July 22, 2018
Ask the Veterinarian What is Equine Cushing’s disease?
How do I know if my horse has “Cushing’s?” Horse folks: do any of you have that horse that just seems to take forever to shed that winter hair coat or maybe not at all? Does your horse slick off except those long hairs on the back of the legs or under the chin? Even after deworming? What about muscle loss along the back despite good feed and teeth? These are some of the questions and concerns horse owners may have as our horse ages. Ever heard of Equine Cushing’s disease?
Equine Cushing’s disease seems to be more common today than ever before. This trend is most likely due to the average horse living longer and veterinary medicine has increased its recognition of the disease. Cushing’s is statistically a disease of older horses with the median age of 23 yrs, but has been diagnosed as young as 9 yrs. Younger horses may have the disease, but the clinical signs (symptoms) may not yet be detectable and/or diagnostic testing cannot detect very early onset of disease.
What is Equine Cushing’s disease? I’ll try to keep a rather complex disease as simplified as possible, but it will get a little confusing. To begin, Cushing’s is one of the two primary endocrine (hormonal) diseases of our equine friends. The second common endocrine disease is Equine Metabolic Syndrome (EMS)/Insulin Resistance (IR)/”Easy Keepers.” The two are biologically different, but not always separate from one another. I’ll eventually explain EMS to a certain extent to help you differentiate the two. Cushing’s is quite common in ponies, but otherwise has no predominated breed predilection. So here goes, first of all Equine Cushing’s is not exactly like Cushing’s disease in people and dogs. Which, is known as HyperAndrenoCorticism “HAC” or the body’s over production of a steroid called cortisol. Although, excessive cortisol is produced in horses, the cause of the two diseases is different. Told you this gets confusing.
Because Equine Cushing’s has a different cause, we need to more accurately rename it as PPID (Pituitary Pars Intermedia Dysfunction). PPID originates from a tumor or adenoma (benign glandular tumor) located on the pituitary gland, which is a very small gland located near the hypothalamus at the base of the brain. This region is basically the central hormone control regulator for the entire body. Signals are sent to and from this region to turn on and turn off chemicals that control our horses’ hormonal functions. It is best described as functioning like the thermostat in your home. PPID, furthermore, affects the intermediate lobe of the pituitary gland. This small region is responsible for hormones that help regulate seasonal hair growth and shedding (αMSH = alpha MelanocyteStimulatingHormone) and the indirect production of ACTH AdrenoCorticoTropicHormone which causes cortisol (steroid) production. There are other hormones produced, but these are most important for this article.
As a horse ages another hormone called dopamine begins to decline. Dopamine helps to regulate the pituitary gland from producing too much ACTH and αMSH. When dopamine decreases, the secretion of these two hormones can increase and a small tumor (adenoma) may begin to form. At this point the “thermostat” has begun to fail, meaning your house’s temperature or “hormonal” regulation becomes uncontrollable. To help explain, the heat is set to 72 degrees F, but the temperature is now 89 degrees in the house and the heat is still rolling. You get hot and mad because the holiday chocolate is melting and the control mechanism (Pituitary = thermostat) isn’t working any more. With the hormones now out of control and being overproduced, the effects of the disease process begin.
The clinical signs of PPID in the horse can include any or all of the following: abnormally long hair coat and delayed shedding (hirsutism or hypertrichosis), muscle wasting, regional fat deposits, laminitis (a.k.a. founder), exercise intolerance, abnormal estrous (heat) cycles, excessive drinking/urinating, persistent secondary infections, and sweating abnormalities (too much = hyperhydrosis, none = anhidrosis, not enough = hypohydrosis). The over-secretion of the involved hormones can cause these clinical signs to eventually become evident in horses with PPID. Generally, by the time hypertrichosis occurs, the disease is advanced. On the contrary, there may be other disease conditions that can cause some of these signs such as poor teeth, poor diet, protein deficiency or parasites. But, the older horse that is otherwise healthy and on a good diet and health management program that starts looking tired, poor, and all shaggy may be a suspect for PPID.
Now, I must briefly explain the “easy keeper” or EMS condition in our horses. A horse with EMS is that horse that maintains a heavy BCS (6/9 to 9/9) on very little feed and forage. These horses become very obese when exposed to lush pasture and/or feed. They often have big, cresty necks, fat pads on the tail base and shoulders, and big bellies. EMS was once thought to be a condition caused primarily by low production of thyroid hormone (hypothyroidism), but that has now been determined not to be the case. Although, the use of thyroid supplementation (Thyro-L) helps the horses lose weight. It actually works by increasing metabolism to help burn calories. The EMS horse is very susceptible to getting sore feet or laminitis when spring forages are growing. These horses have a condition in which they have altered sugar (glucose) and insulin metabolism called Insulin Resistance, IR. The most important side effects of EMS/IR are increased risk for pasture associated laminitis and their predisposition PPID. This is why it is important to understand both conditions. A horse can be an “easy keeper” most of its life; then one year its shedding is delayed beyond herd mates and as the years progress it starts losing muscle mass and has increased episodes of laminitis. Not all EMS horses develop PPID, not all horses with PPID had EMS at one time, and not all old horses will develop PPID or EMS. But, you need to watch for some of these changes as your horse ages and mention them to your veterinarian when they are on the farm performing annual vaccinations or dentistry.
If you or your veterinarian suspect PPID based on long hair coat or weight loss with your ageing horse, then there are some diagnostics and exams that should be performed. After performing a physical exam with otherwise normal findings, your veterinarian may recommend collecting blood and running a minimum database including a CBC, Chemistry, and T4 to check baseline values and monitor for other suspect conditions. A fecal may performed as well to rule out parasitism. If these tests are not diagnostic for any other conditions, more specific tests for PPID will likely be performed. Your veterinarian will determine the test they are most comfortable using to help diagnose PPID. A resting plasma ACTH, Low Dose Dexamethasone Suppression Test, TRH stimulation/ACTH Combo test can be performed, or the practical presence of Hypertrichosis (long hair coat) is often diagnostic itself. Unfortunately, the tests are not always reliable and a good relationship with your veterinarian and his/her understanding your horse’s clinical signs of PPID are often the diagnosis. The only definitive diagnostic is the post-mortem evaluation of the pituitary adenoma.
Once diagnosed, PPID is managed with a drug called Pergolide. Pergolide increases production of dopamine. Dopamine is the hormone that, when levels are low, it may cause PPID. By increasing dopamine with the use of pergolide, there are two potential effects. It may decrease PPID tumor activity and may decrease growth of the tumor. Your veterinarian will advise you on dosage, follow-up testing, and any dietary changes that may be needed to improve your horse’s life quality and longevity.
What happens if your horse has PPID and you don’t treat it? Since horses are individuals, your horse may live a long, hairy life into his 30s or your horse may progressively deteriorate, lose his ability to thermoregulate during the summer or winter, develop debilitating laminitis and chronic infections (skin, sinus, hoof abcess, etc.). He may ultimately succumb to his body breaking down under elevated cortisol levels.
So, to keep such a complex condition simple, you as a horse owner should pay special attention to your ageing horse. Have routine dentistry performed by your veterinarian; utilize high quality senior feeds such as Purina Senior, SafeChoice Senior, or Triple Crown Senior; deworm appropriately; and provide the needed basics for your horse. If they are starting to shed hair coat slowly, develop laminitis, and lose weight in spite of providing the above care, then contact your veterinarian to examine your horse and ask about “Equine Cushing’s” disease.
Make sure your horse is drinking plenty of water this winter! Have a Merry Christmas!
If you have any specific equine health related questions you would like me to address just email them to email@example.com. Mention if you wish to remain anonymous, otherwise I look forward to helping you understand any health questions you may have concerning our wonderful equine companions.
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