Equine metabolic syndrome (EMS) is a condition that affects how horses metabolize sugars from the diet and use them for energy.

It involves an inability to properly respond to the hormone insulin, similar to Type 2 diabetes in humans. In addition to insulin resistance, EMS also results in increased obesity, abnormal fat deposits (such as cresty neck), and a higher risk of laminitis.

Metabolic syndrome commonly affects ponies and horses referred to as “easy keepers“. But not all horses that are obese have EMS, which is why testing is important to diagnose this disorder.

The treatment for EMS includes reducing starches and sugars in the diet, decreasing caloric intake to promote weight loss, safely increasing exercise and, in some cases, using medication. Regular veterinary checks are important for preventing and treating EMS.

If your horse has been diagnosed with EMS or you are looking for strategies to prevent EMS, contact our equine nutritionists to formulate an appropriate feeding plan.

Equine Metabolic Syndrome

Equine metabolic syndrome (EMS) is a collection of risk factors related to metabolic dysfunction. [1][2][3]

The clinical signs associated with EMS were previously referred to as peripheral Cushing’s disease, hypothyroidism, prelaminitic syndrome, or Syndrome X. The term Equine Metabolic Syndrome was first suggested in research in 2002. [4]

The predominant feature of EMS is hyperinsulinemia, which refers to abnormally high levels of insulin in the blood.

In horses with EMS, cells do not respond to the hormone insulin properly which results in higher than normal insulin required to bring glucose into cells. Blood sugar levels remain normal in most cases.

Horses may have one or more of the following:

  • High insulin levels (hyperinsulinemia)
  • Insulin resistance (IR)
  • High triglyceride levels (hyperlipidemia)
  • Fatty neck crest
  • Other abnormal fat deposits, including above the eyes, at the tail base, on the withers or anywhere on the body
  • Abnormal estrus cycles, sometimes with pain and laminitis flare-ups

This often coincides with obesity and increases the risk of debilitating laminitis.

Additionally, horses with IR and EMS may be more sensitive to steroid administration (e.g. joint injections, steroids given for skin or respiratory allergies) and could be inadvertently pushed into a laminitic episode in situations of otherwise routine steroid use. [5]

Veterinary examination is necessary to diagnose EMS in horses. Common diagnostic tests used to evaluate horses for the condition include:

  • Baseline insulin, glucose, and leptin or adiponectin
  • ACTH to rule out PPID / Cushing’s disease as the cause of hyperinsulinemia
  • Oral Sugar Test (OST) or Oral Glucose Tolerance Test (OGTT)
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Risk Factors

Multiple factors including genetics, environment, diet, exercise, sex and possibly the gut microbiome influence the development of EMS.

Genetics appears to play a strong role in the risk of EMS. A fat pony has always been the poster child for pasture laminitis. In 2006, Treiber et al. were the first to examine the genetics of EMS in a year-long study of ponies at pasture. [23] In comparison, modern breeds such as Thoroughbreds, Standardbreds, Quarter horses, full Drafts and Warmbloods are unlikely to develop EMS unless they have PPID.

Research indicates insulin resistance is associated with age. [6] Insulin concentrations are significantly higher in older horses and ponies compared to those that are younger. Horses between the ages of 5 and 16 are most likely to be affected by EMS. However, signs of EMS have emerged in horses as young as 3 years old, when the period of rapid growth has passed.

Other endocrine diseases, particularly pituitary pars intermedia dysfunction (PPID), can increase the risk of developing concurrent EMS. PPID was formally referred to as Equine Cushing’s Disease. PPID occurs in horses 15 years and older (although it has been diagnosed in horses as young as 7), and becomes more likely to develop as the horse ages.

Younger horses with EMS typically have a normal pituitary gland, whereas older horses with EMS and PPID are likely to have lesions on their pituitary gland. [7]

EMS is far less likely to occur in stallions than geldings or mares, unless the horse has PPID. Dr. Kellon of the Equine Cushing’s and Insulin Resistance Group reports than in over 2 decades of following thousands of metabolic horses there has never been an incident of EMS in a stallion that did not have PPID.

Prevalence

Over 90% of horses presenting with laminitis as their predominant clinical sign have EMS. [8]

Horses with EMS often have an obese body condition, although the condition can also occur in thin horses. Thin horses with EMS typically exhibit abnormal fatty deposits on various regions of their body, such as their neck and tailhead regions.

Objectively monitoring your horse’s body condition over time is critical for assessing their risk of developing EMS.

EMS occurs in donkeys, ponies, and equine breeds such as Saddlebred, Tennessee Walking, Paso Fino, Morgan, Mustang, and Quarter horses. The condition is less common in Thoroughbreds and Standardbreds.

Gender does not predispose horses to have EMS.

Insulin Resistance – A Key Feature of EMS

Most horses diagnosed with EMS are insulin resistant, which means they are unable to properly metabolize carbohydrates in the diet.

This puts them at risk of laminitis when fed high-starch or sugar-rich feeds or when given access to lush pasture.

Insulin is a hormone produced by the pancreas in response to high levels of sugar in the blood. Insulin enables the body to use sugar for energy by helping your muscle and fat cells take up glucose from the blood.

When a horse ingests starch, it gets broken down into glucose in the stomach and absorbed into the blood. Glucose subsequently stimulates the pancreas to release insulin.

Insulin has many roles in the body, including enabling the movement of glucose into muscle and adipose (fat) cells of the body. When insulin binds its receptor on these cells, it makes channels available to move glucose out of the blood and into the cell.

Insulin resistance refers to a condition in which normal insulin concentrations fail to stimulate the tissues to uptake glucose.

When cells become resistant to insulin, the pancreas responds by releasing more of the hormone to prevent blood glucose levels from becoming too high.

This leads to high blood levels of insulin (hyperinsulinemia) in horses with metabolic syndrome.

Signs and Symptoms of EMS

Horses affected by EMS are typically overweight with increased fat deposition on the neck, over the ribs and topline, above the eyes, and at the base of the tail. [9] Their body condition score typically ranges between six to nine on a nine-point scale.

Geldings with EMS often have increased fat deposition in their sheath, whereas mares with the condition may have fatty deposits around the mammary gland. Affected horses often have a history of gaining weight easily after reaching adulthood.

Laminitis commonly occurs in horses with EMS following the ingestion of feed sources that are high in soluble carbohydrates. Bouts of laminitis are most likely to occur in horses with EMS after consuming lush pasture or high carbohydrate feeds.

Signs of EMS-related laminitis in the hooves include abnormal hoof growth rings, radiographic evidence of rotation in the coffin bone, and demineralization in the coffin bone (pedal osteitis).

Horses with this condition may also show symptoms related to high levels of circulating glucose and insulin resistance including:

  • Excessive drinking and urination
  • Loss of muscle mass
  • Changes in appetite
  • Changes in exercise tolerance
  • Infertility or abnormal ovarian activity

Supporting findings on bloodwork submitted by your veterinarian may include:

  • High blood insulin levels (hyperinsulinemia)
  • High fatty acids in the blood (hyperlipidemia)
  • High leptin (a hormone produced by adipose or fat cells)
  • Blood glucose levels may be normal or high (hyperglycemia)

Causes of EMS

The exact cause of metabolic syndrome in horses is unknown. However, the development of the condition is believed to be influenced by multiple factors including:

Genetics

Research demonstrates that horses affected by EMS have genes that predispose them to develop the condition. [10] These genes may have assisted the survival of the ancestors of modern-day horses by promoting increased energy efficiency.

Storing energy reserves in the form of excess fat may have enabled ancestral equines to live in harsh environments. However, this can cause problems for modern horses that are provided with readily available calorie-dense food sources.

Obesity

When obesity develops in horses with EMS, the amount of fat stored in their liver increases. Excess fat stored in the liver may promote insulin resistance by interfering with the normal signaling activities of the hormone.

The fatty tissues of obese horses with EMS produce high levels of adipokines, a group of hormones produced by fat cells that have a variety of effects on the body, including an increase in inflammation.

A prolonged inflammatory state in the body affects how cells respond to insulin and leads to insulin resistance and high blood glucose and insulin levels.

Elevated insulin levels is a known cause of laminitis in horses and ponies. An abnormal insulin level may alter the function of the epidermal laminar cells of the hooves and its ability to uptake glucose, increasing the risk for horses with EMS to develop laminitis.

Management Practices

The influence of management practices on the development of metabolic issues is greatest during the first decade of a horse’s life. [7]

Horses that are relatively inactive and fed high-calorie diets have a greater risk of developing EMS.

Gut Microbiome

The gut microbiome is believed to play a role in the development of metabolic disease and obesity in humans and other species.

A small study of EMS horses showed less diversity in their fecal microbiota compared to healthy horses and an increase in biomarkers that are associated with glucose intolerance in humans. [11]

Environmental Factors

Endocrine disrupting chemicals (EDCs) have been proposed as a risk factor for the development of EMS in horses.

Data collected from horses living on farms located within 30 miles of EDC disposal sites showed a higher incidence of laminitis and insulin dysregulation. [12]

How is EMS Diagnosed?

Definitive diagnosis of EMS requires an examination from a veterinarian. Diagnosis of EMS is based on physical examination, diagnostic testing, and an evaluation of the horse’s dietary history. [13]

Physical Examination

Prior to starting a treatment plan, a baseline neck circumference and body condition score should be established for horses suspected of having EMS.

The body condition scoring system assesses horses’ body condition based on the amount of fat that has accumulated in areas of their body including the ribs, shoulder, withers, loin, tailhead, and neck.

Body condition score refers to a numerical value ranging from 1 to 9. A score of 5 is considered the ideal body condition for a mature horse.

Hoof Evaluation

Horses suspected of having EMS should have their hooves carefully examined. Signs of laminitis include growth rings in the feet that widen at the heels, increased digital pulse, and warm hooves.

When laminitis is severe enough, horses may be unable to walk or have a shortened stride.

Radiographs

Radiographs are useful for determining if a horse has previously experienced a bout of laminitis.

Lateral images of the pedal bone are recommended to detect changes such as rotation and sinking of the coffin bone, reduced sole depth, and accumulated pockets of gas/fluid in the hoof capsule.

Diagnostic Blood Testing

The most common diagnostic blood tests used to diagnose and monitor EMS and the hyperinsulinemia that occurs with the condition are Baseline Insulin and Glucose, the Oral Sugar Test (OST), and Baseline Leptin or Adiponectin.

Baseline Insulin

High insulin levels (hyperinsulinemia) are often present in horses with EMS. A blood insulin concentration of over 20 uIU/mL indicates insulin resistance. [13]

However, this is an upper limit determined mathematically and may generate many false negatives. For example, in a pony field study, ponies not prone to developing laminitis had insulin levels of 13 uIU/mL or lower, even on Spring pastures. [23]

Previous guidelines recommended measuring insulin levels after a period of fasting. However, it is no longer recommended to fast horses prior to insulin testing because of the high number of falsely negative tests. Testing should be done with the horse having constant access to pasture or hay but fed nothing else. This forage-only diet should commence the night before and continue on the day of testing.

If the horse has been fasted the reference level for a normal blood insulin is lower. Insulin in a fasted horse should be under 10 uIU/mL. [24][25]

Testing should be completed in a low-stress environment and not during active laminitis to avoid false results. Insulin is affected by numerous factors including diet, exposure to stress, and pain.

Baseline Glucose

Horses with EMS typically present with blood glucose (sugar) concentrations in the normal range or slightly elevated (hyperglycemia).

PPID or diabetes mellitus may be present if glucose levels are well above the normal range.

Oral Sugar Test (OST)

A more sensitive diagnostic test, the Oral Sugar Test involves feeding a sugary solution such as corn syrup to a horse after a fast of three to twelve hours.

Blood samples are collected after 60 and 90 minutes and are analyzed to determine insulin concentration. An insulin concentration over 60 uIU/mL is abnormal. [13] Cutoffs as low as 40 uIU/ml have been used. [26]

Oral Glucose Tolerance Test (OGTT)

The Oral Glucose Tolerance Test (OGTT) assesses blood insulin concentration after the horse has consumed feed with dextrose powder added to it at a rate of 1g/kg. However, this test is rarely done because acceptance of the meal and gastric emptying rate impact the results. [13]

IV Glucose Tolerance Test (IVGTT)

The intravenous glucose tolerance test is the most sensitive test available for diagnosing insulin resistance.

This test requires taking a blood sample prior to IV administration of sugar (dextrose) and subsequent collection of additional samples at 1, 15-, 30-, 45-, and 60-minutes post-injection.

Blood samples are analyzed to determine glucose and insulin values. In healthy horses, the concentrations of these substances return to normal quickly. Glucose and insulin levels remain elevated for a longer time in horses with EMS.

Baseline Leptin

A hormone produced by adipose or fat cells, leptin is typically present at a higher concentration in the blood of horses affected by EMS. [14]

This test is useful for tracking weight loss in horses because reductions in weight result in a lower leptin concentration.

Baseline Adiponectin

High molecular weight adiponectin is another hormone produced by fat and is used instead of leptin as a marker for EMS in the EU, UK and AU. Adiponectin increases insulin sensitivity and is low in EMS horses.

Adiponectin has been shown to be more closely linked to insulin resistance than leptin. [26]. High molecular weight adiponectin in lean horses ranged from 2.8 – 24.2 ug/mL versus 0.7-4.9 ug/mL in obese horses. [27]

Testing to Rule-Out PPID

It is important to test horses with EMS for PPID because this condition can worsen insulin resistance.

Horses over the age of 15 years, or those that show signs of increased drinking and urination or laminitis in the Fall should be tested for PPID.

Tests for PPID include measuring adrenocorticotropic hormone (ACTH) or the more sensitive TRH Stimulation test which checks ACTH after stimulation with thyrotropin-releasing hormone.

Treatment of EMS

The first step in treating Equine Metabolic Syndrome is to make changes to your horse’s feeding program to help them achieve a normal body weight. Dietary management strategies may involve selecting lower energy density forages and removing excess grains and sugars.

In addition, exercise can help horses maintain a healthy body condition and improve insulin sensitivity.

Some horses affected by EMS may require medication to improve their condition if diet and exercise strategies are insufficient.

Dietary Management

Overweight horses should be fed carbohydrate-restricted diets to reduce their body weight and improve insulin regulation. Studies show that horses with EMS that lose excess weight see improved insulin response. [15]

Grain, fruits, vegetables and sugary treats should be removed from the diet because they have high sugar and/or starch content. High-fat feeds may need to be reduced in the diet due to their high energy content and because the safety of high fat feeding has not been established in EMS horses.

Overweight horses with EMS require restricted or no access to pasture until they achieve a normal body weight. The use of a grazing muzzle can help to reduce intake while still providing turnout. Pasture should especially be restricted when the grasses are actively growing and contain high sugar content.

Forages provided to horses with EMS should be analyzed to determine starch and sugar content. A hay sample report will show values for the percentages of starch and ethanol-soluble carbohydrates (ESC) in the forage. The sum of starch plus ethanol-soluble carbohydrates (ESC) is the hydrolyzable carbohydrate (HC) fraction.

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The HC value of forage fed to horses with EMS should ideally be less than 10% of dry matter. Soaking hay reduces the concentration of sugar, but may also lead to loss of some nutrients if soaking time is excessive. [16] A 30 minute soak in hot water or 60 minutes in cold water is sufficient to lower the sugar levels.

The ideal rate of weight loss in obese horses with EMS is between 0.5% and 1.0% of body mass per week.

It is recommended to consult with your veterinarian and an equine nutritionist to design a weight loss feeding program for your horse. A nutritionist can help your horse achieve weight management goals while ensuring nutrient requirements for vitamins and minerals are met.

Supplements

Horses affected by EMS and on restricted diets may require supplemental vitamins and minerals to avoid nutrient deficiencies that result from reducing intake or imbalances in the forage.

In some cases, additional dietary protein may be required if the forage fed contains an insufficient amount.

Research demonstrates that supplements including magnesium, chromium, and short-chain fructo-oligosaccharides (a prebiotic) help to improve insulin sensitivity in normal horses or EMS horses fed a high starch challenge.. [17][18][19]

Mad Barn’s AminoTrace+ mineral and vitamin supplement is specifically formulated for horses with EMS or inuslin resistance. It is a low-NSC concentrated formula that provides comprehensive coverage of the nutrients that EMS horses require to support healthy metabolic function.

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Exercise

Research shows a number of benefits of exercise in horses with EMS, including reduced inflammation and blood insulin concentrations. [20][21][22]

Designing the right exercise program for your horse will depend on body condition, fitness level, and the stability of the hooves if laminitis is present or has occurred in the past.

Medications

If diet and exercise do not sufficiently improve the condition of horses with EMS, drug therapies may be considered as an additional strategy. The following medications may be used to treat horses with EMS.

Metformin: Used to treat horses with severe insulin resistance, this drug prevents increases in blood glucose and insulin levels post-feeding. [13] However, its effects may decrease over time.

Levothyroxine: This drug is beneficial in horses that are resistant to weight loss. [13]

Pergolide: This drug is only necessary in horses diagnosed with concurrent PPID (Equine Cushing’s Disease). Pergolide works by reregulating the overproduction of hormones produced by the pituitary gland of the brain.

SGLT2 inhibitors: Sodium-glucose co-transporter-2 (SGLT2) inhibitors block the reuptake of glucose in the kidneys so that it is lost into the urine, decreasing the amount of insulin the pancreas needs to produce.

To date, canagliflozin (Invokana) and ertugliflozin (Steglatro) are SGLT2 inhibitors that have been used in horses. [28][29] They are usually very effective in lowering insulin and universally effective in relieving laminitis pain.

Increased triglycerides is a side effect that must be managed with diet. [30] Treated horses need generous salt supplementation to keep them drinking as the drug increases urine production. NSAIDs such as banamine or bute, should be used with caution in horses on SGLT2 inhibitors. Human studies show that this drug combination increases the risk of kidney damage. [#]

EMS Prevention & Recovery

Horses being treated for EMS should be assessed regularly by a veterinarian to determine if management strategies are helping. Retesting for elevated insulin is also required to monitor improvements in health.

It is possible to reverse hyperinsulinemia and prevent recurrence of this condition! However, many horses can redevelop EMS, particularly if they’re genetically predisposed to it. The following strategies are recommended to help your horse recover from EMS and stay healthy:

  • Provide a forage-based diet with low-HC content to prevent obesity and avoid stimulating high insulin production.
  • Avoid grain and balancers fed in 1-lb amounts or higher
  • Use well rinsed and soaked beet pulp as a supplement carrier, if needed
  • Adopt a suitable exercise program for sound horses to aid in body condition management.
  • Work with an equine nutritionist to create a nutritionally balanced diet with an appropriate energy intake.
  • Regularly check the hooves of your horse to detect early signs of laminitis such as an increased digital pulse.
  • Reduce or eliminate access to lush pastures that are high in sugars.
  • Use a grazing muzzle to reduce grass consumption. Use a hay net when providing free choice hay.
  • Keep affected horses in a dry lot to permit exercise while preventing grass consumption.
  • Soak hay for 30 minutes in hot water or 60 minutes in cold water to reduce the amount of sugar.

For a comprehensive diet plan to prevent EMS and/or support improved insulin sensitivity, submit your horse’s diet for evaluation by our equine nutritionists.

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References

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  2. McCue, M. et al.Equine metabolic syndrome: a complex disease influenced by genetics and the environment. J Equine Vet Sci. 2015.
  3. Vick, MM. et al.Relationships among inflammatory cytokines, obesity, and insulin sensitivity in the horse. J Anim Sci. 2007.
  4. Frank, N.Equine metabolic syndrome. Vet Clin North Am Equine Pract. 2011.
  5. Cornelisse, CJ, Robinson, NE. Glucocorticoid therapy and the risk of equine laminitis. Equine Vet Ed. 2013.
  6. Morgan, RA. et al.Prevalence and risk factors for hyperinsulinaemia in ponies in Queensland, Australia. Aust Vet J. 2014.
  7. Johnson, PJ.The equine metabolic syndrome peripheral Cushing’s syndrome Vet Clin North Am Equine Pract.
  8. Morgan, R. et al.Equine metabolic syndrome. Vet Rec. 2015.
  9. Frank N. et al.Insulin dysregulation. Equine Vet J. 2014.
  10. Lewis, SL. et al.Genomewide association study reveals a risk locus for equine metabolic syndrome in the Arabian horse. J Anim Sci. 2017.
  11. Elzinga, SE. et al.Comparison of the fecal microbiota in horses with equine metabolic syndrome and metabolically normal controls fed a similar all-forage diet. J Equine Vet Sci. 2016.
  12. Durward-Akhurst, SA. et al.The association between endocrine disrupting chemicals and equine metabolic syndrome. Dorothy Russell Havemeyer Foundation Equine Endocrinology Symposium. 2017.
  13. Frank, N. et al.Current best practice in clinical management of equine endocrine patients. Equine Vet Educ. 2014.
  14. Pleasant, RS. et al. Adiposity, plasma insulin, leptin, lipids, and oxidative stress in mature light breed horses. J Vet Intern Med. 2013.
  15. Delarocque, J. et al.Weight loss is linearly associated with a reduction of the insulin response to an oral glucose test in Icelandic horses. BMC Veterinary Research. 2020.
  16. Bochnia, M. et al.Effect of Hay Soaking Duration on Metabolizable Energy, Total and Prececal Digestible Crude Protein and Amino Acids, Non-Starch Carbohydrates, Macronutrients and Trace Elements. J Equine Vet Sci. 2021.
  17. Vervuert, I. et al.Effects of chromium yeast supplementation on postprandial glycaemic and insulinaemic responses in insulin-resistant ponies and horses.
  18. Respondek, F. et al.Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses. J Anim Sci. 2011.
  19. Winter, JC. et al.Oral supplementation of magnesium aspartate hydrochloride in horses with equine metabolic syndrome.
  20. Morgan, RA. et al.Treatment of Equine Metabolic Syndrome: a clinical case series. Equine Veterinary Journal.
  21. Menzies-Gow, NJ. Et al.The effect of exercise on plasma concentrations of inflammatory markers in normal and previously laminitic ponies. Equine Vet J. 2014.
  22. Bonelli, F. et al.How swimming affects plasma insulin and glucose concentration in Thoroughbreds: a pilot study.Vet J. 2017.
  23. Treiber, K.H. et al. Evaluation of genetic and metabolic predispositions and nutritional risk factors for pasture-associated laminitis in ponies. J Am Vet Med Assoc. 2006.
  24. Lindase, S. et al. Evaluation of fasting plasma insulin and proxy measurements to assess insulin sensitivity in horses. BMC Vet Res. 2021.
  25. Olley, R.B. et al. Comparison of fasted basal insulin with the combined
    glucose-insulin test in horses and ponies with suspected insulin dysregulation
    . Vet J. 2019.
  26. Ungru, J. et al.Effects of body weight reduction on blood adipokines and subcutaneous adipose tissue adipokine mRNA expression profiles in obese ponies. Vet Rec. 2012 .
  27. Durham, A.E. The potential usefulness and applications of adiponectin testing in horses with suspected EMS. Liphook Equine Hospital. Accessed November 6, 2023.
  28. Kellon, E.M. and Gustafson, K.M. Use of the SGLT2 inhibitor canagliflozin for control of refractory equine hyperinsulinemia and laminitis. Open Vet J. 2022.
  29. Sundra, T. et al. Preliminary observations on the use of ertugliflozin in the management of hyperinsulinaemia and laminitis in 51 horses: A case series. Equine Vet Educ. 2022.
  30. Kellon, E.M. and Gustafson, K.M. Hypertriglyceridemia in equines with refractory hyperinsulinemia treated with SGLT2 inhibitors. Open Vet J. 2023.