Equine Metabolic Syndrome (EMS) and Equine Cushing’s Disease (PPID) are the two most common metabolic disturbances in horses. I recently wrote a two-part blog about PPID (Part One and Part Two.) This post will be about EMS, and next week I will discuss dietary recommendations for horses with various metabolic challenges.
Typically, EMS occurs in younger horses, while PPID is more common in horses 15 years of age or older. When Alan and I were in the market for a new trail buddy, we traveled to Texas to check out Sadie. She ultimately came home with us, but not before we initiated testing for Equine Metabolic Syndrome.
Sadie had been hanging out all summer on a lush Oklahoma pasture. Additionally, she was rarely ridden. Not only that, but she’s one of those horses that can merely think about eating and gain weight. She was, shall we say, “fluffy!”
Her bloodwork indicated that she was moving in the direction of becoming insulin resistant, but not yet abnormal enough to receive the diagnosis of EMS.
The article I referenced in the opening paragraph is technical and written for clinicians. The Merck Manual is a time-honored, valuable reference for practicing veterinarians. The article on EMS was written by Janice E. Kritchevsky, a Purdue DVM that I interviewed on our podcast about PPID. This paragraph contains crucial bullet points about EMS.
Horses with EMS respond to high carbohydrate meals with an exaggerated increase in insulin, a higher than expected blood glucose level, and a very slow return of blood glucose concentrations to baseline values. This indicates a resistance to the peripheral effects of insulin (EMS) and/or an inability to metabolize oral carbohydrate normally (insulin dysregulation).
By Janice E. Kritchevsky, VMD, MS, DACVIM, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Purdue University
EMS vs. Insulin Resistance vs. Insulin Dysregulation
The paragraph above differentiates Equine Metabolic Syndrome (EMS) from insulin dysregulation. EMS is a lack of response to insulin in the bloodstream, whereas insulin dysregulation indicates abnormalities in the way the horse processes and uses carbohydrates in the diet.
It can get to the point of splitting hairs when trying to understand the disease processes and the terminology. As Dr. Kritchevsky states above, EMS is a lack of response to insulin, or in other words, insulin resistance. However, insulin resistance is only one facet of EMS. Horses that get too fat (adiposity) develop insulin resistance and consequently produce higher levels of insulin (hyperinsulinemia) in their attempt to drive the glucose into their cells.
This is Mother Nature at play… “I have plenty of fat stored up, so I don’t need as many nutrients delivered to my cells. Therefore, I will “downregulate” or diminish the body’s response to insulin,” (the hormone that is necessary to get glucose into cells.)
Laminitis
Horses with abnormalities in insulin production or response are predisposed to laminitis. This is because insulin has vasoregulatory properties. That means it influences the dilation and constriction of blood vessels. “Hot feet” are a sign of laminitis, and that is a result of dilation of the blood vessels supplying the hoof.
Too much insulin also damages the cellular bonds of the lamellae in the hoof, resulting in separation and subsequent laminitis (inflammation of the lamellae.)
No one knows for sure why some horses develop EMS and others do not. There does appear to be a genetic predisposition at play. Horses that developed a genetic ability to survive in spite of poor environmental conditions are now at a disadvantage when managed with abundant and rich sources of nutrition.
Dr. Kritchevsky goes on to say…
EMS may be a predisposing factor for pituitary pars intermedia dysfunction (PPID; also called equine Cushing disease). Both endocrine disorders can occur concurrently in middle-aged and older horses. Horses with EMS should therefore be monitored to detect the onset of PPID.
What are the indications that a horse has EMS?
No clinician is going to suggest testing a slim or appropriate-weight horse for EMS! One of the first red flags of EMS is an overweight horse with fat deposits around the neck (cresty neck), topline, ribs, and tail head. On a scale of 1-9, a body score >6 is typical for an EMS candidate.
Laminitis is another indicator of metabolic disease, and/or evidence of previous bouts of undiagnosed laminitis. Unfortunately, laminitic horses are understandably reluctant to exercise, making weight control even more challenging!
What tests are available?
As mentioned above, there are many aspects to insulin abnormalities. Is the insulin level normal but the body doesn’t respond to it? Is there too much insulin in the blood? What, exactly, is happening? What tests will help us figure out the puzzle?
Testing can be challenging because we are measuring glucose and insulin levels, both of which are heavily influenced by pain and stress. Horses that are painful from laminitis need to be stabilized first. The testing should be done in as stress-free a manner as possible.
The Oral Sugar Test is performed by giving the horse a specified amount of corn syrup orally and then collecting blood at a specified time to measure the level of insulin generated by the ingested sugar.
The Insulin Tolerance Test measures changes in glucose concentration after the horse is given a specified amount of insulin. Blood is drawn before the insulin is given (a baseline level), and then again 30 minutes after the insulin is administered. The blood glucose level needs to drop to less than 50% of what it was at the baseline level.
A horse that consistently shows abnormally high blood sugar levels also needs to be tested for PPID. Dr. Kritchevsky states:
Tests for PPID such as measuring endogenous (naturally occurring) ACTH concentration or thyroid releasing hormone response test are normal in horses with EMS. Positive results indicate that the horse is concurrently affected by EMS and PPID, which can occur in older horses. Detection of PPID is important, because it is thought that PPID exacerbates insulin resistance in horses previously affected by EMS.
There is a wonderful group available on the internet that is an absolutely incredible source of information about metabolic diseases of horses. Check out theEquine Cushing’s and Insulin Resistance Group Inc.
Next week I will talk about the nutritional and husbandry management needed for horses with metabolic diseases.
Lastly, an update on our much-delayed podcast. I am seriously disappointed that Dr. Kritchevsky’s interview is not yet available to hear. We are having ongoing technical and practical issues getting the podcast launched. It is an excellent interview and when we finally have it available I will let you know. It is currently not under my control.
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