Update on Equine Metabolic Syndrome

Obese horses with cresty necks, insulin resistance, and a tendency to develop laminitis have long been a management challenge for owners and veterinarians. It seems that no matter the feeding regimen, these horses don’t lose weight easily and are always on the verge of foundering. This cluster of characteristics has been termed equine metabolic syndrome, or EMS, and a lecture presented at the 2013 annual convention of the American Association of Equine Practitioners contained some recent information on the condition.

In the lecture, Ray Geor, B.V.Sc., Ph.D., from Michigan State University explained that horses with EMS characteristically show fat deposits in the neck area and also around the tailhead and behind the shoulder. While any horse can develop laminitis after gorging on carbohydrates or in response to body-wide inflammation or infection, horses with EMS are at risk for laminitis even without these precipitating events. They also may have abnormal responses to oral or intravenous glucose or insulin challenges. However, the expression of these characteristics is not found in every horse with EMS, making diagnosis a challenge.

Some evidence suggests that equines of Dartmoor, Welsh, Morgan, Tennessee Walking Horse, Saddlebred, Arabian, and Paso Fino breeding may be somewhat more likely to develop EMS than those of other breeds. Horses that are “easy keepers,” meaning that they require fewer calories to maintain body condition, may also have a genetic makeup that puts them at increased risk.

Insulin resistance found in EMS horses has been linked in some equine studies to increased arterial pressure during the summer, but not in winter months. However, in studies conducted by Geor’s group, blood pressure was not different in ponies with or without EMS during spring, summer, and fall.

Obesity and laminitis are often seen in horses with EMS, but the relationship between these factors is not clear. In a study of more than 600 horses and ponies with or without laminitis, triglyceride level, fasting insulin concentration, and insulin after administration of corn syrup were most consistently elevated in the equines that had a history of laminitis. However, girth-to-height ratios and body condition scores did not indicate a clear correlation with laminitic and nonlaminitic animals.

Furthermore, laminitic equines that had recently lost weight and were no longer obese had the same variation in metabolic measurements such as insulin and triglyceride measurements as their obese cohorts. This suggests that obesity, in and of itself, does not cause laminitis. However, obesity may be an indication of metabolic dysfunction that can manifest in several ways. For horses and ponies thought to have EMS, Geor stated, “First, the presence or absence of obesity cannot be used as a diagnostic criterion. Second, whereas dietary restriction and weight loss may result in some improvement in insulin sensitivity and so forth in affected animals, the underlying metabolic dysfunction is likely to persist.”

In other words, EMS is more complicated than simply whether a horse is fat or thin, and reducing body condition in obese horses is not a cure for EMS. Dietary management that results in weight loss will, however, most likely help horses by decreasing strain on joints, reducing discomfort in mildly laminitic hooves, and moderating inflammation linked to some types of adipose tissue.