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Equine Metabolic Syndrome (EMS)

What is Equine Metabolic Syndrome?

Equine metabolic syndrome is a health problem that affects many horses that are considered easy keepers. Symptoms include large regional fat deposits, obesity, insulin resistance, and recurrent episodes of laminitis. Unfortunately in this day and age, obese horses are more the rule than the exception. Despite education by veterinarians and nutritionists on keeping horses in a healthy body weight, horse owners continue to feed high calorie concentrates and hay. More often than not, the result is a plump middle-aged horse that is anything but healthy or athletic. He or she gasps for breath when subjected to mild exercise, and his or her limbs bear the brunt of unnecessary pounds.

Now researchers have uncovered yet another reason to keep mature horses slim and conditioned – equine metabolic syndrome. Because it’s a relatively recent discovery, veterinarians are just beginning to learn the intricacies of this disorder.

If this disease is not controlled, it can be debilitating for the horse, emotionally taxing for the owner, and it may even lead to euthanasia if meticulous diet and exercise management is not initiated. I hope to provide the facts that are presently known about the disease in order to help horse owners use preventative medicine to keep their horses as healthy as possible for as long as possible.

Insulin Resistance

Equine metabolic syndrome (EMS) is characterized by insulin resistance (IR) or high fasting insulin concentrations. The physiology of insulin starts with the body’s response to eating foods that are broken down through the gastrointestinal system into glucose or other sugar molecules. Abundant in certain feedstuffs commonly fed to horses, glucose is absorbed from the intestine and results in elevated blood sugar. High blood sugar leads to the release of insulin from the pancreas, which encourages the removal of glucose from the bloodstream by fat or skeletal muscle cells in normal animals. Once in the cells, glucose can be put to work immediately in order to fuel exercise or growth, or it can be stored as glycogen or fat for later use. Insulin resistance suggests that either the central tissue (liver) or the peripheral tissues (skeletal muscle or fat cells) are insensitive to the action of insulin, or the quantity of insulin released by the pancreas in response to hyperglycemia has declined. This leaves glucose circulating in the bloodstream. Because glucose levels do not drop, the pancreas continues to discharge insulin, leading to elevated concentrations of insulin in the bloodstream – a condition known as hyperinsulinemia.

Little is known about what predisposes a horse to this disease, however the responsibility may rest on genetic and environmental factors. EMS is similar to type II diabetes in humans, where the action of insulin is impaired despite often elevated concentrations. In humans, causative factors for this disease are well documented: aging, pregnancy, smoking, reduced physical activity, and obesity.

In horses, obesity appears to be related to the onset of metabolic syndrome. Age and diet may also be directly related to the development of this disease. It is believed that as a horse ages, their sensitivity to insulin decreases. In addition, diets high in starch and sugar cause significant spikes in blood glucose and insulin, causing problems to develop over time.

It is important to note that not all fat horses are insulin resistant or hyperinsulinemic. Current postulation is that horses whose fat cells produce high levels of the hormone leptin (which plays a key role in regulating energy intake and expenditure, including appetite and hunger, metabolism, and behavior) are prone to insulin resistance. Leptin is not believed to cause insulin resistance, but it is found to be higher in horses that are insulin resistant.

Laminitis (“Founder”)

An unwelcome clinical sign of EMS is laminitis. The type of laminitis exhibited by these obese middle aged horses tends to be mild in contrast to that seen with other diseases. Recurrent bouts of lameness that are mild in nature may be observed during the early stages of EMS. Most of the time, these horses have such mild clinical signs of pain that even knowledgeable and attentive horse owners can not definitively detect lameness. If the disease is not meticulously controlled, the chronic form of recurrent laminitis can be identified by abnormal hoof growth, unusual growth lines, prolapsed soles, and separation of the white line. End stage laminitis may be obvious on radiographs, which is where the term “founder” comes into play. Founder is the shifting of the coffin bone within the hoof capsule, and it is clearly seen through the use of radiography. Not all horses that experience laminitis will founder.

Regardless of the cause, laminitis is the result of alterations in structure of the laminae, the interconnected layers of tissue that secure the integrity of the foot. Laminae hold the coffin bone in the appropriate position until disrupted, when they do not hold as well and result in rotation or sinking (i.e. founder). In obese horses, researchers believe that insulin sensitivity and vascular spasms may incite changes within the laminae. Regardless of current theories however, a definitive cause for obesity associated laminitis remains an uncertainty.


Currently the diagnosis of EMS is based on clinical signs and physical traits, along with blood glucose and hormone concentrations (including insulin and leptin). Measuring glucose and insulin after a glucose challenge test can be helpful for diagnosis in clinically suspect horses that have normal fasting glucose and insulin levels. Affected horses usually are between the ages of eight and eighteen, and no breeds are particularly prone more than others. The most apparent physical characteristic that all EMS patients share is the distribution of body fat. Regional areas of unusual fat accumulation include the top of the neck (cresty neck) and over the top of the shoulders, croup, and tail head. Significant fat sometimes settles in the sheaths of geldings and in the mammary glands of mares. Some of the physical traits are similar to horses with Pituitary Pars Intermedia Dysfunction (PPID) or Cushing’s disease — and to confuse things even more, some horses can be affected by both diseases simultaneously. In fact, EMS was previously falsely diagnosed as Cushing’s because of the abundance of similarities between the two in appearance. The treatment for Cushing’s/PPID and EMS do not correlate and need to be managed quite differently, so it is of the utmost importance to get an accurate diagnosis.

Managing EMS

Once EMS is officially diagnosed by your veterinarian, the two most important factors to control in order to prevent the disease from progressing are diet and exercise.

Horses become overweight because they consume too many calories in relation to the work asked of them — just like humans or any other living creature. Mature horses diagnosed with metabolic syndrome should not be given grain, feed mixed with molasses, or added sugars of any kind. Access to pasture should be limited, and a balanced rationed feed (concentrated protein, minerals, and vitamins) can be given to provide essential nutrients without Non-Structural Carbohydrates (NSC). Important minerals to include in the diet are magnesium and chromium. Magnesium is involved with the secretion of insulin, and chromium has been thought to reduce blood glucose levels in horses.

2014-05-nibblenetIf the affected horse needs to lose weight, placing the hay in a NibbleNet will allow them to eat small amounts of hay throughout the day instead of large amounts in a single serving. It is recommended to soak hay for at least 30 minutes before feeding in order to reduce the amount of sugar within the hay. One of the hardest things owners of horses diagnosed with EMS must contend with is restricting pasture access. Having diagnosed this disease to a number of horses that have access to beautiful lush pastures, it is heartbreaking for their owners to not allow them on their normal territory. One way around this is to place a grazing muzzle on the horse to restrict their grass intake while still allowing them to be turned out.

In addition to changes in diet, exercise should be increased in order to slim down overweight horses. Exercise programs must be designed with the individual horse in mind. If the horse has suffered from laminitis, consultation with your veterinarian and farrier is warranted before a new exercise regimen is implemented.

Sometimes horses that need to lose weight are prescribed with levothyroxine (Thyro-L), which is a synthetic thyroid hormone that is used to increase metabolism and facilitate weight loss, but it can also reverse insulin resistance. Thyro-L should only be used for 3-6 months, and efficacy after 6 months is limited. If the horse is undergoing a laminitis episode, he or she may be placed on NSAIDS to help counteract the inflammatory process and provide pain relief. With all this said, there is no magic pill that can stop the debilitating path of EMS. A diet and daily exercise program that emphasizes optimal body condition may be just enough to dodge the effects of this precarious disease.

Katy Raynor

Written by Katy Raynor, DVM, New England Equine Medical and Surgical Center. Katy can be reached at 603-749-9112 or


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