Bit & Bridle

Equine Metabolic Syndrome (EMS)

Other Names

  • Insulin Resistance Syndrome
  • Obesity-Associated Insulin Resistance
  • Pre-laminitic Metabolic Syndrome
  • Syndrome X

What is Equine Metabolic Syndrome (EMS)?

Equine metabolic syndrome (EMS) is a hormonal and metabolic disorder characterised by insulin dysregulation — the horse’s cells become less responsive to insulin, leading to abnormally high blood insulin concentrations after feeding. This disrupts normal glucose metabolism and, critically, makes affected horses highly prone to developing laminitis, often triggered by exposure to dietary sugars and starches that would have no effect on a metabolically normal horse.

EMS is distinct from PPID (Cushing’s disease), though the two conditions share features and can co-exist. EMS predominantly affects younger to middle-aged horses and ponies (typically under 15 years), whereas PPID is primarily a disease of older animals. The two conditions can — and sometimes do — occur together, which must be considered during diagnosis and treatment planning.

What Causes EMS?

EMS is a complex multifactorial condition with both genetic and environmental components:

  • Genetic predisposition: Certain breeds and types have evolved on nutrient-poor environments and are metabolically very efficient — they thrive on minimal calories and accumulate fat easily. When these animals are managed with modern feeding practices, the mismatch creates insulin dysregulation. Native British and Irish pony breeds are particularly at risk.
  • Obesity: Accumulation of excess adipose tissue — particularly in the characteristic locations of crest, tailhead, and behind the shoulder (regional adiposity) — drives insulin resistance. Adipose tissue is not metabolically inert; it actively secretes inflammatory mediators and hormones that interfere with insulin signalling.
  • Dietary factors: Diets high in non-structural carbohydrates (NSC) — particularly the combination of sugars and starches — chronically stimulate insulin secretion. In susceptible horses, this leads to progressive loss of insulin sensitivity.
  • Lack of exercise: Physical activity improves insulin sensitivity in horses as it does in humans. Horses in sedentary management — particularly those on box rest or restricted turnout — lose the metabolic benefit of regular exercise.

High-Risk Breeds and Types

  • Native British and Irish pony breeds: Welsh, Dartmoor, New Forest, Exmoor, Shetland, Fell, Dales, and Highland ponies
  • Donkeys and mules
  • Arabians and Morgan horses (in some studies)
  • Easy-keeper crossbreds and cobs
  • Mustangs and feral breed types

Signs and Symptoms

EMS often goes unrecognised until a horse develops laminitis, at which point it may already be well-established. Signs to watch for include:

  • Obesity and cresty neck: A body condition score above 6/9, with particular fat deposition on the crest of the neck (which becomes hard and firm rather than soft and moveable), over the shoulder, at the tailhead, and behind the girth.
  • Laminitis (current or historical): Particularly pasture-associated laminitis in spring and autumn. A horse with unexplained recurrent laminitis should be tested for EMS.
  • “Easy-keeper” status: The horse gains weight easily and is difficult to keep in lean condition despite restricted feeding.
  • Lethargy: Some EMS horses show reduced energy and enthusiasm.
  • Unusual fat distribution: Pockets of fat in the hollow above the eye, sheath area, or other abnormal locations.

Unlike PPID, EMS horses do not typically show hypertrichosis (abnormal coat), polydipsia, or polyuria. These signs suggest PPID rather than (or in addition to) EMS.

Diagnosis

Diagnosis is based on a combination of clinical assessment and blood testing:

  • Body condition scoring: Systematic assessment of fat deposition using the 1–9 Henneke scale or similar system. Regional adiposity (particularly crest scoring) is particularly informative.
  • Resting insulin and glucose: Blood samples taken after an appropriate fast (typically 4–6 hours, no concentrates for 12 hours) to measure baseline insulin and glucose. An elevated resting insulin strongly suggests insulin dysregulation.
  • Dynamic insulin testing (oral sugar test / insulin tolerance test): Considered more sensitive than resting insulin for detecting early EMS. The oral sugar test measures insulin response to a standard dose of corn syrup (Karo syrup), mimicking the insulin response to a sugary meal. An exaggerated insulin response confirms insulin dysregulation even when resting values are normal.
  • ACTH testing: To rule out or confirm concurrent PPID, particularly in horses over 10 years.

Treatment and Management

There is no pharmaceutical cure for EMS. Management centres entirely on dietary control, weight loss, and exercise — and requires sustained, long-term owner commitment.

Weight Loss and Dietary Management

Reducing body condition score to 4–5/9 is the single most effective intervention. This requires:

  • Strict calorie restriction: Feed hay at approximately 1.5% of current body weight (dry matter) per day. This must be combined with a low-NSC hay — not all hay is equal, and high-sugar meadow hay will not support weight loss even if restricted by weight.
  • Soak hay, but have it analysed: The ECEIM consensus on EMS (Durham and colleagues, 2019) recommends forage with a non-structural carbohydrate (sugar plus starch) content below 10%, and recommends soaking for horses with insulin dysregulation, but no fixed soak time makes hay safe. When Longland and colleagues soaked a range of UK hays for up to 16 hours, an average of 27% of the water-soluble carbohydrate was removed, with a range of 6% to 54%, and seven of the hays tested were still above the level suggested for laminitic animals afterwards (Veterinary Record, 2011). If the analysis is still too high after soaking, find different hay. The consensus also warns that long soaks in warm weather encourage bacterial growth, so keep soaks short when it is warm.
  • Eliminate concentrates and treats: No grain, no hard feed, no apples or carrots, no sugary supplements. A good-quality vitamin and mineral balancer or ration balancer (low in starch and sugar) is appropriate to prevent nutritional deficiencies during restriction.
  • Restrict pasture access: Grass, particularly lush spring or autumn grass, is typically too high in NSC for EMS horses, and even partial grazing is hard to measure. For that reason the ECEIM consensus advises preventing pasture access altogether for the first 6 to 12 weeks of dietary restriction in horses with insulin dysregulation. When grazing comes back, a grazing muzzle reduces intake (it does not stop it) and can increase foraging time and movement, and strip-grazing on sparse, rested pasture or a track system that limits total grass intake gives more control. Cutting turnout time alone has a limited effect, because horses eat quickly when time is short.

Exercise

Regular exercise is a powerful tool for improving insulin sensitivity. Even moderate daily exercise (20–30 minutes of active walking or light schooling) has measurable metabolic benefit. However, exercise during an acute laminitic episode is contraindicated — wait until the horse is sound and comfortable before gradually reintroducing work.

Pharmacological Treatment

Levothyroxine sodium (a thyroid hormone supplement) is used in some EMS horses — it promotes weight loss and improves insulin sensitivity, particularly in horses where dietary management alone is insufficient. It is used off-licence in horses and prescribed by vets for a time-limited period (typically 3–6 months) as a “jumpstart” to weight loss, not as a permanent solution.

Prevention

  • Avoid obesity from the outset — manage body condition proactively in high-risk breeds, even when young
  • Feed forage-based, low-NSC diets to native breeds and easy-keepers from the start of ownership
  • Provide regular exercise — daily turnout with movement is preferable to box rest
  • Restrict spring and autumn pasture in high-risk horses as a standard management practice, not just after a laminitis episode
  • Monitor body condition score monthly and adjust management before weight becomes problematic

When to Call the Vet

Discuss EMS with your vet if:

  • A pony or horse develops laminitis — particularly if it’s pasture-associated and occurs in a native breed
  • A horse is very difficult to keep in lean condition despite restricted feeding
  • You notice characteristic fat deposition on the crest, shoulder, or tailhead in an easy-keeper horse
  • A horse has had a laminitic episode and you want to investigate whether metabolic disease is the underlying cause

Frequently Asked Questions About EMS in Horses

What is the difference between EMS and PPID?

EMS is a metabolic disorder primarily affecting younger horses; it is driven by obesity and dietary factors and managed through lifestyle changes. PPID (Cushing’s disease) is caused by brain degeneration affecting pituitary function; it predominantly affects horses over 15 and is treated with daily medication (pergolide). Both cause insulin dysregulation and laminitis risk. They can co-exist in the same horse, which is why testing for both is recommended when one is suspected.

Can an EMS horse ever go back on pasture?

Many EMS horses can safely access pasture under controlled conditions once weight is optimised and insulin levels are stable. A grazing muzzle during high-risk periods, strip-grazing on rested pasture, and avoiding early morning turnout (when grass sugar content peaks) allows most EMS horses to benefit from some pasture time. For severely affected horses, dry lot management with forage-only feeding may be necessary long-term.

Can EMS cause laminitis in the absence of obvious obesity?

Yes. Not all horses with EMS are overtly obese — lean horses can have insulin dysregulation, particularly if on high-NSC diets or in certain high-risk breeds. This is why blood testing is important rather than relying on body condition alone. Lean EMS horses are sometimes called “metabolically obese” — their fat percentage or regional fat distribution may be disproportionate even without obvious generalised obesity.

Is EMS curable?

EMS is not cured, but it can be managed into remission. With successful weight loss, appropriate diet, and regular exercise, many horses’ insulin levels normalise and laminitis risk reduces substantially. However, genetic predisposition remains — the metabolic efficiency that caused the problem does not disappear, and management must continue for life. EMS horses are always at higher risk of relapse during times of stress, illness, or if management slips.