In recent decades, sedentary lifestyles and inordinate calorie intake have caused many Americans’ waistlines to expand. Subsequently, obesity and Type 2 diabetes have become two of the most pressing health threats to humans today. “From a public health perspective, these are public enemy No. 1,” says Philip Schauer, MD, Director of Cleveland Clinic’s Bariatric and Metabolic Institute, in Ohio.
And just as we overeat, we lovingly overfeed our pets. Small animal veterinarians report that obesity in dogs and cats is one of the most common medical issues that these species face. Horses can be overfed as well, ending up obese and afflicted with associated health problems.
Early horses were free-roaming grazing animals with digestive systems designed for constant consumption of native grasses. They also got sufficient exercise walking great distances to find this forage. Today’s horses are still equipped to graze for 10-12 hours per day with no complications, but horse owners that do not have adequate pasture acreage to accommodate this type of lifestyle (and nowadays that seems to include nearly all of us) keep their horses in stalls with daily turnout into small paddocks.
Furthermore, we force many stalled horses to adapt to our busy schedules and feed them grain meals twice daily with hay. If, under these circumstances, we don’t manage horses’ diets and exercise properly, they can become susceptible to obesity. It’s probably no surprise that the most common causes of equine obesity are similar to those of human obesity: overfeeding and under-exercising.
Similar to how we commit to exercising and watching what we eat to lower numbers on the bathroom scale, we can resolve many equine weight problems simply by increasing horses’ exercise and reducing their caloric intake. Most horses respond to this prescription; however, some are resistant to weight loss and require greater reductions in energy intake.
Equine metabolic syndrome, or EMS, has become the universal veterinary medical term for the collection of metabolic abnormalities seen in such horses and ponies. Similarities between the human metabolic syndrome of Type 2 diabetes mellitus and EMS gave rise to the latter condition’s name. A huge concern, with regard to an EMS diagnosis, is the ever-present risk of developing the hoof disease laminitis—it might be the precursor to laminitis, or it might actually promote its start.
“Given the potential for loss of function and/or life that can result following the onset of laminitis, it is difficult to overstate the central role that lamellar failure plays in the need for effective management of EMS,” says Melody de Laat, PhD, a research fellow in the Earth, Environmental and Biological Sciences department at the Queensland University of Technology, in Brisbane, Australia. “As a result, the need for focused and translatable (to horse owners) research on equine obesity and endocrine dysfunction has never been greater.”
The collection of metabolic conditions that comprise EMS share underlying mechanisms and pathways. Researchers and veterinarians have proposed a typical EMS phenotype, or cluster of physical traits, which ties together:
- General obesity;
- Insulin dysregulation; and
- Predisposition to laminitis.
Additional conditions EMS horses can develop include elevated triglyceride (fat) levels and/or leptin (a fat-derived hormone) concentrations in the blood and arterial hypertension (high blood pressure).
Even though obesity is a physically obvious risk factor for horses developing EMS, many owners overlook it. In fact, some horse owners deem a degree of obesity as normal, acceptable, and even desirable. A widely used model by which horses are scored, in terms of whole-body adiposity, is the 1-9 Henneke scoring system, a scientific method of evaluating a horse’s body condition regardless of breed, body type, sex, or age. A score of 1 is considered poor or emaciated with no body fat, while a score of 9 is extremely fat or obese. Veterinarians consider body scores from 4 to 7 acceptable. General obesity can be described as an overall accumulation of fat, with a score of 8 or 9.
EMS horses suffer more specifically from regional adiposity, an accumulation of fat in certain areas. These include:
- Around the nuchal ligament in the neck (appearing as a cresty neck);
- Fat pads close to the tailhead;
- Just above the eye;
- Behind the shoulder; and/or
- In the prepuce (sheath) or mammary gland (teat) region.
The mechanisms underlying generalized obesity or regional adiposity in EMS are unknown, but chronic overfeeding in association with limited physical activity appears to be a major contributing factor.
This mouthful of a term refers collectively to two components of EMS: insulin resistance and hyperinsulinemia. The pancreas secretes the hormone insulin when blood glucose levels rise after a meal. In a normal body, insulin’s actions decrease blood glucose concentration to maintain a normal blood glucose level. Insulin resistance occurs when body cells lose their sensitivity to insulin; consequently, the body cannot properly use insulin to reduce the blood glucose levels. At first, the pancreas produces extra insulin. But over time the pancreas is unable to keep up and cannot make enough insulin to keep blood glucose levels normal.
Hyperinsulinemia (elevated insulin in the blood) is caused by an increase in insulin secretion or a delayed insulin clearance. Inadequate insulin clearance can be a direct consequence of insulin resistance, a condition in which the body produces insulin but does not use it effectively. Consequently, there is an abundant amount of insulin with a delayed clearance from the bloodstream. A horse doesn’t have to be obese to develop insulin dysregulation. Lean horses and ponies might suffer from insulin resistance and hyperinsulemia, while some obese equids’ insulin regulation is completely normal.
“These inconsistencies can be explained by considering obesity as a modifying factor, with the genetics of the individual animal determining the magnitude of insulin dysregulation,” says Nicholas Frank, DVM, PhD, Dipl. ACVIM, a professor of large animal internal medicine and chair of the department of clinical sciences at the Cummings School of Veterinary Medicine at Tufts University, in North Grafton, Massachusetts. In other words, obesity can lead to and aggravate the problem, but ultimately genetics play the determining role in the severity of insulin dysregulation.
Predisposition to Laminitis
The coffin or pedal bone is at the bottom end of the horse’s digit, protected by the hoof capsule. Interlocking layers of connecting tissues called laminae suspend the coffin bone from the hoof wall, and these tissues require a continuous blood and nutrient supply. Any condition that alters this supply, causing an inflammation of the laminae, can weaken this vital attachment and initiate laminitis. Causes are numerous, including grain overload, colic, colitis (inflammation of the colon), stress, trauma, and retained placenta. However, clinical (apparent) or subclinical laminitis can also develop with no conspicuous cause or injury.
Such is the case in EMS horses—research results suggest the condition can predispose horses and ponies to laminitis and might even induce it. But it’s important to remember that while obesity, insulin resistance, hyperinsulinemia, and laminitis are closely associated in horses, some obese animals, as with insulin dysregulation, can be unaffected while some nonobese horses are susceptible to EMS and laminitis.
“Increased adiposity is evident before laminitis in most animals, but a lean EMS phenotype also exists, with insulin dysregulation and laminitis occurring in horses and ponies with normal appearance,” explains Frank.
While scientists have not yet pinpointed the exact mechanisms involved with EMS-related laminitis, they have shown hyperinsulinemia to induce laminitis in experimental models.
State of the Syndrome
Today veterinarians are identifying EMS that has developed in many horses and ponies due to the animals’ limited physical activity and high calorie rations. Warmblood and pony breeds appear to be more susceptible to EMS, although the syndrome occurs in light horse breeds as well. As seen with past and present research, a range of metabolic conditions can be associated with EMS, from general obesity to insulin dysregulation. The most common and crippling EMS complication is laminitis. Therefore, EMS remains an important focus for equine veterinary researchers and practitioners alike.
“Long-term goals need to be achievable, and we, as horse owners and veterinarians, need to address the ‘human element’ that contributes to the disease,” says de Laat.
To that effect, “Most horses and ponies with EMS can be effectively managed by controlling the horse’s diet, instituting an exercise program, and limiting or eliminating access to pasture,” Frank says.
Proper nutrition involves reducing the amount of energy you provide in your horse’s diet to promote weight loss if necessary. Always consult with your veterinarian if you suspect your horse has EMS, because some affected horses require pharmaceutical products in addition to diet and exercise to manage the syndrome effectively.
As always, the best practice is preventive medicine. We can prevent EMS development in many horses with good management practices, with body condition scoring as part of your horse’s wellness evaluation, and by intervening before metabolic problems arise.