California Riding Magazine • April, 2014

Ask the Vet

by Lydia F. Gray, DVM, MA

My friends and I have a small wager resting on your answer to: Can Insulin Resistance be present in both Cushing's Disease AND Equine Metabolic Syndrome? Also any advice you can share to help us remember which condition is which and how to tell them apart would be most appreciated! CB

Dear CB,

Fortunately, I've just returned from the 2013 AAEP Annual Convention which featured an entire half day on Geriatric Medicine/Metabolics. While it can be challenging to keep up with all the new information about these two separate but related conditions, the fact that multiple teams of researchers are actively researching the underlying causes, best diagnostics, and most appropriate treatments is a good thing for our aging equine population.

In a nutshell, Cushing's Disease, or Pituitary Pars Intermedia Dysfunction (PPID), is a collection of clinical signs due to overproduction of certain pituitary hormones. It is caused by an enlarged and overactive pituitary gland normally kept in check by inhibitory dopamine from the hypothalamus. PPID is a commonly diagnosed disease in the senior equine population, affecting over 20% of horses more than 15 years of age. Common signs include weight/muscle loss, behavioral changes, secondary infections, and changes in haircoat. Laminitis may also occur.

Equine Metabolic Syndrome, or EMS, is a collection of endocrine and metabolic abnormalities associated with the development of laminitis in horses. Faulty insulin metabolism is a key component of EMS and can include both insulin resistance (IR) and hyperinsulinemia. IR is the failure of tissues to respond appropriately to insulin while hyperinsulinemia means elevated levels of insulin in the blood. Collectively they are referred to as "insulin dysregulation" the new term you may hear veterinarians using or read in magazine articles which simply means an excessive insulin response to sugar.

Here's where it gets tricky: horses with EMS always have insulin resistance while horses with PPID may or may not have it. Since researchers now believe the laminitis associated with these two conditions stems from faulty insulin metabolism, horses suspected of having either EMS or PPID should be tested for insulin resistance. This means that in addition to whatever testing your veterinarian suggests to diagnose PPID—such as a resting ACTH sample or the TRH Stimulation Test—a separate test should also be performed to check insulin status.

There are two recommended tests to check insulin status in the horse: the oral sugar test and the fasting insulin concentration. Both are easy to perform on site (no hospital stay required), however the oral sugar test is more sensitive and therefore recommended as the first choice.

This may seem like a lot of complicated information when all you really want to know is if your horse has one of these conditions and if so, how to treat and manage it. However, it's important to know if the underlying problem is EMS, PPID with insulin resistance, or PPID without insulin resistance so that the appropriate nutrition, management, and prescription medication (if necessary) can be started. For example, while oxidative stress is a factor in all three conditions—and for which antioxidants like Vitamin E are recommended—only horses with faulty insulin metabolism really need the sugars and starches in their diets limited. So a horse with PPID that has normal insulin function can and should have access to high-quality hay, pasture, and other forage sources to help maintain gut health and good weight.

While I may not have simplified things for you and your friends, hopefully this cutting edge information on two very common conditions in horses has given you some food for thought!

Article provided by SmartPak.
Dr. Lydia Gray is the Medical Director/Staff Veterinarian for SmartPak, where she guides research and new product development, answers questions on her Ask the Vet blog, and speaks around the country at various events such as Equine Affaire, Dressage at Devon, and the USHJA Trainers’ Symposiums.