Typically, overweight, middle- aged horses have been considered the prime candidates for developing equine metabolic syndrome (EMS). Because insulin resistance is one of the diagnostic criteria for EMS, the disorder can often be confused with Cushing’s Disease.
The Ohio State University Galbreath Equine Center Associate Professor Teresa A. Burns, DVM, PhD, DACVIM, has emphasized that breed predisposition to EMS might be as important as body condition in predicting the disease.
Some breeds are simply more prone to developing EMS than others. For example, Arabians, Tennessee Walking Horses, American Saddlebreds, Paso Finos, ponies, donkeys and mules are more susceptible to the syndrome. Individuals of predisposed breeds can be affected even when the horse is lean.
“This likely has to do with a genetic tendency toward insulin resistance/ dysregulation that is common in these breeds,” she said. “Horses of predisposed breeds can be affected even when they are of normal body condition (or even thin); in this case, the breed is ‘typical,’ but the degree of adiposity is not.”
EMS was first described in 2002, but it is perhaps the most common endocrine disorder encountered in equine veterinary practice. It can have diverse effects on the horse’s reproductive physiology and fertility, including effects on reproductive seasonality, ovulation efficiency, implantation, early pregnancy loss and lactation. Like the human metabolic syndrome, EMS is a constellation of clinical findings that is predictive of an adverse clinical event/outcome.
While metabolic syndrome in humans is predictive of type II diabetes mellitus, atherosclerotic cardiovascular disease, stroke and certain types of cancer, EMS is predictive of laminitis in affected equids. The reasons underlying this species difference in the complications of obesity remain obscure, but dietary composition and genetics are likely involved. It is clear, however, that nutritional obesity is associated with disease risk in both horses and humans.
Despite the potential for a genetic connection, risk of EMS is still higher in horses that are overweight. This is due to the effects of their increased body fat mass on systemic insulin and glucose dynamics.
“Adipose tissue tends to become more insulin resistant at the tissue level as it expands when storing more lipid (such as in diet-induced obesity) in parallel with macrophage infiltration of the tissue and increased production of inflammatory cytokines,” Burns said.
These local changes in adipose tissue can ultimately have body-wide effects on insulin sensitivity, with horses that have a greater degree of adiposity being more likely to be insulin resistant. Insulin resistance increases the likelihood of elevated serum insulin concentrations (hyperinsulinemia) following ingestion of soluble carbohydrate, which is linked tightly with the risk of laminitis. This is one way in which increased adiposity can be associated with EMS.
EMS is an active area of research for several groups around the world. Current studies are attempting to identify the link(s) between insulin/growth factor signaling and derangements of the cytoskeletal structure of the epithelial cells of the laminae, she said. This ultimately results in displacement of the coffin bone within the hoof capsule in laminitis.
Additional work is being done to optimize diagnostic testing for insulin dysregulation in horses. This would make testing more accurate and convenient for performing on-farm. It would also allow researchers to evaluate therapies that have been shown to be effective for other forms of laminitis (such as distal limb cryotherapy) to gauge their efficacy in the treatment of EMS-associated laminitis.
“Identification of the pathophysiological players in laminitis associated with EMS will hopefully lead to new points at which we can intervene therapeutically,” she said.
As researchers continue to investigate endocrine disorders, advancements in diagnostic testing and management methods enable veterinarians to more readily identify EMS. Burns provided an update on both that can aid veterinarians working with EMS patients.
Determining whether or not a horse has EMS can be challenging. Commonly, diagnosis begins with ruling out other disorders such as pituitary pars intermedia dysfunction (PPID, or equine Cushing’s syndrome). Obesity tends to be the most widely recognized clinical sign of EMS. However, even horses with a normal or poor body condition can be candidates for EMS.
“Thin animals of breeds predisposed to EMS often retain regional fat deposits, such as a cresty neck and/or tail head fat pads, even if they have little visible fat deposition elsewhere,” Burns said.
Because obesity isn’t always directly correlated to EMS, laminitis provides another hint that a horse might have an endocrine disorder.
In recent years, the development of more dynamic testing to measure insulin dysregulation has made the diagnosis of EMS more efficient and effective. Specifically, the combined glucose-insulin tolerance test (CGIT) and the oral sugar test (OST) have been optimized for field use, so it is easier for veterinarians to make a diagnosis on-farm.
“The lab evidence of insulin dysregulation, such as increased basal insulin concentration or abnormal combined glucose and insulin test or oral sugar test, are clearer indicators of EMS,” she said.
She cautioned that limitations of the sensitivity of some available tests, such as the basal insulin concentrations and the OST, have been identified.
“They are specific tests, but they may miss some affected horses due to their relatively poor sensitivity,” she said.
Creating a treatment plan depends on the horse and its current body condition. The plan of action will likely be directly correlated to the specific horse’s genetic risk of insulin dysregulation.
Dietary changes are the most effective method for managing the majority of horses with EMS. Burns recommended minimizing dietary non-structural carbohydrate content to minimize the risk of elevated serum insulin concentrations and the risk of laminitis.
“It’s important to encourage weight loss through careful dietary restriction and reducing the horse’s total caloric intake,” she said.
Like people, horses can effectively lose weight when fed less and exercised more. Low-carbohydrate diets are now more readily available commercially and have made the dietary management of horses with EMS easier in the past several years. Limiting access to fresh, lush grass either through use of a grazing muzzle or dry-lot turnout is as important as choosing the right feed.
“Horses that are undergoing a weight loss program can be initially offered 1.5- 2.0% of their body weight in hay per day,” Burns said. “Ideally, hay that has been analyzed and shown to contain <10% NSC on a dry matter basis should be offered. If this reduction does not yield weight loss after four to six weeks, then further reduction of the daily hay ration to 1.0-1.5% of body weight is recommended.”
Feeding less than 1.0% of body weight in hay daily is not recommended, and EMS horses that fail to lose weight with reasonable dietary caloric restriction might be good candidates for the use of drugs such as levothyroxine. This medication is used until the weight loss is achieved, then discontinued.
Horses that are overweight with EMS rarely need the extra calories contained in concentrate feed; instead, a small amount of a high-protein ration balancer product can be used to supplement hay. Lean horses with EMS, particularly those that are in regular work, might require supplemental calories in addition to their daily forage ration to maintain (or safely increase) their body weight.
In those cases, supplemental concentrate feeds that provide calories from fat and fiber, rather than non-structural carbohydrates, are safer choices. Beet pulp, vegetable oils, rice bran and coconut meal might be good options for supplemental calories, depending on the circumstances.
Horses that are not actively laminitic benefit from regular aerobic exercise. A horse owner’s time constraints can make it difficult to increase the frequency or duration of exercise. Encouraging a client to consider a part-lease or an arrangement with an appropriately skilled rider can provide benefit to the horse by increasing its activity.
Medications can be used in some cases to enhance a horse’s insulin sensitivity. Thyroid hormones increase insulin sensitivity as well as enhancing basal metabolic rate, which allows the horse to burn more calories and enhances weight loss if dietary intake is appropriately restricted at the same time.
“If needed, medications such as levothyroxine or metformin can be used,” Burns said. “These drugs are particularly useful in horses that are laminitic and therefore can’t humanely be subjected to an exercise program.”
An actively laminitic horse, or one that has historical evidence of laminitis such as morphological changes in the hoof capsule and radiographic changes, will likely benefit from therapeutic podiatry and analgesia. Regular trimming and shoeing to encourage more appropriate alignment between the bones of the digit and the hoof capsule and ease breakover are very important to the rehabilitation of a horse with EMS-associated laminitis. With appropriate foot care, affected horses can recover the ability to perform athletically.
Pain management, including medications such as non-steroidal anti-inflammatory drugs, acetaminophen, gabapentin and/or opioids, are also important for the comfort of affected horses. Regular supervision of the program by a veterinarian is important to maximize efficacy and minimize the risk of adverse effects.
Once a veterinarian has determined that a horse has EMS, there is no magical cure. Management is an ongoing, long-term process that requires buy-in from the horse owner.
However, a treatment plan that is comprehensive, regularly monitored and optimized as needed in response to clinical findings can enhance the quality of life of horses with EMS and maximize the likelihood that they can return to their intended functions after they are properly diagnosed and treated.
“Helping horses heal after an EMS diagnosis can be achieved through the team effort of horse owners and veterinarians,” Burns concluded.