horse with EMS

Equine Metabolic Syndrome

What is equine metabolic syndrome?


  • Equine metabolic syndrome (EMS) is a disorder associated with an inability to regulate blood insulin levels (insulin dysregulation).
  • Affected horses commonly show increased regional fat deposition and a reduced ability to lose weight.
  • It often affects “thrifty” equids such as ponies, donkeys, Arabians, and mustangs.
  • Horses with EMS are at a high risk for laminitis.
  • EMS is primarily treated through diet and exercise management.

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Equine metabolic syndrome (EMS) is a disorder associated with inappropriate blood insulin levels (insulin dysregulation) along with a combination of increased fat deposition and a reduced ability to lose weight. When affected horses consume meals high in specific carbohydrates, their bodies produce higher than normal levels of insulin and are slow to return to baseline values. These clinical signs were previously referred to as hypothyroidism, peripheral Cushing’s disease, prelaminitic syndrome, or Syndrome X.

This disorder often affects a subset of horses bred to survive in harsh climates, including ponies, donkeys, Arabians and mustangs. These breeds utilize glucose very efficiently to ensure that they have plenty of energy reserves when food is scarce. When these “thrifty” horses are placed in an environment where they have access to an abundance of carbohydrates and do not get as much exercise as they would in their ancestral habitats, they suddenly consume too many calories.

One of the most serious consequences of EMS is the development of laminitis, which can result in devastating separation of the hoof from the underlying coffin bone. This can cause excruciating pain and mechanical damage that at best requires special management and at worst may be life threatening. Laminitis can happen in a variety of circumstances, but EMS lowers the threshold for laminitis and makes it easier to develop.

Another potentially negative consequence of EMS is that normally when horses are sick and not eating, their fat stores are mobilized into the blood stream and transported to the liver where they are broken down and used as energy. When these horses have a lot of stored fat, more fat is released into the blood than the liver can handle at one time (hyperlipemia). Consequently, the liver is infiltrated with fat to the point that it becomes diseased and the horses feel even worse and even less like eating. This turns into a vicious cycle where the horse does not eat and more fat is mobilized which makes the horse not want to eat. Emergency veterinary intervention is often needed to reverse this downward metabolic spiral.

There appears to be a genetic predisposition to EMS, but the underlying reason why some horses develop EMS and others do not is currently unknown.

Horse with fat depositions on the neck
Horses with EMS often exhibit fat deposits on the neck, giving it a "cresty" appearance.

What are the clinical signs of equine metabolic syndrome?

The hallmark of EMS is insulin dysregulation, an abnormal insulin response to oral or intravenous (IV) glucose and other similar sugars. Most affected horses are obese, with increased body condition scores (BCS) overall (BCS >6 out of 9) and increased regional fat deposits in the neck (“cresty” appearance), ribs, and tailhead regions. However, insulin dysregulation can occur in thinner animals as well and not all horses with an increased BCS have EMS.

Affected horses are at a high risk for laminitis, especially when provided with access to pasture or high-carbohydrate feeds.

How is equine metabolic syndrome diagnosed?

Blood glucose and insulin level measurements at single time points are screening tools for EMS and insulin dysregulation. Blood samples are typically taken first thing in the morning before the horse is fed to avoid spikes in glucose and insulin associated with eating. If the insulin concentration is above a certain level (>50 µU/mL), the horse is diagnosed with insulin dysregulation. If insulin concentrations are <50 µU/mL, additional tests may be indicated to decide if the horse has insulin dysregulation or not. Blood glucose and insulin can be influenced by many factors, including stress, so it is best to obtain these blood samples when the horse is at home in a comfortable environment and routine.

An oral sugar test (OST) measures glucose-induced insulin response to oral carbohydrates. Insulin is measured 60 and 90 minutes after administration of 0.15 mL/kg corn syrup to fasted horses. This test may exhibit poor repeatability. An oral glucose test, in which glucose powder is administered, may have better repeatability than the oral sugar test.

An insulin tolerance test (ITT) measures the ability of tissues to take up glucose (i.e. insulin sensitivity). The glucose concentration of a baseline blood sample is compared to the concentration in a blood sample taken 30 minutes after a dose of insulin is administered. The horse is insulin resistant if the second blood glucose concentration does not decrease to 50% or less of the baseline glucose value.

A combined glucose/insulin tolerance test measures the rate of decrease in glucose concentrations after IV infusions of dextrose and insulin. The results reflect tissue insulin sensitivity. Insulin concentration at 45 minutes after administration reflects insulin clearance rate and pancreatic insulin output.

Tests for PPID such as measuring ACTH concentration or thyroid releasing hormone response are normal in horses with EMS. Positive results indicate that the horse is affected by both EMS and PPID, which can occur in older horses. Detection of PPID is important, because it is thought that PPID exacerbates insulin resistance in horses previously affected by EMS.

How is equine metabolic syndrome treated?

Equine metabolic syndrome is treated with dietary management in the form of non-structural carbohydrate (NSC) restriction, restriction of total calorie intake, and a reduction (grazing muzzle) or elimination of pasture access. Management also may include increased exercise depending on the horse’s physical condition and soundness.

Feed analysis can determine the NSC content of the forage. Many companies will analyze hay samples quickly and inexpensively. Ideally, NSCs should comprise <10% of the hay dry matter for horses with EMS. A common misconception is that all alfalfas have “bad” carbohydrates and grass hays have “good” carbohydrates; however, this is not always the case.  Feed analysis is very important for horses with EMS as hays can be variable in the amount of NSCs they contain. If a hay analysis is not possible, soaking hay in water can be recommended to lower water-soluble carbohydrate concentrations. However, the actual amount reduced is variable, so this is not a reliable method to produce a low-NSC forage.

If a horse is overweight, a diet plan should be discussed with a veterinarian to ensure weight loss occurs at the appropriate speed (slower weight loss is typically better). Horses should initially be fed 1.5% of their ideal body weight in forage per day. This can be lowered to 1.25% after 30 days, if necessary. Sudden feed restriction should be avoided, because it may lead to increased fats in the bloodstream and further exacerbate insulin resistance. Increasing the amount and level of exercise will increase the rate of weight loss. Purchasing a scale and weighing out rations of hay daily is important since each flake of hay has a different weight depending on how compacted the bale is and the hay type. Horses with controlled and stable laminitis may benefit from walking if approved by a veterinarian; however, exercise is not recommended for horses with active laminitis. Scales or weight tapes should be utilized to document weight reduction. In addition, neck thickness and diameter can be monitored over time. If increased exercise and dietary modification are not sufficient to decrease body weight, medical therapy (thyroxine or metformin) may be beneficial.

Thin horses with EMS should receive increased calories in the form of roughage, fat, and possibly protein supplementation. Molasses-free beet pulp, vegetable oils, and low-carbohydrate, moderate protein, higher fat supplements can be used until a desired BCS is reached.

Supplements should be provided to add needed vitamins and minerals, but not additional calories. Horses fed a primarily hay based diet should receive a daily ration balancer (pelleted feed that acts as a multivitamin). Complete feeds that are formulated to be low in digestible energy and carbohydrates specifically designed for horses with insulin resistance may be used in place of forage and supplements if fed according to the label. Numerous dietary supplements have been suggested to increase insulin sensitivity, including cinnamon, chromium, and magnesium. None of these have been shown to improve insulin sensitivity in horses in experimental situations. It is particularly important to give a mineral supplement to animals fed soaked hay, because minerals leech out in the water along with the soluble carbohydrates. Labels for pelleted feeds should also be checked to ensure the NSC content is <10%.

Retesting may need to be performed at regular intervals to identify necessary adjustments to the management program.

What is the prognosis for equine metabolic syndrome?

The prognosis for horses with EMS varies from horse to horse. Many horses respond well to management through diet and exercise. Horses that are “easy-keepers” or have persistently high insulin levels can be more challenging to manage and may require medical treatments. Although proper treatment can reduce clinical signs, there is no “cure” for EMS. Long-term care requires diligence along with support and guidance from a veterinarian.

How can equine metabolic syndrome be prevented?

Prevention of equine metabolic syndrome should focus on maintaining normal weight in horses, particularly high-risk breeds. Since these horses may be more efficient users of calories, it is imperative to feed appropriately to maintain an ideal BCS and not to use arbitrary feeding guidelines. Exercise particular care when turning horses out on pasture during times of high-soluble carbohydrate content (spring and autumn).

For more information:

Equine Endocrinology Group, Recommendations for the Diagnosis and Treatment of Equine Metabolic Syndrome (EMS)

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