Note: While Cushing's disease can contribute to other metabolic disorders (such as insulin resistance), we're going to focus solely on Cushing's disease for the purpose of this post. We'll tackle insulin resistance in another blog post!
The term Equine Cushing's Disease was coined for the similarity to the syndrome in humans and dogs. (Cushing's disease in humans was originally described by a neurosurgeon at Johns Hopkins University named Harvey Cushing.) In humans and dogs, Cushing's is most commonly caused by either 1) an adrenal tumor or 2) a tumor in the anterior lobe of the pituitary gland which causes hyperplasia (excessive growth) of the adrenal gland. Both of these cause an increase in cortisol (a steroid). In horses, Cushing's is primarily due to a tumor in the intermediate lobe of the pituitary gland, but there is no associated hyperplasia of the adrenal gland. That's why the technical name of it is Pituitary Pars Intermedia Dysfunction (PPID). Whew! If you thought that was scientific, just wait!
So what exactly is going on to make this disease such a concern? Well, as the pituitary tumor grows, it secretes a hormone called ACTH (Adreno-Cortico-Trophic Hormone. Yes, it's a mouthful!) This hormone signals the adrenal gland to secrete cortisol, also known as a "stress hormone". The chronically increased level of steroids in the body wreaks havoc on the balance that the system usually tries to attain. This is what leads to the common signs of PPID.
So on to the tests we do use! The first (which we typically rely on) is to test the resting ACTH levels. This is simple enough - if the measured level of ACTH is above or below a certain threshhold, the horse is positive or negative, respectively. Caution must be used with this as there is a 3-fold increase in the fall, but this has been measured in normal horses, so we can adjust values for that. The second test is known as a Dex Suppression Test, whereby a blood sample is taken, then a small amount of dexamethasone (a steroid) is administered. A second blood sample is taken approximately 20 hours later. This test should cause a significant drop in the amount of ACTH present. If it does not, then the horse is positive for Cushings disease. However, this test can be concerning as we are giving steroids to a horse with possibly high circulating levels of steroids. Since excessive levels of steroids can rarely cause laminitis, we reserve this test for specific cases.
For a long time, pergolide was only available as a compounded formulation. Now that we have an FDA approved form, we don't recommend or even dispense the compounded drug. This is because studies have shown that compounded pergolide is not stable and does not have a very long shelf life. The concentration of compounded drugs is also quite questionable and is not always what the label states. For more on the discussion of Prascend vs. compounded pergolide, you can read our blog from when Prascend was first introduced. Check out this study published in the Journal of the American Veterinary Medical Association on stability of compounded pergolide. In addition, compounded pergolide is no longer allowed under FDA rules.
The other aspect of treating Cushing's is management. Since Cushing's disease will often lead to insulin resistance (again, more on that in another blog!), it is usually important to control the intake of starches and sugars in the diet. These horses should obtain most of their calories from low-starch hay and fat (since fat is a much safer form of calories than starch/sugar for horses with metabolic conditions). An appropriate diet should be outlined with your veterinarian for best results. Appropriate foot care is important, to reduce the incidence of hoof abscesses and laminitis. Finally, horses with Cushing's disease should always be managed more carefully with regards to wounds, parasite control - their immune system is often not 100% normal due to the effects of cortisol, so they will always be prone to chronic infection, non-healing wounds, or higher parasite burdens. Your veterinarian can help you come up with a plan to reduce these risks.
While Cushing's disease is common and incurable, it can be successfully managed. Through owner and veterinary diligence, we can help keep these horses happy and relatively healthy for many years.