Equine metabolic syndrome (EMS) is a group of risk factors that together have a strong association with the most common form of laminitis. This laminitis is caused by sustained high insulin levels (sometimes termed hyperinsulinemia-associated laminitis or endocrine laminitis).
These high insulin levels might be caused by too high levels of starch and/or sugar in the pasture, hay, or in complementary feeds.
Insulin Dysregulation
At the core of the equine metabolic syndrome group of factors is insulin dysregulation (ID). In this disease, animals become super-sensitive to starch and sugar in their diet. It also can mean their insulin becomes less effective at clearing blood glucose, so more has to be produced (from the pancreas). Either way, blood insulin levels can rise dramatically.
Generalized or regional adiposity is also commonly recognised in this syndrome because insulin is also a fat-promoting and -sustaining hormone. Fat levels in the blood might also be slightly increased.
Although obese animals are more at risk, it is important to emphasise that lean animals can also show insulin dysregulation. It is not possible to know whether an individual suffers from insulin dysregulation or not just by looking at them.
It is now recognised that high insulin levels are at the heart of this syndrome. All of the other components might be secondary. Equine metabolic syndrome might occur over a wide age range. In older animals, it might coexist with pituitary pars intermedia dysfunction (PPID or equine Cushing’s disease), which could develop independently.
Equine Metabolic Syndrome Predispositions
Many animals, particularly those of pony breeds and certain horse breeds, might be genetically predisposed to producing high insulin responses to dietary hydrolysable carbohydrates (i.e., starch and sugars, typically called non-structural carbohydrates or NSC). This is possibly because they evolved or adapted in areas with sparse or seasonally variable grazing.
It is fascinating to look back into the evolutionary origins of the modern equine breeds. At the end of the last ice age (20+ thousand years ago), the origins of the pony and Spanish breeds developed in Western Europe. They were a separate subspecies to the ‘hot-blooded’ breeds such as Thoroughbreds, Standardbreds, and Arabians that developed in the middle east.
At that time, grazing in the cold Western European regions might have been sparse. Having high insulin levels could have enabled animals to maximise the storage and conservation of energy (both hydrolysable carbohydrates and fats) from their feed.
But now cut to the present day, when we keep our animals on modern pastures (mostly developed to maximise growth in cattle and sheep) and other diets often high in hydrolysable carbohydrates. These environmental factors interact cumulatively with the underlying genetic characteristics of certain breeds to result in insulin dysregulation and EMS. This phenomenon is termed ‘epigenetics’.
Breeds At Risk of Equine Metabolic Syndrome
The breeds most at risk of developing equine metabolic syndrome include the pony breeds, Spanish Breeds (e.g., Andalusians), Gaited breeds (e.g., Saddlebreds, Paso Finos), Morgans, Miniature horses, and Warmbloods.
However, while many of these animals with a high genetic risk might develop equine metabolic syndrome with only mild environmental influences, other horses with lower genetic risk can still develop EMS if they are kept for a sufficient period of time on diets that are high in non-structural carbohydrates.
Therefore, any horse can potentially develop equine metabolic syndrome if exposed to sufficient environmental influences (i.e., diet and lack of exercise). For example, occasionally even Thoroughbred broodmares kept on lush pasture and high starch and/or sugar diets might become ‘foot sore’ with low-grade endocrine laminitis.
Why is EMS important?
Equine metabolic syndrome is extremely common, particularly in the breeds listed above. In a study of ponies and cobs in the UK, the reported prevalence of EMS was 23.3%. This is the proportion of the population with the condition at any one point in time.
The most important reason why equine metabolic syndrome is of concern for health and welfare is the increased risk of laminitis.
There is a direct effect of increased and sustained plasma insulin on the lamellar cells in the foot. These laminae transfer and spread the weight of the animal from the pedal bone onto the hoof wall. This causes a loss of structural integrity of the lamellar bonds. This ‘stretching’ (and eventually breaking of the bonds) is associated with pain since there are nerve endings in the lamellar tissues, just like in the human nail-bed.
Laminitis is one of the most common reasons for owners to seek veterinary attention for their equids. Studies in pleasure horses have indicated that around 15% had suffered from at least one episode of laminitis.
Equine Metabolic Syndrome Characteristics
Typically, animals with equine metabolic syndrome exhibit obesity. This can be seen either as generalised or regional adiposity, with a cresty neck and deposits of adipose tissue behind the shoulders and at the tail head.
There might be preputial or mammary gland enlargement due to a combination of adipose tissue and some fluid swelling.
These equids are commonly weight loss resistant. Once obese, they might maintain their body fat on a remarkably modest caloric intake (referred to as easy keepers or good doers). These animals often show some degree of lameness (particularly on hard surfaces) due to active laminitis. They also might show divergent hoof rings, which are a sign of previous or subclinical laminitis.
Some animals might be quite lean, yet are still considered to have equine metabolic syndrome (the so-called non-obese manifestation of the disease). They might be extremely insulin dysregulated and at high risk of laminitis. These lean animals often have clinical signs of lameness or evidence of subclinical laminitis (i.e., divergent hoof rings).
These animals might have previously been obese or are genetically predisposed animals that have had their diet and exercise closely regulated. However, it is sometimes unclear why these individuals do not exhibit obesity or regional adiposity.
Body Morphometric Assessments
High plasma insulin levels tend to promote obesity and weight loss resistance. Managing body condition with therapy and dietary management is a very good adjunct to monitoring improvements in insulin dysregulation in animals with the typical obese equine metabolic syndrome characteristics.
Body condition scoring can be a very accurate index of body fat percentage when undertaken by experienced assessors. In particular, the 9-point condition scoring scale does specifically assess areas of regional adiposity (although it was not designed with ponies in mind).
However, at the upper end of the scale there might be large differences in percent of body fat between grades. In addition, the scoring system only assesses the superficial fat deposits beneath the skin. There might be disproportionate increases in internal body fat as animals become very obese.
At a simpler level, owners might monitor an increase or decrease in body fat in their animals by measuring the heart girth or belly girth using a tape measure.
A more integrated objective ‘body condition index’ has been developed using belly girth, heart girth, and neck circumference measurements that also could be more sensitive than condition scoring to progressive minor changes in body fat.
The veterinary use of ultrasound to measure fat depth at certain areas tends to be a slightly variable measure of overall adiposity. Other more novel techniques such as bioelectrical impedance analysis require validation in a range of different breeds and sizes before they can be considered accurate.
Cresty Neck
The cresty neck is a common feature of regional adiposity associated with equine metabolic syndrome. A cresty neck scoring system has been developed.
In one recent study, horses with a cresty neck score greater than or equal to 3 out of 5 had 11.3 times higher odds of having insulin dysregulation.
Whether the neck crest fat (or indeed the internal abdominal fat) is contributing significantly to insulin dysregulation is unclear. There does not seem to be a clear link between cresty neck and risk of future laminitis. However, these outward signs of equine metabolic syndrome all point to the effects of high insulin.
Weight loss programs and weight control diets will all be simultaneously keeping insulin levels under control (effective weight loss cannot be achieved if insulin levels remain high). Therefore, from the owner point of view, monitoring body fat will often help when managing EMS cases to reduce the risk of laminitis.
Final Words
It is really important with respect to senior horses to note that some horses, particularly those with PPID, can be lean or show weight loss while also being insulin dysregulated. These present a more difficult challenge to reduce laminitis risk.
Veterinary advice as well as nutritional support should be sought in managing these cases.
Further Reading
Insulin Dysregulation and Cresty Neck Score. MySeniorHorse.com
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Simon Bailey, BVMS, PhD, FHEA, DECVPT, FRCVS, is a Professor of Veterinary Biosciences and Deputy Head of School at the Melbourne Veterinary School (University of Melbourne) in Australia. His areas of interest are in equine metabolic syndrome, equine laminitis, PPID, and equine physiology/endocrinology, nutrition.View all posts