Insulin Resistance and Equine Metabolic Syndrome

Equine Metabolic Syndrome (EMS) is a relatively new term to veterinary medicine. EMS along with Pituitary Pars Intermedia Dysfunction (PPID) are the most common endocrine disorders associated with horses and ponies. The term was first used in 2002 to describe a condition in horses that consisted of obesity, insulin resistance (IR) and laminitis. The term EMS was used due to the similarities seen between horses and humans who have metabolic syndrome and shares some attributes similar to Type II diabetes in humans.  EMS is a complex disorder and the complete mechanism of how EMS occurs is not currently fully understood.

What causes Equine Metobolic Syndrome?

EMS is thought to be relatively common; however, exact data regarding the number of cases has not been adequately studied. The increasing incidence of the disease is thought to be multifactorial. While there does seem to be a genetic component associated with the syndrome and certain breeds such as ponies, Morgan horses, Paso Finos, American Saddlebreds and donkeys appear predisposed, any breed can be affected by EMS. The other major factors associated with the development of the disease are the environmental components, particularly those that lead to obesity. So while recognition of the syndrome is on the rise, it is estimated due to increased obesity levels, the overall incidence of the disease is increasing as well. Currently, laminitis is the most devastating consequence of EMS, however, the possibility that risk factors associated with metabolic syndrome in other species, such as heart disease, may increase as well. It is therefore very important that EMS cases be diagnosed as early as possible and proper management practices are put into place to minimise potential damage to your horse or pony.

Diagnosing EMS

Horses and ponies affected by EMS are normally known as ‘easy keepers’; these are the horses that only need to look at a blade of grass and gain 5kg. This does not mean that all EMS ponies are fat, but that they do not need a lot of feed to maintain weight and if overfeeding, overgrazing or lack of exercise occurs they become obese quite quickly. When considering if your horse or pony might be affected by EMS it is important to look at the 3 aspects that comprise a diagnosis of EMS.


There are many challenges to assessing and maintaining the weight of your horse or pony since they don’t normally have easy access to scales. There are also many different breed predilections and no two animals are ever the same. So while we might all want to believe that it isn’t fat, but muscle, it often tends to be fat. Another aspect of EMS is that the horse or pony may not be overweight but have oddly developed fat deposits; this might lead to them looking like they have lumps. This is due to their inability to properly process fats. The most frequently used method of evaluating equine obesity is by evaluating body condition scoring (BCS).  Remember not all EMS horses are obese they could just have abnormal fat deposits, and not all obese horses have EMS so it is important to have your horse or pony evaluated by your veterinarian to assess their risk of EMS.

Insulin Resistance

Insulin resistance is the failure of tissues to adequately respond to circulating insulin and thus control the blood glucose concentration4. Horses with EMS have compensated insulin resistance where they produce more insulin in response to resistance, which combined with decreased insulin clearance results in hyperinsulinaemia. These horses and ponies have marked responses to carbohydrate challenge which is referred to as insulin dysregulation. There are currently a couple of simple blood tests available for evaluating insulin resistance. A resting or basal insulin measure can be taken through a simple blood draw. These results are an important aid in diagnosing EMS but also offer information when monitoring progress. However, it might be necessary to use dynamic test to diagnose EMS. These tests give a more complete picture of insulin and glucose dynamics and are more sensitive than resting insulin levels. Dynamic tests usually involve drawing an initial blood sample followed be administering an oral glucose or sugar to the patient and then taking a second blood sample a few hours later. These blood samples are then compared to evaluate how well the horse or pony responded to the oral glucose or sugar.

Laminitis in EMS horses

While the mechanism for EMS is not fully understood, it has been well documented that high insulin levels in the blood will lead to bouts of laminitis. It is currently thought that laminitis in EMS horses is associated primarily to vascular dysfunction; which means the blood supply to the hoof has been affected. Exactly why this occurs is still being examined, however, it is hypothesized that high insulin levels affect blood vessel tone and therefore, change the blood supply to the hoof. This means that laminitis due to endocrine disorders differs from other laminitis which occurs due to acute inflammation. This is important because horses with EMS can have both painful and non-painful bouts of laminitis for many years prior to diagnosis and if the EMS is not managed properly the laminitis can continue to occur.  Additionally, research has shown that once laminitis has occurred a horse or pony is more likely to have recurring bouts of laminitis, this means diagnosing and managing EMS before laminitis develops is key in preventing chronic, recurrent laminitis.

Grass related laminitis

Often there is an increase in cases of laminitis during the spring months due to the higher levels of sugars in the grass. These high levels often result in increased circulating glucose and insulin, which exacerbates existing insulin resistance in EMS horses and ponies, thereby increasing the risk of laminitis. However, it is important to remember that grazing alone is not the cause of the laminitis, there is an underlying endocrine problem. This makes sense since you might have multiple horses on the same pasture land, and only some of them get laminitis. If it was only the grass causing the laminitis you would expect all horses grazing on the same pasture to get laminitis.

Treatment of your EMS horse or pony

The key success factors to controlling EMS in your horse or pony is proper management. This includes obtaining and maintaining your horse or pony’s correct body weight. For this to occur you may need to have the feed and pasture quality evaluated to determine how much food your horse or pony should be given or whether a complete change in feed intake is needed. Restricted paddock time or a grazing muzzle may be needed to help control your horse or pony’s weight. It might be recommended that hay be soaked to help reduce sugar content. Exercise, when laminitis is not a limiting factor, will also help control your horse or pony’s weight, normally routine exercise for about 30 minutes a day is adequate. Other medications or supplements might be recommended in some cases. Since many aspects of EMS can look similar to PPID and the conditions can often occur in conjunction with each other, it is important to consider having your horse or pony tested for PPID when being evaluated for EMS. Remember each horse and pony is individual and therefore needs to be individually evaluated. Any changes in management or conditioning should be discussed with your veterinarian. Caring for you horse or pony needs to be a joint effort between everyone involved to ensure the best possible outcome.


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  5. Basinska et al The production and distribution of IL-6 and TNF-a in subcutaneous adipose tissue and their correlation with serum concentrations in Welsh ponies with EMS J Vet Sci 2015;16(1):113-120
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  7. Hollands T Obesity & laminitis: minimising the impact of nutrition. Laminitis Awareness 2010 Proceedings. 28-39


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