The Insidious Onset Of Equine Metabolic Syndrome

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.