Management of metabolic diseases

Metabolic diseases are becoming increasingly more common in the modern equine. Equine metabolic syndrome (EMS) is a term coined to describe a collection of metabolic risk factors, mainly insulin dysregulation (ID), hyperinsulinemia-associated laminitis (HAL), and obesity. Insulin dysregulation is characterized by high resting insulin, high postprandial insulin, or insulin resistance at the tissue level. The severity of ID can vary greatly between individuals and is often seen in obese horses while less commonly in non-obese horses. Hyperinsulinemia-associated laminitis, also known as pasture-associated laminitis, is thought to be a result of chronically high insulin levels having adverse effects on laminar tissues, and causing mild to moderate lameness which can progress to classical laminitis. Divergent hoof rings can be indicative of insidious HAL and disruption of the internal structural changes in the hoof. Dietary sugar, starch and fructans can be instigators of HAL in susceptible animals. Although the exact mechanism has not been completely elucidated, the most popular theory is that hyperinsulinemia induces inappropriate stimulation of insulin-like growth factor-1 receptors on lamellar epidermal cells. Obesity is a common characteristic of the EMS horse.

With the clinical signs of EMS in obese individuals, some or all the following may be present: weight loss resistance, clinical laminitis, divergent hoof rings (subclinical laminitis), cresty neck, subcutaneous adipose tissue deposits, preputial or mammary gland enlargement. Manifestation of EMS in non-obese individuals commonly presents as clinical or subclinical laminitis. Many genetically at-risk non-obese EMS horses are not obese because they are maintained in controlled environment and/or have been obese historically. EMS may coexist with pituitary pars intermedia dysfunction (PPID) in older horses. More information on PPID will be discussed in the accompanying abstract on Feeding Old Horses by this author.

While there are a number of tests currently used for diagnosing EMS and ID, the two recommended tests are the Oral Sugar Test (OST) and Insulin Tolerance Test (ITT). Resting insulin and glucose blood levels can be used alongside the dynamic tests as a two-step approach to ID testing. The combined glucose-insulin test, frequently-sampled intravenous glucose tolerance test, and euglycemic-hyperinsulinemic clamp procedure are considered too complex and expensive for routine clinical use but can provide relevant information in a research setting.

Management of EMS in obese individuals involves dietary changes and restrictions such as restricting or eliminating grazing access, as well as elimination of treats, grain and most concentrate feeds (except a ration balancer type feed). Hay fed should be low non-structural carbohydrate (NSC) content, preferably < 10%. For weight loss, offer the equivalent of 1.2-1.5% BW in hay, on a dry matter basis, and avoid higher calorie hays such as alfalfa. For hays with higher NSC content, soaking for 60 minutes can leach out some of the sugar but the amount lost can be variable dependent on the hay and is not 100% reliable for sugar reduction. For situations where lower NSC hay is not available, then up to 50% of the hay can be replaced with good quality straw but needs to be introduced into the diet very gradually to avoid colic. To avoid prolonged periods of fasting, slow feeder hay nets or small frequent meals can be used. If small amounts of grazing are tolerated, then a grazing muzzle can be used to limit the amount of grass consumed. To balance out the vitamin and mineral shortcomings of the hay/pasture, particularly if the hay is being soaked, the diet should include either a low NSC ration balancer type concentrate feed for when the protein content of the hay is low or questionable, or a vitamin mineral supplement when protein is adequate in the hay or horses that are extremely sensitive to any concentrate. If desired weight loss is achieved, then amounts of hay and pasture access can be gradually increased to a level that maintains the weight loss but does not cause an increase in body condition. Supplements commonly recommended for ID are magnesium, chromium, and resveratrol as well as some herbal blends. While minerals like magnesium are required in the diet, the above-requirement supplementation has not been studied very well as treatment for ID. Off label use of the following drugs are currently being used in the treatment of EMS: high dose levothyroxine, metformin hydrochloride, and sodium-glucose co-transporter 2 (SGLT2) inhibitors. As exercise is likely to be beneficial for weight loss and maintenance of reduced body condition as well as improving insulin sensitivity, any level of exercise tolerated is recommended unless laminitis is present. Ideally, horses should be maintained in a low stress environment and housed with a companion if restricted to a drylot. Avoiding confinement in a stall, if at all possible, except during laminitis treatment, is desirable. Conscientious hoof care is also essential in the treatment of EMS.

Treatment of non-obese EMS horses is similar except weight loss is not desirable. The horses should still be maintained on a low-glycemic diet, such as low NSC hay and a low NSC concentrate feed or ration balancer. However, the non-obese EMS horse may be able to tolerate higher amounts of a low NSC, high fat, and high-quality fiber concentrate feed such as one with beet pulp and soy hulls. Medication with metformin or SGLT2 inhibitors can be used if dietary restrictions are not effective at controlling ID. Exercise and stress management are equally important as long as the horse is not laminitic.

Frank N, Bailey S, Bertin F, Burns T, de Laat M, Durham A, Kritchevsky J, Menzies-Gow N. 2022 Recommendations for the Diagnosis and Management of Equine Metabolic Syndrome. Equine Endocrinology Group, https://sites.tufts.edu/equineendogroup/files/2022/10/EMS-EEG-Recommendations-2022.pdf.

Dr. Kathleen Crandell earned her MS in Equine Nutrition and Exercise Physiology and PhD in Equine Nutrition and Reproduction from Virginia Polytechnic Institute and State University (Virginia Tech). Dr. Crandell was a Superintendent of the Virginia Tech Middleburg Agricultural Research and Extension Center in Middleburg, Virginia, where she ran an equine research and breeding program with over 100 Thoroughbred horses. For the past 26 years, she has been an equine nutrition consultant for Kentucky Equine Research (KER), responsible for support of the national and international feed mills associated with KER with feed formulation, advising on issues of manufacturing and packaging of equine feeds and customer equine feeding recommendations. She also provides recommendations for nutritional programs of individual horses or farms and equine publications, and has consulted for some of the top equine competitors in the US. Dr. Crandell has presented nutritional seminars for education of the general equine public and scientific community around the world. Dr. Crandell has authored book chapters and articles in refereed journals. She also writes or is interviewed for articles on specific topics in equine nutrition for KER publications, KER.com Equinews website and popular magazines. In addition, Dr. Crandell is an Instructor for Equine Guelph, University of Guelph, Ontario, Canada, teaching an equine nutrition course “Advanced Equine Health through Nutrition”, an in-depth study of nutritional related diseases and disorders. Dr. Crandell has been awarded 3 John H. Daniels Fellowships by the National Sporting Library in Middleburg, Virginia, to study historical perspectives on equine nutrition, supplementation and laminitis. Having previously owned a small breeding farm and competed in endurance and competitive trail, she is currently an avid trail rider with her Arabian and half-Arabian horses.