Equine Metabolic Syndrome: Understanding EMS and Metabolic Testing

Equine Metabolic Syndrome (EMS) is an increasingly recognised condition in horses and ponies and, most importantly, is a major risk factor for laminitis. Traditionally, we have tended to associate EMS with overweight horses and ponies - particularly those with a characteristic cresty neck and increased body condition score. However, our understanding of the condition has developed considerably.

Not every overweight horse has EMS, and not every horse with EMS is overweight.

This is important because simply looking at a horse is not enough to determine its metabolic health. Appropriate blood testing can help us identify horses with abnormal insulin regulation and, crucially, recognise those at increased risk of laminitis before they experience their first episode.

What is Equine Metabolic Syndrome?

At the centre of EMS is Insulin Dysregulation. Insulin is a hormone produced by the pancreas and plays an essential role in regulating how the body responds to glucose following a meal. After carbohydrates are digested and glucose enters the bloodstream, insulin helps tissues such as muscle and fat take up and use or store that energy.

In horses with insulin dysregulation, this system does not function normally. A horse may have abnormally high insulin concentrations at rest, produce an excessive insulin response following consumption of sugar or carbohydrate, or have tissues that do not respond appropriately to insulin – known as insulin resistance. Some horses have a combination of these abnormalities.

The most significant consequence is hyperinsulinaemia-associated laminitis (HAL). Sustained high concentrations of insulin have been shown to cause laminitis, establishing a direct link between insulin dysregulation and laminitis risk. In fact, hyperinsulinaemia is thought to be responsible for many laminitis cases in horses and ponies. This is why recognising insulin dysregulation before laminitis develops is so valuable.

Does a horse have to be overweight to have EMS?

No - and this is an important change in how we think about EMS. Obesity is common in affected horses and can exacerbate metabolic dysfunction, but it is not essential for a diagnosis. Some horses with significant insulin dysregulation can have a relatively normal overall body condition.

Regional fat deposits can also be important. A pronounced cresty neck, or abnormal fat accumulation around the tail head, sheath or mammary region, can be associated with metabolic dysfunction even when the horse is not generally obese.

Other factors can increase the likelihood of EMS, including diets high in sugar and starch, unrestricted access to pasture and reduced physical activity. Age and genetics also play a role, with native British pony breeds and some other breeds showing increased susceptibility.

How do we test for EMS?

There isn't one single blood result that tells us everything about a horse's metabolism. When investigating EMS, we interpret laboratory results alongside the horse's relevant clinical history. A metabolic assessment may include insulin, glucose, triglycerides and adiponectin, with further dynamic testing recommended in some horses.

Insulin is central to the investigation of EMS because insulin dysregulation is the defining feature of the condition. A basal or resting insulin concentration is one of the simplest tests we can perform. A significantly elevated result provides strong evidence of insulin dysregulation and can identify horses at increased risk of laminitis.

Some horses can have a normal resting insulin concentration and still have insulin dysregulation. Basal insulin testing is relatively specific when the result is high, but it is not particularly sensitive. In other words, an abnormal result is useful, but a normal result cannot always give a horse the all-clear.

Dynamic insulin testing - where there is clinical suspicion despite a normal resting insulin result, we may recommend a dynamic test, such as an Oral Sugar Test (OST). This involves giving the horse a carefully measured amount of sugar orally and then taking a blood sample after a set period to assess its insulin response. The principle is relatively straightforward: rather than asking what the horse's insulin concentration is when very little is happening metabolically, we deliberately provide a controlled carbohydrate challenge and assess how the body responds.

Dynamic testing is more sensitive than basal insulin measurement and can identify horses whose abnormal insulin response would otherwise remain undetected. Importantly, the response to an oral sugar challenge is also particularly useful when assessing a horse's risk of developing laminitis.

Glucose can be measured alongside insulin, but EMS in horses is quite different from type 2 diabetes in people. Even horses with significant insulin resistance will often maintain their blood glucose within the normal range. The pancreas compensates by producing additional insulin, allowing glucose to remain relatively well controlled. Consequently, a normal glucose concentration does not rule out EMS or insulin dysregulation.

Triglycerides are fats circulating within the bloodstream and provide us with additional information about how a horse is handling and mobilising its energy reserves. Increased triglyceride concentrations can occur in horses with EMS, although they are not present in every affected horse. Monitoring triglycerides can become particularly important in horses and ponies undergoing significant dietary restriction or weight loss and, in some horses receiving medication for severe insulin dysregulation.

Newer medications used in selected EMS cases can substantially reduce insulin concentrations but may also cause an increase in circulating triglycerides. Measuring triglycerides therefore gives us another useful piece of the metabolic picture and can be particularly important when monitoring treatment.

Adiponectin - fat tissue isn't simply an energy store, it is metabolically active and produces hormones known as adipokines. One of these is adiponectin, a hormone with several beneficial effects. It helps improve insulin sensitivity, promotes fatty acid metabolism and has protective effects on blood vessels. Unlike some other hormones associated with fat tissue, adiponectin concentrations tend to decrease as body fat increases. Therefore, measuring adiponectin may provide additional information about a horse's metabolic health and insulin sensitivity. It does not replace insulin testing, but alongside insulin, glucose and triglycerides it can contribute to a more complete assessment of the individual horse.

Which horses should we consider testing?

Metabolic testing may be particularly worthwhile in horses and ponies that are overweight or have a pronounced cresty neck or other regional fat deposits; have experienced laminitis previously; have abnormal hoof growth rings or other evidence suggestive of previous laminitis; gain weight very easily; or belong to a breed with an increased susceptibility to insulin dysregulation.

However, one of the key messages from current research is that appearance alone isn't enough. Lean horses can also have significant insulin dysregulation. Testing may therefore be appropriate whenever a horse's history or clinical examination raises concerns, even if they don't fit the stereotypical appearance of an “EMS pony”.

Perhaps the most important reason to investigate EMS is that we would much rather identify a horse at increased risk before its first episode of laminitis. The prognosis for horses diagnosed and appropriately managed before developing hyperinsulinaemia-associated laminitis is excellent. Once recurrent or chronic laminitis develops, management can become considerably more challenging.

For horses diagnosed with EMS, testing is not necessarily a one-off event. Changes in diet, grazing, body condition and exercise can all influence insulin responses, so ongoing monitoring allows us to determine whether management changes are improving metabolic health.

Ultimately, EMS isn't simply about whether a horse or pony is fat, it is about how that individual horse responds metabolically to its diet and environment.

If you're concerned that your horse or pony may be at risk of EMS, please speak to our veterinary team. We can assess their history, body condition score and individual risk factors and advise whether metabolic testing - including insulin, glucose, triglycerides and adiponectin would be appropriate.