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July 17, 2009

The Insidious Onset Of Equine Metabolic Syndrome

As more horses are recognized as being overweight, practitioners are recommending management changes to ward off the risk of EMS.

The sight of a fat pony feasting on lush grass has long been a red flag for laminitis and founder. But obesity and excess eating are risks no longer reserved for pint-sized ponies. Nor is laminitis the sole concern. Lately, horses have begun to follow a similar tract to their human counterparts in areas of eating and obesity and related health problems.

As with people, many horses are more sedentary than they were decades ago. In addition, they are eating high-sugar diets and consuming more calories than needed to maintain condition and energy requirements. In human medicine, doctors have trumpeted warnings of the relationship between obesity and type II diabetes. Equine veterinarians are raising similar concerns for a condition known as equine metabolic syndrome.

“What we’re looking at is a complex syndrome, and I think we’ve just scratched the surface,” said Harold Schott, DVM, PhD, a professor at Michigan State University with an interest in equine metabolic disorders. “But what I tell owners is if your horse is obese, EMS is a concern.”

Three Components

Though veterinarians have recognized the association between obesity and laminitis for more than 50 years, the term EMS was only coined in the early 2000s. It is defined as a cluster of problems including these three components:

• Obesity. Typically, horses affected by this syndrome are easy keepers; they maintain weight on relatively sparse forage and readily gain pounds on rich diets, be it pasture or concentrated feeds. Some EMS horses display general obesity while others have regional adiposity—fat deposits along the neck and tail head. Said Schott, “These horses in particular are often more challenging to manage than those with generalized obesity.”

• Insulin resistance. Secreted by the pancreas, insulin is a hormone responsible for regulating blood glucose concentration. When glucose concentration is high, insulin stimulates rapid glucose absorption by cells, especially skeletal muscle cells, for fuel. Insulin also stabilizes body fat tissue and inhibits glucose production and secretion by the liver.
   
IR leads to a cascade of events in which tissues fail to respond to insulin, causing higher than normal blood glucose concentration (hyperglycemia) and elevated insulin (hyperinsulinemia).
   
“Imagine that insulin is the key that unlocks the cells to put glucose in,” said Rhonda Hoffman, PhD, associate professor of horse science at Middle Tennessee State University. “With IR, the horse is making tons of insulin, but the key just won’t fit in the door. By law of averages, a little insulin gets through but most doesn’t, so the body continues to make more and more.”

• Laminitis. Inflammation of the sensitive laminae of the hoof is another factor in this syndrome, though levels of severity vary widely. Some horses remain relatively comfortable but display abnormal hoof growth and dropped soles. Others suffer shifting of the coffin bone and show classic signs of laminitis pain.
   
Experts are unsure exactly what triggers EMS or how it progresses, though excessive weight may be a precipitating factor.
   
“In other species, fat cells in the abdomen produce an excessive quantity of hormones that block action of insulin, therefore contributing to IR,” said Philip Johnson, BVSc, MRCVS, professor of equine medicine and surgery at the University of Missouri. “Also, excess dietary energy can be stored in muscle cells in obese individuals. The accumulation of fat in muscle also interferes with the action of insulin.” But whether these findings in other species also apply to horses is not known.
   
Experts also theorize that IR may predispose horses to laminitis, either by altering blood flow to the foot or impeding glucose delivery to hoof keratinocytes.
   
“Frankly, there are several theories as to why the laminae fail, and this is just one,” said Schott. “In the hoof cells have several energy sources, and some rely more on glucose. In cases of insulin resistance, the cells may suffer glucose deprivation, and the basement membrane—the junction of dermis and epidermis in the hoof—loses integrity.”
   
Researchers are also investigating the relationship between cortisol activity, with respect to IR and laminitis.

Identifying And Diagnosing At-Risk Horses


EMS most frequently occurs, though is not limited, to older horses between the ages of 10 to 20. In addition, breeds predisposed to weight gain appear also at greater risk to EMS. They include ponies, warmblood breeds, domesticated Spanish Mustangs, Peruvian Pasos, Paso Finos, Andalusians, Saddlebreds and Morgans.
   
Prevalence is not well documented, though studies on equine obesity also recognize signs of EMS. In 2006 researchers at the Virginia-Maryland Regional College of Veterinary Medicine evaluated and assessed the body condition of 300 horses. Though ponies, draft breeds and pregnant or lactating mares were excluded, researchers noted that 51 percent of the horses were overweight and of those, 19 percent were obese. Blood samples drawn and tested revealed 18 percent of the overweight horses and 32 percent of the obese horses were hyperinsulinemic.
   
But because many owners either equate equine obesity with health and nutrition, or they prefer the appearance of a heavy horse, initial signs of EMS may not go unnoticed. “EMS is usually not recognized until insidious-onset laminitis develops in overweight horses that have no obvious risk factors for laminitis such as grain overload,” said Schott.
   
Johnson agreed that laminitis is usually the first tip-off: “Horses with IR often develop laminitis that hasn’t been recognized by the owner because there isn’t any pain necessarily in early stages.”
   
Veterinarians often use these clinical signs—weight and changes to the foot suggestive of laminitis—to diagnose EMS cases, but diagnostic tests to detect IR are also available. In the field, most veterinarians use an insulin-glucose concentration test. A horse’s diet is restricted to either hay or nothing at night, then blood is drawn in the morning; elevated insulin levels suggest IR. However, these findings are not always conclusive in affected animals.
   
“The problem is that an overweight horse can be insulin resistant and be found to have what is called fasting hyperinsulinemia, meaning the test results do not confirm the condition,” said Schott.
   
More specific but labor-intensive tests are also available. In field and hospital settings, a veterinarian may administer a bolus of glucose intravenously, then blood samples are collected over four hours. An exaggerated insulin response suggests IR.
   
While Schott recognizes the benefits of testing, he believes that clinical signs are often all that’s needed. “If the horse is overweight, he’s at risk. You don’t need a test to tell the owner what to do.”

Managing For Better Health

No drug treatment exists for EMS, however managing diet and exercise can improve conditions of affected horses.
   
If you are unsure whether or not your horse is overweight, contact your veterinarian for a wellness exam. You may also evaluate a horse using a body condition score (see sidebar). This system does not measure weight by pounds but offers an overall body condition rating and helps to identify underweight and overweight animals.
   
For weight loss, consider consulting with a veterinarian, an equine nutritionist or a feed consultant about necessary changes to your horse’s diet. The primary goal, said Hoffman, is to reduce the intake of nonstructural carbohydrates.
   
“NSC is a lab term for sugar and starches that exist in plant cell walls and help give it structure. It’s where the plants store nutrients,” she said. Consuming them increases glucose and insulin concentrations.
   
Traditional sweet feeds are rich in carbohydrates, with rates as high as 65 percent NSC, said Hoffman. Many horses do not require any concentrated feed in their diets and receive adequate nutrition simply with forage and access to a mineral block. For overweight horses, consider cutting grain meals entirely or offer a small serving of a low-starch, low-calorie pelleted feed.
   
Unfortunately, among some easy keepers, free choice forage also contributes to obesity. The study by the Virginia-Maryland Regional College of Veterinary Medicine found that many of the overweight horses consumed only hay or pasture grass.
   
Gauge NSC levels in your forage by sending a sample to a laboratory for analysis. State extension centers may have access to testing, as do some feed suppliers who offer consulting services. Dairy One Cooperative, based in Ithaca, N.Y., offers forage analysis, including NSC levels, among its services (www.dairyone.com).
   
How much NSC is too much? “No one knows for sure. That’s a big debate over what’s appropriate,” said Hoffman.

“Generally in the industry, feeds that are 12 percent or less NSC are considered low-starch. It’s also a good target for hay.”
   
Hoffman also recommends soaking hay in water before offering it to EMS horses. “Sugars dissolve in water,” she explained, “so soaking hay for 20 to 30 minutes will significantly reduce sugars and starches.”
   
Consider fitting overweight, pasture-kept horses with muzzles, which greatly restrict the amount of grass consumed in each bite. Without a muzzle, affected horses may need to be restricted to a dry lot.
   
Exercise is another vital component for weight loss and EMS management, but often it’s more difficult to execute.

“Let’s face it, horses are more sedentary than they used to be,” said Schott. “And some affected horses have seen a decrease in exercise, for example, if their rider has gone to college, the horse isn’t ridden as much. Exercise is a time commitment.”
   
Fitness work may not be feasible for horses suffering from chronic laminitis, but for all others, a regular routine that incorporates riding, longeing or round pen work can make a difference.

“Exercise is most effective in horses who are sedentary, especially when you’ve already made dietary changes,” said Hoffman. “I’ve looked at IR in obese horses and for those who were [really big], it didn’t matter what they ate, they were stuck at that weight. But exercise helped push them along.”
   
Fitness plans for weight loss vary according to a horse’s previous work load; certainly horses who have been out of work  must start slowly with short, light sessions and over several weeks build up to more frequent, intense exercise.
   
Even if your schedule does not lend itself to routine, daily workouts, studies show that even short periods of light exercise or training improve insulin sensitivity in horses. In a 2002 study of 12 aged mares, including six obese, IR horses, researchers noted that insulin infusion rates improved on the days when the horses were worked at the trot for 30 minutes.
   
To monitor improvements in your horse’s condition, talk to your veterinarian about testing blood glucose levels every six months and assessing body condition score. Photographing your horse every 90 days may help monitor changes in his condition.
   
Whether an owner prefers a horse’s fleshy, round appearance, or excessive gain is unintentional, veterinarians caution their clients to heed their horses’ weighty warnings. Fortunately exercise, dietary changes and regular hoof care may be all that it takes to control EMS.
   
Said Schott, “If people are really willing to give it a try for three months, they are conscientious about getting the weight off, they can turn a horse around.”


What’s In A Name?

Years ago, overweight horses that appeared insulin resistant and laminitic were believed to suffer from hypothyroidism or dysfunction of the pituitary gland, known as Cushing’s disease (also called pituitary pars intermedia dysfunction). The clinical signs were certainly similar to Cushing’s, which also occurs in aged horses and is sometimes accompanied by insulin resistance. But these horses did not display the trademark hairy coat associated with Cushing’s, endocrinologic tests didn’t support the diagnosis, nor did horses respond to drugs prescribed for the disease.

Up until about 2000, practitioners referred to the condition as Peripheral Cushing’s Disease or Equine Syndrome X (named for insulin resistance in humans), but a few years later decided that Equine Metabolic Syndrome best described the condition.


Assessing Weight

Is your horse too fat, too thin or just right? Most veterinarians recommend using the body conditioning score, sometimes called the Henneke Scale, named for Don Henneke, PhD, who developed the system during graduate study in 1983 at Texas A&M.

The system employs visual appearance and palpation to assign a score of 1 (poor) to 9 (extremely fat) based on the presence or lack of fat deposits along the horse’s neck, withers, ribs, crease down the back, area behind the shoulder and along the tailhead.

For the average horse (excluding broodmares and lactating mares), Rhonda Hoffman, PhD, considers a score of 6 as a yellow warning light. “Generally, we like horses to be in the 5 or 6 range. A 6 tells you to watch out, while a horse with a 7 or above needs to lose weight."

 

 
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