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Cooperative Extension Fact Sheet FS1067

Metabolic Problems in the Horse: Sorting out the Diagnosis

  • Carey Williams, Extension Specialist in Equine Management
  • Nettie Liburt, Ph.D., former Animal Science Doctoral Student

Equine Metabolic Syndrome (EMS) and pituitary pars intermedia dysfunction (PPID), also known as Equine Cushing's Disease, are the most common endocrine disorders of horses. Insulin dysregulation (ID) can be a consequence of one or both conditions. These endocrine disorders put a horse at higher risk of developing laminitis, which is inflammation of the sensitive laminae that hold the coffin bone to the hoof wall. Founder occurs when the coffin bone rotates and is a result of, but not the same as, laminitis. There is a lot of confusion regarding the definition of these terms, and the diagnosis and treatment of EMS, PPID and ID. This factsheet will provide the fact-based information available to clarify the issues surrounding diagnosis and dietary treatment of EMS.

Terminology

Historically, these terms came from human literature, but as research and knowledge has evolved, definitions have been clarified to reflect the horse more accurately.

Equine Metabolic Syndrome

Equine Metabolic Syndrome, or EMS, is defined by the Equine Endocrinology Group as both an endocrine and a metabolic disorder where horses tend to gain weight easily, have increased body fat, insulin dysregulation and hyperinsulinemia (abnormally high concentration of insulin circulating in the blood). Hyperinsulinemia is a known risk factor for the development of laminitis. Any horse could potentially develop EMS regardless of age, gender, or breed. However, there are certain classes/breeds that are thought to have a genetic component that puts them at higher risk of developing EMS; these include ponies, Warmbloods, Morgans, Paso Finos, and Arabians. Development of a cresty neck and/or fat pads around the tail head are physical signs of EMS in horses.

Zoom in Figure 1.

Figure 1. Horse with PPID, displaying a long hair coat.

Pituitary Pars Intermedia Dysfunction (PPID)

Pituitary pars intermedia dysfunction (PPID) develops when neurons that extend to the pars intermedia (middle lobe of the horse's pituitary gland) that secrete dopamine begin to degenerate, causing a lack of inhibition of the pars intermedia. The result is an increase in size of the pars intermedia as well as increased hormone secretion, in particular, adrenocorticotrophic hormone (ACTH). This pituitary dysfunction and resulting ACTH cause abnormally high cortisol release from the adrenal gland. The high cortisol is the cause of the clinical signs of ID, which include hyperglycemia, excessive thirst and urination and hair coat changes. Horses with PPID are more susceptible to the development of EMS due to high cortisol, but PPID and EMS are separate conditions. In other words, a horse can have either PPID, EMS or both at the same time.  PPID also occurs in approximately 20% of horses over the age of 15 years (Kirkwood, et al., 2022), but it has been diagnosed in younger animals.

Currently, the recommended testing for PPID is the Thyrotropin Hormone (TRH) Stimulation Test. After an overnight fast (a horse can have hay and water, just not a concentrate meal within 12 hours), a veterinarian will collect a baseline blood sample. The veterinarian will then administer a dose of TRH, followed by a second blood sample exactly 10 minutes later. The TRH stimulation test is much more precise than a baseline ACTH-only test, as ACTH concentration can vary by season and by itself may not yield accurate results.

Insulin Dysregulation (ID)

Insulin Dysregulation (ID) is a term coined to encompass abnormalities of insulin metabolism including both insulin resistance (IR) and hyperinsulinemia (Frank and Tadros, 2013). Insulin is a hormone secreted from the pancreas in response to an increase in blood glucose (usually from a meal) that signals tissues to absorb glucose from the blood for storage. IR is the condition where tissue (such as muscle, liver and adipose) do not respond to insulin. When this signal fails, the concentration of glucose in the blood remains high. Hence, the body attempts to compensate by secreting more insulin, leading to hyperinsulinemia and increased risk for laminitis. Diagnosis of ID typically involves either "clamp" techniques or an oral glucose challenge.

  • Zoom in Figure 3.

    Figure 2. Oats.

    Clamp techniques involve infusing both insulin and glucose intravenously and taking blood samples at frequent intervals for glucose and insulin analyses. The resultant data are then analyzed via complex mathematical models (the "minimal model" analysis) to calculate the amount of insulin required to reduce blood glucose or keep blood glucose constant. These are direct measures of cellular sensitivity and are the "gold standard" for diagnostic purposes. However, this technique is most often used in research settings as very few veterinary clinics are equipped to run these tests. These tests are also cost-prohibitive for the owner.
  • Zoom in Figure 2.

    Figure 3. Sweet Feed.

    A simpler test of insulin sensitivity is to administer, either orally or intravenously, a standardized glucose challenge, or oral glucose tolerance test, and measure the glucose and insulin responses at less frequent intervals. This method involves collecting a baseline blood sample after an overnight fast (hay and water allowed), then giving the horse either a measured dose of Karo syrup or a challenge meal (either oats or a sweet feed). Blood samples are collected 60- and 90-minutes after the horse has consumed the syrup/meal. Samples are sent to the lab for measurement of blood glucose and insulin. The resulting lab values are used to diagnose a horse's glucose and insulin status.

Both techniques, however, assume normal pancreatic beta cell function (necessary for insulin release), which is influenced by a variety of factors such as plasma cortisol concentrations, time of day, diet to which the horse was adapted and even season of the year.

Where Does Hyperinsulinemia Fall?

Hyperinsulinemia can be defined as an unusually high blood insulin concentration, with or without concomitant hyperglycemia. This is a key feature of pituitary dysfunction, obesity, chronic laminitis, and IR in horses. Hyperinsulinemia is also correlated with osteochondrosis dissecans or OCD in weanlings less than 12 months of age. OCD is the failure of proper mineralization of growing bones which results in cartilage defects at the articular surface of major joints such as the stifle, hock, and fetlock. Hyperinsulinemia appears to be prevalent in the equine population, though many of these hyperinsulinemic horses will appear clinically normal. Therefore, hyperinsulinemia alone does not prove the presence of a clinically significant disorder.

Zoom in Figure 4.

Figure 4. Blood samples taken in the morning may contain higher glucose and insulin.

Hyperinsulinemia can be transiently induced by the following conditions in horses that are otherwise clinically 'normal':

  1. Stress: The high cortisol release in certain situations causes transitory insulin resistance. Therefore, horses that are in severe pain (i.e., acute laminitis) or extremely upset may have abnormally high insulin and glucose in their blood.
  2. High glycemic index feeds: A meal of grain or sweet feed can cause elevated blood insulin/glucose for over 3 hours after feeding. The amount of non-structural carbohydrates (NSC) is a major contributing factor to this response, along with the horse's individual metabolic status. If a blood sample is taken within 3 to 4 hours of feeding, results may be interpreted erroneously as being high.
  3. Time of day: Blood cortisol concentration is generally higher in the morning than in the afternoon in normal horses. Blood samples taken in the morning may consequently have higher concentrations of glucose and insulin than those taken in the afternoon or early evening even in fasting horses.
  4. Zoom in Figure 5.

    Figure 5. Obesity is commonly associated with IR.

    High starch/sugar rations: Horses consuming a ration that contains large amounts of starch and sugar may have a higher glucose and insulin response compared to a ration of only forage or minimal amounts of low carbohydrate concentrate. Again, the glucose and insulin response will depend on the horse's individual physiology and overall content of the diet.
  5. Obesity:Though not all obese horses are ID, it is commonly associated with excess body fat. Horses with a body condition score (BCS) of 7 or higher on the Henneke BCS Scale are considered obese.

What Could It Mean?

If you take a single blood sample the glucose/insulin results will be meaningless unless you know:

  1. When the animal was last fed
  2. Type of feed it ate last
  3. Feed ration it was accustomed to eating
  4. Time of day
  5. Level of stress


Dietary Recommendations

When a single blood sample is taken that measures abnormally high blood insulin and/or glucose levels, the above factors need to be considered first before definitive testing is considered. If the above factors are ruled out, definitive testing includes oral glucose tolerance test, dexamethasone suppression test and/or ACTH measurements. One of these definitive tests should be run before instituting drug or drastic dietary measures.

If the horse does have pituitary dysfunction, drugs such as pergolide may be prescribed to control the clinical signs. Dietary starch and sugar should be restricted by feeding forage predominantly. Grazing may need to be limited by either utilizing a dry lot or a grazing muzzle. Hay/forage may need to be soaked to help reduce the amount of starch and sugar. Target level of NSC in the forage portion of the diet for severely ID horses should be <10% (Frank et al., 2022). If concentrates are fed, they should be formulated to provide restricted starch and sugar. ID horses may need to be restricted to 0.1 g (if severe) - 0.5 g (if at risk) of NSC per kilogram of body weight per meal (Macon et al., 2023).

If the horse is not obese or has pituitary dysfunction but is truly IR, the only treatment would be to restrict starch and sugar intake as above. However, NSC intake can be slightly higher than those in with severe problems, i.e., <12% total NSC in forage. High fat concentrate formulations can be used if necessary to maintain body weight/condition. Intakes that are high in fat, however, will not significantly affect insulin sensitivity, as the consumption of dietary fat does not elicit insulin secretion from the pancreas. Maintaining an exercise program appropriate for the individual horse will help support proper glucose and insulin metabolism (Liburt et al., 2013).

Generally speaking, if a horse is obese, it should be fed restricted amounts of forage (approximately 1.5% body weight on a dry matter basis) divided into three or four feedings a day to achieve weight loss. Use of slow feed hay nets or other slow feeders will extend the time it takes a horse to consume a portion of hay, lessening the time the stomach is empty between meals. Water and salt should also be available as a free choice. A ration balancer may be necessary to ensure proper amounts of protein, vitamins and minerals are consumed without adding excess calories. A reduction in the need for calories does not equate to a reduction in the need for nutrients, thus the reason ration balancers are so useful.

Despite popular literature, there is no scientific evidence that reduction of iron intake, high fat intake or various herbal or "natural" supplements have any effect at all on insulin sensitivity in horses. Consult a qualified equine nutritionist if you have concerns about your horse's diet.

References and Other Reading

  1. Frank, N., S. Bailey, F.R. Bertin, T. Burns, M. de Laat, A.E. Durham, J. Kritchevsky, and N. Menzies-Gow. 2022. Recommendations for the diagnosis and Management of Equine Metabolic Syndrome (EMS). Equine Endocrinology Group.
  2. Frank, N. and Tadros, 2013. Insulin dysregulation. Equine Veterinary Journal. DOI: 10.1111/evj.12169
  3. Heaton, C.P., Cavinder, C.A., McClure, E.N., Smith, T., Smith, W.B., Liburt, N., Krotky, A, Harris, P. 2024. Prevalence of insulin dysregulation in the non-obese stock-type horse and relationship with morphometric neck measurements. Journal of Equine Veterinary Science. 141. DOI: 10.1016/j.jevs.2024.105151
  4. Kirkwood, N.C., Hughes, K.J., and Stewart, A.J. 2022. Pituitary Pars Intermedia Dysfunction (PPID) in horses. Veterinary Sciences. 9(10): 556. DOI: 10.3390/vetsci9100556
  5. Liburt, N.R., McKeever, K.H., Malinowski, K., Smarsh, D.N, and Avenatti, R. 2013. Response of the hypothalamic-pituitary -adrenal axis and glucose homeostasis during and in recovery from acute exercise before and after training in old and young Standardbred mares. Journal of Equine Veterinary Science. 33(5): 327-328. DOI: 10.1016/j.jevs.2013.03.024
  6. Liburt, N.R., Mastellar, S.L., Share, E.R. and Harris, P.A. 2023. How challenging is it to find non-insulin dysregulated horses in an apparently clinically healthy herd of university horses? Journal of Equine Veterinary Science. Vo. 124. DOI: 10.1016/j.jevs.2023.104389
  7. Macon, E.L., Harris, P.A., Bailey, S., Barker, A.C., Adams, A. 2023. Identifying possible thresholds for nonstructural carbohydrate in the insulin dysregulated horse. Equine Veterinary Journal. 55(6):1069-1077. DOI: 10.1111/evj.13910

Acknowledgements

The authors would like to thank original co-author Sarah L. Ralston, VMD, Ph.D., DACVN, a retired Animal Sciences faculty member, for her contribution to pieces of this updated version.

Photo Credits: C. Williams, Equine Extension Specialist, Rutgers University.

January 2025