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Editorial Information

David Rendle BVSc MVM CertEM(IntMed) DipECEIM MRCVS

Published 2012

Metabolic Syndrome


The term equine metabolic syndrome (EMS) has only been in use for the last 10 years and replaces previous terms such as peripheral Cushing's syndrome, Cushings X and pre-laminitic metabolic syndrome. The term describes a common syndrome of obesity and predisposition to laminitis that affects horses and in particular ponies. The condition has similarities with human metabolic syndrome and Type II Diabetes.

Aetiopathogenesis

A reduction in the normal response to insulin (insulin resistance) is central to EMS. Insulin's most important function is the control of glucose levels in the blood and when insulin resistance occurs the normal relationship between insulin and glucose levels becomes disrupted. In addition to insulin resistance the syndrome also encompasses a number of other potential metabolic derangements including altered energy metabolism, changes in fat composition, clotting disorders, inflammation and damage to blood vessels. In horses the blood vessels in the feet are thought to be especially susceptible to these metabolic changes as a result of their unique anatomy potentially giving rise to laminitis. The precise mechanisms by which insulin resistance and other metabolic changes result in disease in the feet are complex and not fully understood; however possibilities include dysfunction of the cells lining the blood vessels of the foot, constriction of the blood vessels, reduced glucose uptake in the foot, altered function of the cells that produce hoof horn, and increased activity of potentially damaging enzymes called matrix metalloproteinases.

Certain breeds have been recognised to be at greater risk of EMS including Welsh, Dartmoor and Shetland ponies and Morgan, Arabian and Warmblood horses. Any breed can be affected if management and particularly diet are inappropriate. Furthermore, this is a disease induced by diet and management factors and can be prevented in all breeds with appropriate dietary restriction.

Clinical signs

  • Obesity is the classical sign of EMS and may be generalised (Figure 1) or localised. Common sites of regional fat deposition are surrounding the nuchal ligament in the neck or "cresty neck" (Figure 2), around the tail head, behind the shoulder, around the eyes (Figure 3) and in the prepuce or mammary gland region. The absence of obesity does not rule out the presence of the condition, occasionally horses that appear to be lean may have insulin resistance and other changes characteristic of EMS.

fig 1

Figure 1:  Obesity: a characteristic sign of EMS

fig 2

Figure 2:  Cresty neck in a pony with EMS

fig 3

Figure 3:  Excessive fat deposition around the eye in a pony with EMS
  • Lameness and evidence of current or previous laminitis may be present. Indicators of laminitis include poor horn quality, uneven or divergent growth rings on the hooves (Figure 4), flat or convex soles (Figure 5), divergent white lines, bruising of the white line and chronic foot infections.

fig 4

Figure 4:   Divergent and irregular growth rings characteristic of chronic laminitis

fig 5

Figure 5:  The convex sole of a pony with EMS and chronic laminitis
  • Abnormal cycling may be observed in mares. This is uncommon but mares may lose their seasonal anovulatory period and have prolonged interovulatory periods.

Equine metabolic syndrome is often confused with Equine Cushing's Disease or more correctly, Pituitary Pars Intermedia Dysfunction (PPID). Although these two diseases have different causes they may both result in insulin resistance and laminitis. There are however important differences:

  • EMS horses are typically young or middle aged, horses with PPID are generally in their teens or older
  • Horses with PPID may demonstrate delayed or failed shedding of the winter haircoat, excessive sweating, increased thirst, increased urination, muscle wasting and a number of other signs that are not seen with EMS.
  • Horses with PPID have abnormal pituitary gland function; horses with EMS do not.

Diagnosis

A presumptive diagnosis can often be made from the appearance of the horse (overweight) and a history of laminitis. In order to confirm the presence of EMS insulin resistance has to be identified. This can be done in a number of ways but whichever testing method is used it is important that the horse is starved prior to testing and is not painful or stressed at the time of testing. Therefore, testing should not be performed when a horse develops an episode of painful laminitis.

Testing methods include:

  • Single blood samples for insulin and glucose concentrations. These results can also be used to calculate "proxies" which estimate the risk of laminitis developing. Insulin should be less than 20 iu/ml in normal horses but some horses with EMS will also have normal levels making the single blood tests slightly unreliable and prompting the use of more accurate tests of insulin function in these cases. In most horses, glucose stays within the normal range so on its own it is not reliable for diagnosis.
  • AnOral Glucose Challenge Test assesses the insulin response to a meal of glucose and is therefore more accurate than a single blood test. A blood test is performed (usually 2 hours) after the horse is fed glucose and the resulting insulin concentration is measured.
  • Intravenous glucose and insulin tests are considered to be the most accurate but are also the most involved and therefore the most expensive. Glucose and insulin are usually administered together and the horse's glucose and insulin responses to them are measured over the next few hours. Because samples have to be taken frequently an intravenous catheter is generally placed and the tests are generally performed in a veterinary hospital.

Blood samples may also be useful in identifying increased levels of fat and hormones other than insulin that are also characteristic of EMS. Diagnosis of EMS if often dependent upon specific tests to rule-out PPID as a cause of insulin resistance.

Radiographs may be taken to confirm the presence of laminitis and to determine the severity of any structural changes that may have occurred in the feet.

Prevention and Treatment

The principles of prevention and treatment are very straightforward; dietary restriction and exercise. Not only do these measures result in weight loss but also increased fitness which improves the way the body responds to insulin. Horses have evolved to lose weight annually through the winter and preventing this from happening is damaging to the metabolism and results in EMS. Horses with, or at risk of, EMS should be fed a diet that is low in soluble sugars and starches. In many cases this means feeding forage with a high fibre and low sugar level only; most native breeds do not require hard feed to maintain their condition. Access to pasture should also be limited especially when grass is lush and growing. Sugars will also accumulate when the days are sunny and the nights are cold in the winter so these periods are also to be avoided. Some horses with established EMS may not be able to tolerate any access to pasture.

Horses that have EMS and need to lose weight should be restricted to a diet of grass hay at 1-2% of their body weight with all treats eliminated from the diet. More radical dietary restriction is often necessary but should be done under veterinary guidance as excessive weight loss over too short a period of time may result in other metabolic disorders. Provision of a vitamin and mineral balancer is advisable.

Affected or at risk horses should be exercised as much as possible. If laminitis develops exercise may not be possible and this presents major difficulties in management. It is therefore worth being proactive and tackling the condition before laminitis occurs. If laminitis does develop, this will need specific treatment. (See laminitis bulletin).

In humans with metabolic syndrome there are a number of drugs that are used to increase insulin sensitivity. These have been tried in horses but results are mixed. Metformin is one such drug that is popular in the UK but there are concerns over whether it is absorbed from the intestine and whether it is effective. Levothyroxine is a thyroid hormone that increases metabolic rate and in so doing induces weight loss. It is important to realise that these horses are not deficient in thyroid hormones and that administration of levothyroxine results in abnormally high levels of thyroid hormones. Levothyroxine is effective but prohibitively expensive in most cases.

A number of supplements have been suggested to be of benefit in horses with EMS including chromium, magnesium, cinnamon, and chasteberry (Vitex agnus-castus) extract. There is no evidence that they are beneficial and indeed there is some evidence that they are ineffective and so currently their use cannot be recommended.

Welfare implications

The welfare implications of laminitis are huge and the majority of cases in the UK are the result of EMS. Laminitis is one of the most common reasons for euthanasia of horses in the UK and results in extensive suffering in animals that survive the condition. Our desire to keep horses in a fatter condition than they should be and our reluctance to restrict diet and access to grazing in at-risk animals directly result in this preventable suffering.

Summary of key learning points

  • EMS is a syndrome of insulin resistance, risk of laminitis and in most cases obesity
  • The condition is preventable
  • Genetics, diet and exercise influence body fat mass that leads to the disease
  • Young to middle age animals are most commonly affected
  • Ponies and native breeds are at greatest risk
  • The condition has some similarities with, but is distinct from, Equine Cushing's Disease
  • Insulin resistance is diagnosed by blood tests, or preferably, by assessing the response to glucose administration
  • The condition is prevented and managed by dietary restriction and, where laminitis permits, exercise
  • Drug therapies should be a last resort and are less effective than management changes
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