight under control, his work schedule was adjusted to include focused conditioning work in addition to his training for the show ring. They also decided to repeat Cushing’s testing at six-month intervals.