Equine Metabolic Syndrome (EMS) is a hormonal condition in horses defined by three linked problems: abnormal fat deposits, insulin dysregulation, and a heightened risk of laminitis. It’s often compared to metabolic syndrome in humans, where obesity and insulin resistance feed off each other and lead to serious complications. In horses, the most devastating of those complications is laminitis, a painful breakdown of the tissues inside the hoof that can be severe enough to require euthanasia.
The Three Core Features of EMS
EMS isn’t a single disease so much as a cluster of related abnormalities. Understanding how they connect helps explain why the condition is so dangerous.
Abnormal fat deposits: Horses with EMS tend to accumulate fat in specific areas rather than gaining weight evenly. The most recognizable sign is a thick, hard crest along the top of the neck. Fat also builds up behind the shoulders, over the tailhead, and around the sheath or udder. These fat deposits aren’t just cosmetic. They actively release hormones that interfere with the body’s ability to respond to insulin.
Insulin dysregulation: Insulin is the hormone that signals cells to absorb sugar from the bloodstream after a meal. In horses with EMS, the fat-derived hormones make tissues less responsive to insulin. The pancreas compensates by pumping out even more insulin, a state called insulin resistance. The result is chronically high insulin levels, sometimes spiking dramatically after meals and sometimes staying elevated all day long.
Laminitis: High insulin is directly toxic to the hoof. The laminae are thin, interlocking tissues that anchor the hoof wall to the bone inside it. Sustained high insulin triggers inappropriate cell growth in these tissues, weakens their structure, and causes local inflammation. The bone can rotate or sink within the hoof capsule, leading to chronic lameness. Laminitis is the reason EMS demands attention; it transforms a metabolic issue into a potentially life-ending one.
How High Insulin Damages the Hoof
The connection between insulin and hoof damage isn’t fully understood, but several mechanisms are at work. High insulin overstimulates growth factor receptors in the laminae, causing the cells there to multiply abnormally. This weakens the tissue that holds the hoof together. At the same time, prolonged high insulin triggers an inflammatory cascade inside the hoof. Research has documented elevated levels of inflammatory proteins, including interleukin-1β, along with increased cell death and abnormal hardening of the outer hoof cells. Blood flow within the hoof is also disrupted. Together, these changes can quietly erode lamellar integrity before a horse ever shows obvious lameness, which is why catching EMS early matters so much.
Breeds at Higher Risk
EMS has a strong genetic component. Breeds that evolved to thrive on sparse forage are the most susceptible because their metabolisms are extremely efficient at storing energy. Ponies of all types, Morgans, Arabians, Paso Finos, Spanish Mustangs, and warmblood crosses top the risk list. Among ponies, native British breeds like Welsh, Shetland, and Dartmoor ponies are particularly prone. These are the classic “easy keepers” that gain weight on what seems like very little feed. That metabolic thriftiness, an advantage in harsh environments, becomes a liability when horses have steady access to rich pasture or grain-heavy diets.
EMS is generally diagnosed in younger to middle-aged horses, typically between 5 and 15 years old. This distinguishes it from Cushing’s disease, which tends to appear later in life.
How EMS Differs From Cushing’s Disease
EMS and Cushing’s disease (formally called Pituitary Pars Intermedia Dysfunction, or PPID) are the two most common hormonal disorders in horses, and they can look similar on the surface. Both involve insulin problems and increase laminitis risk. But their causes and typical presentations are distinct.
Cushing’s disease is a pituitary gland problem. A tumor or overgrowth in the brain’s pituitary gland causes it to produce excess hormones, particularly ACTH. The hallmark sign is a long, curly coat that fails to shed normally in spring. Horses with Cushing’s also tend to sweat excessively, drink and urinate more than normal, and lose muscle mass over the topline and hindquarters. These signs are not part of EMS.
In EMS, the pituitary gland functions normally. The problem originates in the fat tissue itself, which overproduces hormones that drive insulin resistance. EMS horses don’t develop the shaggy coat or muscle wasting seen in Cushing’s, but they do carry the characteristic regional fat deposits. A horse can have both conditions simultaneously, especially as it ages, which complicates diagnosis. Veterinarians use blood tests measuring ACTH levels and insulin responses to distinguish between the two.
Recognizing EMS: The Cresty Neck Score
One of the simplest screening tools for EMS is the cresty neck scoring system, a 0 to 5 scale that assesses how much fat has accumulated along the top of the neck. A score of 0 means there’s no visible or palpable crest at all. At a score of 2, a noticeable crest is present but can still be cupped in one hand and bent side to side. By score 3, the crest is enlarged and mounded, filling a cupped hand and losing flexibility. At score 4, the crest is so thick it can no longer be cupped or bent. Score 5 means the crest has grown so heavy it permanently flops to one side.
A cresty neck score of 3 or higher is a red flag. Combined with other regional fat pads and a history of unexplained laminitis or foot soreness, it should prompt testing. Veterinarians typically measure resting insulin levels and may also test how insulin responds after a meal or an oral sugar challenge to confirm the diagnosis.
Diet: The Foundation of Management
Dietary changes are the single most important part of managing EMS. The goal is to reduce the sugars and starches (collectively called non-structural carbohydrates, or NSC) that trigger insulin spikes after meals. Hay should be tested and ideally contain less than 10 to 12% NSC. If testing isn’t available, soaking hay in water for 30 to 60 minutes before feeding can leach out a portion of the soluble sugars.
Pasture access is the biggest dietary risk for EMS horses. Grass, especially during spring growth and on sunny afternoons, can contain very high sugar levels. Many EMS horses need restricted or eliminated pasture access, at least during high-risk seasons. Grazing muzzles can reduce intake but don’t eliminate the risk entirely.
Grain and sweet feeds should be removed from the diet. A ration balancer or low-NSC vitamin and mineral supplement ensures nutritional needs are met without the sugar load. The overall calorie intake often needs to come down as well, since weight loss directly improves insulin sensitivity. For overweight horses, feeding 1.5% of ideal body weight in hay per day (not current body weight) is a common starting point.
Exercise and Insulin Sensitivity
Exercise is a powerful tool for improving how a horse’s body handles insulin. Research comparing horses on turnout alone versus structured light or moderate exercise found that both light and moderate activity improved insulin sensitivity significantly compared to turnout. Simply being in a paddock wasn’t enough, particularly when the diet was calorie-dense. This means that for EMS horses able to exercise safely (those not in active laminitis), regular ridden or lunged work makes a real metabolic difference.
Even 20 to 30 minutes of trotting and cantering several times a week can help. For horses that can’t be ridden, hand-walking, ponying from another horse, or using a horse walker are reasonable alternatives. The key is consistency. Exercise improves insulin sensitivity only as long as it continues; the benefits fade quickly if the horse returns to a sedentary routine.
Medications and Their Limitations
When diet and exercise alone aren’t enough, veterinarians may add medication. Metformin, the same drug used in human type 2 diabetes, has been widely prescribed for EMS horses, but its effectiveness is debated. Horses absorb metformin poorly. Studies measuring blood levels after oral dosing found a bioavailability of only about 7% in unfed horses and roughly 4% in fed horses. That means very little of the drug actually reaches the bloodstream. Some veterinarians still use it, hypothesizing that it may act locally in the gut to blunt sugar absorption even if blood levels stay low, but the evidence base for this is thin.
Newer medications in the SGLT2 inhibitor class, which work by causing the kidneys to excrete excess sugar into the urine, have shown more promise. These drugs lower blood insulin levels and are gaining traction in equine practice, though availability varies by region. Medication is always a supplement to diet and exercise, never a replacement.
Long-Term Outlook
EMS is a lifelong condition. It can’t be cured, but it can be managed well enough that many horses live comfortably for years. The horses that do best are those whose owners catch the condition before a serious laminitis episode, commit to a low-sugar diet, and maintain a consistent exercise program. Weight loss alone can dramatically improve insulin numbers and reduce laminitis risk.
The horses that struggle are those with repeated laminitis episodes, which cause cumulative and sometimes irreversible hoof damage. Each episode weakens the laminae further, making the next episode more likely. This is why prevention through metabolic control, rather than treating laminitis after it occurs, is the entire strategy with EMS. Regular monitoring of body condition, cresty neck score, and periodic insulin testing gives the best chance of staying ahead of the disease.