Ask the Vet: Insulin Resistance, Cushing’s Disease, & Equine Metabolic Syndrome

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Understanding each condition, and the relationships between them

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, Nevada

Dear CB,
I hope the wager involves chocolate and that you were the one correctly voting “yes” that insulin resistance CAN be present in both Cushing’s Disease and Equine Metabolic Syndrome.

As luck would have it, I recently attended 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 associated with these two separate but related conditions, the fact that multiple teams of researchers across the US 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 (as if it wasn’t tricky enough already!): 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) but 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 have some grasp of the underlying problem in EMS, PPID with insulin resistance, and 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!

Lydia Gray, DVM MA, is the Staff Veterinarian for SmartPak. Prior to joining SmartPak, Dr. Gray served as the first-ever Director of Owner Education for the American Association of Equine Practitioners. She has authored numerous articles in publications such as The Horse, Horse Illustrated, Western Horseman and a variety of veterinary journals and magazines. Dr. Gray is also a frequent speaker at horse expos, veterinary conventions and other events. After graduating with honors from the University of Illinois College of Veterinary Medicine and receiving her Master's Degree in Interpersonal and Organizational Communication, she practiced at the Tremont Veterinary Clinic for several years. Dr. Gray is active in the American Veterinary Medical Association and Illinois State Veterinary Medical Association. She enjoys training and showing her Trakehner, Newman, in both combined driving and dressage, and is a USDF “L” Program Graduate (with distinction). Find Dr. Gray on Google+

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6 comments on “Ask the Vet: Insulin Resistance, Cushing’s Disease, & Equine Metabolic Syndrome
  1. Jani says:

    I have a 15 y/o American Saddlebred with Cushings and is IR as well, I have a question on how this disease effects the muscles as I see there are many opinions out there that do not specify this as a common side effect….I have intermitent lameness that is not foot related and x-rays were clean….I have therapy for him 2 x’s a month and he does swim quite often in a free swim facility. He’s developed several strange muscle lumps that are not painful to the touch but seem odd. Your help is appreciated.

    • SmartPak SmartPak says:

      Jani,
      your situation proves just how complicated and confusing metabolic and endocrine problems are in horses. In general, Cushing’s Disease leads to muscle atrophy or breakdown, but if you feel your horse has not “read the book” when it comes to these conditions I highly recommend you speak with your veterinarian about your concerns. He or she (sometimes with the help of a second opinion from a referral center or teaching hospital) may recommend additional diagnostic testing or adjustments in prescription medication, diet, and management to cope with unusual findings. – Dr. Lydia Gray

  2. Kelly says:

    Thank you Dr. Gray for you explanation. What is your suggestion on where to learn about horse diseases with out going to vet school. I have been out of horses for a while and there are a lot of new things out here. It can be confusing. Thank you.

    • SmartPak SmartPak says:

      Hi Kelly, thanks for asking!
      I can steer you toward a few more resources if you’re interested in learning more about equine health.
      SmartPak’s Equine Health Library is chock-full of good information, so that would be a fun place to start.
      I’d recommend you continue to browse the SmartPak Blog as well. Also, the most important resource regarding equine health conditions will be your veterinarian. Your vet is the absolute best resource for understanding your horse’s unique health needs, and so I would encourage you to keep that line of communication open. – Dr. Lydia Gray

      Equine Health Library: https://www.smartpakequine.com/content/Health-Library

  3. Chuck says:

    We have a pony that may have Cushing IR. He’s being tested next week. We’ve been giving him the Smartpak’s Smart Tendon, and we’re wondering it this is good for him?

    • SmartPak SmartPak says:

      Hi Chuck, it’s excellent that you’re working with your veterinarian to determine exactly what’s going on with your pony; that is absolutely the most important step! While we are aware of no issues regarding the ingredients in SmartTendon, we encourage you to discuss your pony’s complete diet, including supplements, with your vet, as you work together to develop a complete management program for him. – Dr. Lydia Gray

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