Equine Metabolic Syndrome
by Dr. Janet Han, DACVIM
Great Lakes Equine Wellness Center, Inc.

Equine metabolic syndrome is a disease that’s being recognized with increasing frequency in horses. In the past, equine metabolic syndrome was called many different things, including peripheral Cushing’s disease, prediabetes, or hypothyroidism. Today, the term equine metabolic syndrome is the most accepted name for this disease, and comes from the fact that it shares many similarities with metabolic syndrome in humans.

The three main characteristics of this condition are obesity, insulin resistance, and laminitis. A horse with equine metabolic syndrome can have an overall obese body condition, or can have what is known as regional adiposity, in which fat accumulates at certain sites such as the crest of the neck, the tailhead, or the sheath. Horses with this disease are also insulin resistant. Insulin is a hormone in the body that regulates carbohydrate, fat, and protein metabolism. In response to increased levels of glucose in the blood (typically following a meal), insulin is secreted from the pancreas and causes the uptake of glucose by tissues, particularly adipose tissue (fat), skeletal muscle, and the liver. With insulin resistance, the body has a decreased response to insulin, and this can result in high levels of glucose in the blood, since the tissues are not taking up as much glucose from the blood as normal. In response, the pancreas will secrete more and more insulin, leading to elevated insulin levels in the blood. The third characteristic of equine metabolic syndrome is laminitis, or founder. This might be the initial reason the horse is first seen by the veterinarian, or it may go unnoticed. Some horses may show evidence of previous episodes of laminitis, such as having abnormal growth rings on the hooves.

Horses with equine metabolic syndrome typically range in age from 6 to 20 years, and are usually regarded as “easy keepers.” Males and females are equally likely to be affected. There seems to be a breed predilection, with pony breeds, Spanish mustangs, Morgans, Paso Finos, Peruvian Pasos, Saddlebreds, and European warmbloods appearing to be particularly at risk. Diagnosis is made through a combination of clinical signs and specific tests. The most common tests that are performed are measuring the level of glucose and insulin in the blood. Glucose is usually normal to elevated, and insulin is typically increased. Glucose tolerance tests, in which an oral or intravenous solution of dextrose is administered, can also be performed to specifically assess if insulin resistance is present.

Treatment includes both dietary and exercise management, and in some cases medications. The main goal is to promote weight loss. Diet should primarily consist of hay, and sweet feed and grain in general should be eliminated. Access to pasture should be limited, and completely eliminated in horses with active laminitis or those that are particularly at risk for repeat episodes of laminitis. Hay should be weighed, so that the amount the horse is receiving can be directly regulated. Ideally, the nonstructural carbohydrate content (NSC) of the hay should be measured through laboratory testing. The higher the NSC content in the hay, the more likely feeding it will worsen insulin resistance. If hay is not directly analyzed, it should be soaked for 30 to 60 minutes in cold water to reduce NSC content prior to feeding. Diet may also need to consist of a multivitamin supplement, a protein supplement, and/or a vitamin E supplement. Unless actively laminitic, the horse should also receive daily exercise to aid in weight loss.

Levothyroxine, a thyroid hormone supplement, may need to be added to the treatment regimen if weight loss is occurring too slowly or if the horse has reached an adequate body condition but continues to remain insulin resistant. Levothyroxine has been shown to increase weight loss and improve sensitivity to insulin. It’s important to remember that horses with equine metabolic syndrome are not truly hypothyroid, although their thyroid levels may be low on bloodwork. A newer drug called metformin has also started to be looked at for use in treating equine metabolic syndrome. This drug is used fairly extensively in human medicine, and has been shown in humans to help decrease glucose production and improve insulin sensitivity. The efficacy of metformin in treating equine metabolic syndrome still needs to be further investigated. Supplements, such as magnesium and chromium, are also frequently added to the treatment regimen, as these substances may aid in improving sensitivity to insulin.

In general, the prognosis for equine metabolic syndrome is favorable to good, as long as the disease is detected early and appropriate management measures are put into place. The prognosis becomes worse if active laminitis with structural changes in the hoof is present or if appropriate management changes are not carried out. Equine metabolic syndrome is a disease that can frequently be well managed with dietary and exercise modifications. Therefore, this is a disease in which the owner will play a particularly important role in treatment by making sure these changes are made and most importantly, adhered to throughout the horse’s life. 

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Janet Han, DVM, MS, DACVIM, of Great Lakes Equine Wellness Center, completed an equine internship at the Rochester Equine Clinic in Rochester, NH followed by a residency in equine internal medicine at the Marion duPont Scott Equine Medical Center in Leesburg, VA. She has a master's degree in Biomedical and Veterinary Sciences at Virginia Polytechnic Institute and State University. Dr. Han is a Diplomate of the American College of Veterinary Internal Medicine.