Equine Metabolic Syndrome
By Dr. Susannah Lewis, DVM, PhD, Rainland Farm Equine Clinic

Equine metabolic syndrome (EMS) is used to describe horses with excessive fat deposits and reduced insulin sensitivity.  Because insulin isn’t as effective in EMS horses, they produce more insulin after eating carbohydrates (sugars) than healthy horses, which leads to elevated insulin levels in their blood on a regular basis. Increased insulin is linked to the development of laminitis, the most serious complication we see with EMS.  EMS horses will often, but not always, have a cresty neck and fat deposits over their shoulder, and tailhead.

What causes EMS? The cause(s) of EMS are not entirely known.  It is more common in certain breeds, including ponies, Morgans, quarter horses and some warmbloods, and there is suspected to be a genetic link.  It is most common in ‘easy keeper’ horses, and should be considered in horses that are significantly overweight (>6 on the BCS scale).

How do you diagnose EMS? EMS is diagnosed by clinical signs and bloodwork.  Typically, blood will be drawn after an overnight fast to check baseline insulin levels.  In some cases, an oral sugar test may be recommended. In this test, we fast horses overnight, and then measure their blood glucose and insulin levels before and after giving a dose of corn syrup.  An EMS horse will release significantly more insulin and have higher blood glucose levels following the dose of corn syrup than a healthy horse.  Because pituitary pars intermedia dysfunction (PPID, also called equine Cushing’s) can co-occur with EMS, PPID testing is also frequently recommended.

Why is it important to diagnose and treat EMS? Uncontrolled EMS can lead to laminitis, an extremely painful inflammation in the hooves which can be career limiting and even life threatening.  Better control of insulin levels reduces the risk of laminitis developing.  Additionally, EMS horses are prone to being overweight, which increases wear and tear on joints and ligaments and may increase the risk of developing equine Cushings. Screening is strongly recommended for most overweight horses and others at risk.

Managing a horse diagnosed with EMS:

  1. Body Condition Score (BCS) Goal: Ideally, horses should be a 4 to 5 out of 9 on the BCS scale.  Ribs should be easily palpated and fat deposits along crest, shoulder and tail head reduced. Equine Metabolic Syndrome horses tend to have disproportionate fat deposits, so fat deposits may remain even at a healthy weight. For more information on the BCS scale: https://ker.com/tools/library/horse-body-condition-score-chart/
  1. Turnout: Turnout not only improves quality of life, it also promotes exercise and movement. However, grass can contain particularly high levels of sugars and pose a significant risk. Dry lot turnout is ideal for horses diagnosed with EMS. If that is not feasible, turnout only with a grazing muzzle is strongly recommended.  Sugar content in grass is lowest from dawn to noon, which are the ideal hours for turnout with a muzzle.  Sugar content in grass also increases when the grass is ‘stressed’ including early growth in the spring, overgrazing, drought and when nights are cool (<40F), and allowing access to grass in these times is more likely to be problematic.
  2. Diet
    1. Hay: Forage remains a critical element of a horse’s diet, and horses should continue to receive about 1-1.5% of their body weight in forage daily (i.e. 10-15 pounds daily for a 1000lb adult horse).  Hay can be soaked for at least 30 minutes to reduce carbohydrate content. Additionally, feeding in a slow feed net and feeding multiple small meals can slow intake and prevent a spike in insulin following a large meal.  Ideally, hay would be tested to determine the nonstructural carbohydrate content, with a target non-structural carbohydrate level of 10% or lower.
    2. Grain: Most EMS horse are overweight and do not need concentrated calories from grain.  A ration balancer can provide all essential nutrients and amino acids with lower carbohydrate load and fewer calories.  Feed according to package directions to ensure sufficient nutrients. Most senior feeds are forage based and lower in carbohydrates, and may also be a good option.
    3. The above recommendations are intended for easy keeper type horses who need to lose weight, as is the case for a majority of EMS horses.  If you feel your EMS horse is too thin, please talk to your veterinarian to formulate an appropriate diet for your horse.
  3. Exercise: Exercise both improves insulin sensitivity and promotes weight loss. Unless a horse is rehabilitating from a bout of laminitis or other injury, EMS horses should be in a regular exercise program appropriate to their fitness level. Talk to your veterinarian if you’d like specific recommendations for an exercise program. Even miniature horses can enjoy long walks or arena training!
  4. Medication: If the above management changes do not adequately control your horse’s insulin levels, there are some medication options as well.  Thyro-L is given to supplement the thyroid gland and increase metabolism in order to promote weight loss.  Metformin can also be given to improve insulin sensitivity.
  5. Monitoring: Fasting insulin levels should be periodically checked to monitor response to treatment.  Appropriately controlled insulin level is key to reducing the risk of laminitis in these horses. Regularly using a weight tape to estimate your horse’s body weight can also help monitor weight loss and catch any weight gain quickly.