equine metabolic syndrome

Perhaps you’ve had equine metabolic syndrome, or EMS, on your radar for the last decade or so. Or maybe your veterinarian first explained it a few years ago, when he thought your obese pony might be at risk for some of the disease’s dangerous side effects, such as acute laminitis. You might only have realized its importance recently, maybe after seeing friends or barnmates struggling to keep their horses with insulin dysregulation (ID, abnormal insulin responses), chronic laminitis (in which the leaflike laminae that suspend the coffin bone within the foot become inflamed or fail and separate from the coffin bone and the hoof wall, allowing the bone to rotate or sink), or both as healthy as possible through spring grass growth, lush summer pastures, and fall pasture sugar spikes.

In reality, however, veterinarians and researchers have been recognizing, dealing with, and studying this syndrome and its side effects for centuries. At the 2018 Kentucky Equine Research Conference, held Oct. 29-30 in Lexington, Kentucky, Pat Harris, MA, VetMB, PhD, Dipl. ECVCN, MRCVS, reviewed how our understanding of EMS has changed over the years and current thoughts on it. Harris heads the WALTHAM Equine Studies Group, in Leicestershire, England, and is the only RCVS-recognized U.K. specialist in veterinary clinical nutrition (equine).

“Metabolic disease in horses is not new,” she said. “Aristotle, for example, most likely referred to laminitis around 350 BC as ‘barley disease,’ presumably because it was associated even then with excessive consumption of grain.

“The first full description or review was possibly commissioned by Constantine the Great in the fourth century, although it was not published until around AD 900,” she continued. “This noted, as we do today, that the condition could have multiple causes, including traveling on hard surfaces, overeating, and drinking too much cold water when overheated. Interestingly, considering today’s link between obesity and laminitis, this review apparently recommended exercise and diet restriction as part of the treatment and management regimen.”

It wasn’t until the 1980s, however, that researchers identified a potential link between insulin levels, laminitis, and obesity, she said. “That is, obese ponies tended to be less insulin sensitive and, therefore, there could be a link between insulin resistance (IR) and laminitis.”

Obesity and Body Condition Scoring

In humans, obesity can be defined as a pathological (disease-causing) accumulation of fat that has adverse health effects. Harris said the same definition can apply to equids.

This left many questions, Harris said—How much fat is excessive? How can we evaluate it? Does it matter where the fat is?

In the late 1970s, Don Henneke, PhD, and colleagues developed a body condition scoring (BCS) system for evaluating fat cover of pregnant Quarter Horse mares—a 5 on the 9-point scale is typically considered ideal, and the higher or lower the BCS than a 5, the more over- or underweight the horse is, respectively. Because these researchers were perhaps more concerned with underweight mares than overweight ones, Harris said, they described the lower BSCs in more detail than the higher ones.

Harris’ work with colleagues Alex Dugdale, MA, VetMB, PhD, DVA, Dipl. ECVAA, PGCert(LTHE), FHEA, MRCVS, and Caroline Argo, BSc, BVSc, PhD, Dip ECAR, MRCVS, then at the University of Liverpool, suggested that horses with BCSs of 7 or higher should be considered obese, Harris said.

Since Henneke completed his work, researchers have created additional BCS systems tailored to other breeds—a Quarter Horse isn’t built the same as a Warmblood, for instance, and doesn’t necessarily develop fat in the same places—although the original system is still commonly used today. 

Harris said that, compared with its original use in cattle to evaluate fat and muscle cover, body condition scoring in horses was used to measure external fat depots. This, therefore, needed further validation. 

So, in research supported by World Horse Welfare, she and colleagues at Liverpool sought to confirm the relationship between BCS and the amount of fat, muscle, and other components in the equine body. They conducted their initial study in seven pony mares with BCSs ranging from just over 1 to 7 that were euthanized for unrelated clinical reasons, and others have followed it up with studies in horses. The researchers, with the owners’ permission, dissected all the fat, muscle, and bone from the ponies and weighed it. They also analyzed the body’s lipid content.

“This work ultimately confirmed that—in both ponies and horses with a stable BCS—the percentage of fat in the horse’s body increases proportionally as BCS rises up to around a BCS of 7,” Harris said. “This meant we could continue using the BCS scale as an indicator of the fat content of the body up to around a BCS of 7 (above which we can say they are obese). Although BCS can give an indication of the fat content, it remains subjective, so a more objective method to accurately determine the total fat content of a horse/pony was developed (deuterium oxide dilution). It is hoped that this can be further developed into a more accessible clinically applicable test in the future.”

The study also revealed that fat in equids is not just laid down on the outside—it is found internally, as well. And, Harris said, internal and external fat increase together as BCS increases—i.e., there’s as much adipose (fat) tissue on the inside as we can see on the outside—but it’s not a linear increase.

“When you go from BCS from 3 to 4, you’re increasing the body’s fat percentage by about 2-3%,” she said. “When you go from a score of 6 to 7, you’re going to increase by more than 12%.

“An important note to remember is that when obese ponies are put on a weight restriction diet, they may not show initially any change in their external fat deposits,” Harris added. “This means that their BCS may not change even though they are losing weight, as they may lose internal fat first, which we cannot detect through the BCS score. This is why it is important to monitor both weight and BCS when an animal is on a diet.”

equine metabolic syndrome

Do We Have an Obesity Problem?

“Absolutely,” said Harris.

She said researchers have conducted many studies looking at obesity prevalence in equids, and the findings shed clear light on a problem:

  • In one study of 319 pleasure riding horses in Scotland, researchers classified 45% of the animals as fat or obese;
  • In two studies from the U.S., researchers classified 48% of 366 horses from North Carolina as overweight or obese and 51% of 300 mature horses from Virginia as overconditioned or obese;
  • In a study of 127 horses and ponies in England (living at pasture for at least six hours per day), researchers classified about 28% as obese at the end of winter and about 35% as obese at the end of summer; and
  • In a study of more than 300 horses competing at a U.K. national unaffiliated championship, 41% were overweight and 21% were obese, with show and dressage horses being the most likely to be overweight.

But why are so many horses obese? Researchers have studied that, too, Harris said, although there’s still work to be done. Some factors include:

  • Breed—In the one small study in the U.K., native breeds (i.e., Shetland, Exmoor, and Highland ponies) and cobs were 14 times more likely to be obese than others, such as Thoroughbreds, researchers found.
  • Pasture—“In the U.K., many fields now used for horse grazing used to be grazed by dairy cows, and now we’re putting native breeds out on these potentially energy-rich pastures,” Harris said. “(By doing that) it is possible to for a 300-kilogram (660-pound) pony to ingest the same number of calories as a 500-kilogram (1,100-pound) racehorse in training.”
  • They become wise to their daily routine—Even if an owner is trying to limit grass intake, their horse or pony might have “wised up to this,” Harris said. Researchers showed that, by Week 5 of being on limited pasture turnout each day, “ponies ingested an estimated 40% of their daily dry matter intake in grass in three hours—and work has shown that ponies can ingest almost 5% of their body weight in dry matter per day and more than 1% of their body weight in dry matter during a three-hour turnout on pasture, which is all that would be recommended for a day when on a severe weight loss restriction diet,” Harris said. “And, obviously, you cannot leave horses and ponies with no feed for 21 hours.”
  • Too many calories and not enough exercise—Researchers and veterinarians alike have observed that owners often don’t feed appropriately for the exercise level they’re undertaking or they believe their horses are working harder than they are and, thus, require more calories than they actually do.

One factor that does not seem to be that relevant is digestibility differences between obese and non-obese animals, Harris said. She and colleagues recently evaluated this in Welsh pony mares and are in the process of preparing the study for publication.

Perhaps the biggest issue with obesity, Harris said, is that some owners might not realize their horses are overweight.

“They think their overweight horses are in perfect condition, perhaps due to a shift in the perception of what is ideal body condition,” she said.

In one study of nearly 550 owners, only 11% correctly identified all the examples of overweight horses they were shown.

Obesity and Insulin Dysregulation

While obesity is clearly an issue on its own, it also contributes to other EMS-related problems, including insulin dysregulation.

As mentioned previously, researchers began investigating the link between obesity, insulin dysregulation, and laminitis development in the mid-1980s. This led to a series of studies showing that:

  • Obese Thoroughbred geldings were more insulin-resistant than their leaner colleagues;
  • Many laminitis-prone animals needed more insulin to exhibit the same glucose response to meals as healthy horses (i.e., they were showing signs of insulin resistance);
  • Insulin-resistant ponies were more at risk of developing pasture-associated laminitis than their healthy counterparts; and
  • Australian researchers determined that high doses of insulin itself could cause laminitis in ponies and horses.

Around this time researchers were typically using the frequently sampled intravenous (IV) glucose tolerance test (FSIVGTT) to diagnose insulin resistance, but it’s not an overly practical test, Harris said. With it, the practitioner gives IV glucose to the horse, followed by insulin 20 minutes later. Blood glucose and insulin levels are monitored frequently, every minute to every five to 10 minutes for about four hours. The FSIVGTT can be carried out in a field setting but is time-consuming and expensive.

So, researchers began trying to identify other laminitis predictors. Initially, a group from Virginia Tech reported that an increased risk for laminitis in apparently healthy ponies could be characterized by a set of risk factors known as the pre-laminitic metabolic syndrome (PLMS). In one study, ponies were four times more likely to develop laminitis if they had three of the four listed criteria (the first three determined on a single blood test):

  • Insulin resistance;
  • Increased β‐cell response;
  • Elevated serum triglycerides; and
  • Body condition score of 6 or higher.

“Doctors had already suggested, in people, that accumulating belly fat could be worse for health than laying down large amounts of other types of fat,” Harris said.

Taking a lead from those scientists, she and colleagues at Virginia Tech wondered whether fat accumulations on horses’ necks—the “cresty neck”—could have similar, more damaging implications than other localized fat deposits in equids. Therefore, they developed the cresty neck score (CNS)—a scale of 0 to 5, where a score of 0 equals no visible crest and a score of 5 means the crest is so large it droops to one side permanently.

When the PLMS criteria were modified to include CNS, Harris said, in one study animals exceeding three of the four criteria were six times more likely to develop laminitis than healthy horses.

equine metabolic syndrome

Researchers Coin ‘EMS’

It wasn’t until around 2010 that scientists finally agreed to use the phrase ‘equine metabolic syndrome,’ Harris said. At this time EMS was used to describe the typical “easy keeper,” which had three specific traits:

  • Increased general or regional adiposity;
  • Insulin resistance; and
  • A predisposition toward laminitis (clinical or subclinical laminitis that develops in the absence of recognized causes such as grain overload, colitis, or retained placenta).

While there was finally a term for what they’d been studying, they didn’t let up on the research.

“More recent studies have suggested that we may be better to use the term insulin dysregulation rather than insulin resistance,” Harris said. “where ID refers to any combination of fasting hyperinsulinemia, postprandial hyperinsulinemia (response to oral sugar test or a consumed feed), or tissue insulin resistance and led to the development of more practical tests, such as the oral sugar test.” 

More recent work has shown that:

  • Not all obese animals show tissue IR, and lean animals can show IR, but overall, being obese increases laminitis risk and can have other significant adverse effects;
  • It is possible for horses to develop a “normal weight obese syndrome,” when they’re thin on the outside and fat on the inside, Harris said. This could especially be important in older horses with pituitary pars intermedia dysfunction (or PPID) that can lose muscle mass and external fat but still be at increased risk of laminitis. More work is needed in this area;
  • While hyperinsulinemia doesn’t always mean a horse will develop laminitis, it does suggest an increased risk, and the response to an oral glucose/sugar test might be one of the most important predictors of risk; and
  • Low adiponectin concentrations (which can develop in obese horses) are linked to an increased laminitis risk. However, obesity doesn’t always result in decreased adiponectin levels;

“All the research from around the world has led to the conclusion that the risk of laminitis is a complex interaction between genetics and the environment,” Harris said.

She added that the most recent definition of EMS has evolved to: “A clinical syndrome associated with an increased risk of laminitis that includes insulin dysregulation and any combination of increased or regional adiposity, weight loss resistance, dyslipidemia and altered adipokine concentrations.” (Equine Endocrinology Group, 2018)

Moving Forward

While researchers and veterinarians know significantly more about EMS now than they did when they began focusing on it, there’s still much more to learn not only about the link between obesity, insulin, and laminitis but also how to manage animals to help prevent them from becoming obese and/or developing ID. 

“Future work is also likely to continue to focus on improved ways of predicting risk as well as understanding why some animals seem to be resistant to developing laminitis even if their test results would suggest otherwise,” Harris said.