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Brushing Up On Insulin Resistance/Equine Metabolic Syndrome

An overview on what equine experts know about EMS.


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Like the explosive expansion of smartphones and the advent of the selfie and duck lips, information and research pertaining to insulin dysregulation and equine metabolic syndrome (EMS) has grown tremendously over the past few years. It was only a few short years ago that our knowledge of endocrine disorders in horses was akin to the first cordless phone … yes, the one with a 2-foot antenna and measuring approximately the same size as Jerry Seinfeld’s head: archaic at best.

Now, punch the term, “equine metabolic syndrome” into PubMed and over 20,000 matches pop up in a microsecond. But, like all good research fields, the data are frequently conflicting and sometimes the clinical significance of the information remains unclear. 

Here is a quick summary of what equine endocrine experts currently know about EMS, including management options to share with interested clients.

1. EMS is a “Real Thing”

Back in the 1980s, veterinarians began recognizing that some older horses had clinical signs of disease similar to equine Cushing’s disease (pituitary pars intermedia dysfunction, PPID). Similar, but not identical. By 2002, it was widely recognized and accepted that certain breeds and types of horses had a constellation of signs consistent with EMS. I, personally, have dubbed it the “Triad of Terror”: obesity, insulin dysregulation, and laminitis.

Different than PPID, horses with EMS tend to be younger. In fact, any mature horse (as young as 5 years of age, depending on breed) is at risk for EMS. While Thoroughbreds, Standardbreds, and Quarter Horses are less frequently diagnosed with EMS, the list of “at risk” breeds is extensive, including:

  • Morgans;
  • Andalusians;
  • Paso Finos;
  • Arabians;
  • Saddlebreds;
  • Tennessee Walking Horses; and,
  • Ponies, such as Welsh ponies.

In addition to obesity and laminitis, other signs suggestive of EMS include regional adiposity and a cresty neck, much like older horses with PPID.

2. Body Condition and Cresty Necks Aren’t Everything

We’ve already indicated that horses and ponies with EMS are frequently described as being overweight and having a cresty neck. One recent study, however, reported that these two findings do not a diagnosis make. After analyzing data from 164 Andalusians, the study authors found that although over one-quarter of the study population were obese (with a body condition score greater than 7) with a cresty neck, many horses (42 percent) were not obese but did have a cresty neck, and the remaining 31 percent had neither a cresty neck nor were obese. And although obese horses had higher insulin levels than non-obese horses (suggestive of developing or established insulin resistance), the overall incidence of insulin dysregulation measured in obese horses with a cresty neck was less than 10 percent.

As highlighted in another recent article, induction of adiposty and multiple daily loads of hyperinsulinemia induced by repetitive feedings of high-glycemic rations (grain/concentrates, sweet feeds) may also play a role in the development of EMS. Those study authors concluded that providing cereal-rich meals appeared to be more important than induction of obesity in the development of EMS.

Finally, evidence suggests that weight gain achieved via excess intake of even low-glycemic loads (e.g., fat-rich meals) can occur without any alterations in insulin regulation.

3. Diagnose EMS Instead of Guessing

So if we can’t rely on age, breed, obesity, regional adiposity, or the presence of a cresty neck to diagnose EMS, then we are reliant on clinical testing. One of the most recent versions of Veterinary Clinics of North America: Equine Edition suggested several tests for EMS. The central working theory is that insulin resistance develops in the face of a continual intake of sugar-rich foods, and hyperinsulinemia directly causes laminitis.

The best field option is the oral sugar test using corn syrup. Simply fast the patient for 5 to 6 hours if possible (although this is not always possible or necessary), administer 0.15 mL/kg light corn syrup, then measure plasma insulin 60-90 minutes later. Insulin levels >45 mU/L suggest the presence of insulin resistance. See the full-length article for information on using glucose instead of corn syrup. 

4. Not a Lone Rider

Overall, the basic facts remain stable: horses 5 to 15 years of age and ponies of hardier breeds that are frequently obese with cresty necks are at risk for (or already have) insulin dysregulation and are therefore at risk of EMS … some of the time. In addition, it’s important to bear in mind that when it comes to equine endocrine disorders, patients can suffer more than one concurrently. Many horses with EMS will also be diagnosed, ultimately, with PPID. Further, other endocrine disorders, albeit rare, have been described in horses and need to be considered. These include diabetes mellitus, neoplasia-associated hypoglycemia, and thyroid and adrenal disorders.

5. Resistance is Not Futile

For all you Star Trek TNG fans out there, be like our intrepid Jean-Luc Picard and refuse to accept assimilation and complacency. Encourage clients to:

  • Accept the things they cannot change. This includes age and breed;
  • Change the things they can. Eliminate concentrates and grazing on lush pastures (use dry lots or grazing muzzles), soak hay to reduce sugar intake (water-soluble or nonstructural carbohydrates), and consult with an equine nutritionist regarding appropriate diet, ration balances, and hay testing; and,
  • The knowledge to ask their veterinarian for assistance early to avoid life-threatening bouts of laminitis.
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