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Evolutionarily, there are advantages to being a horse that “gets fat on air.” In fact, for centuries, in the heyday of the work horse, horses were selected for just this trait: the ability to maintain body condition and do heavy labor even when food was scant. They are thought of as easy keepers.
Unfortunately, evolution is a slow process. While for the most part our horses no longer haul large wagons, pull plows, or function as the major source of transportation, their feed ‒ though subject to some whims of climate ‒ is largely abundant and affordable. The result is calorie-efficient animals housed in boxes or on irrigated and maintained pastures, fed nutrient-rich diets, and exercised for maybe a couple of hours per day.
At the extreme end of energy conservation is the horse affected with equine metabolic syndrome (EMS). These horses appear to have some metabolic glitches including a propensity toward obesity and associated insulin resistance. Researchers investigating the possibility of a genetic component to EMS suspect that while genetics may play a role in EMS, most likely the expression is complex and has more than one gene involved. In other words, there probably isn’t an EMS on/off switch.
Why do we care about EMS and obesity in horses, anyway? Isn’t a round croup (rump) a sign of a well-fed animal? While physicians and small animal veterinarians have done an exemplary job over the years counseling people and clients about overweight pets, a feedlot mentality still seems to persist in the horse industry. Despite broad marketing of new "low non-structural carbohydrate foods," horse owners still seem perpetually concerned that they are “underfeeding” their horses. Yet the consequences of obesity in horses are as severe as in any species, and in the case of laminitis, rapidly disastrous.
Horses who are overweight for any reason run a high risk of health problems. Overfed horses are being managed far outside their true needs. The horse is a grazer, designed to walk throughout the day and forage on a diet of mixed grasses. Feeding large quantities of alfalfa, hay, and concentrated feeds to a horse is like an all-you-can eat buffet for people; marathoners might be able to burn off buckets of pasta and fried chicken, but for most people it's too much food. For horses with EMS, a poorly regulated diet is closer to feeding a Type II diabetic that same pasta, fried chicken, and soft-serve ice cream feast.
While EMS horses are not strictly diabetic as they don’t dump sugar into their urine, these horses do typically have difficulty properly processing sugars and their blood work is generally characterized by insulin resistance. This means that while the body does just fine at producing insulin, the receptors that would
normally help metabolize these sugars are not doing so well enough.
In horses, this glitch in sugar processing often leads to another physiologic upset: the condition known as laminitis. Laminitis is an inflammatory condition of the lamellae, sensitive tissues that connect between the hoof wall and the coffin bone. When these lamellae become inflamed, several things can happen:
1. Pain Ever had an infected toenail? Ok, now say you weighed over 1000 pounds and your entire foot was that toe. Now, now make it both feet and drop something heavy on the feet. That's what laminitis feels like.
- Rotation of the coffin bone (that triangle-shaped bone inside the horse’s hoof) The lamellae balance the forces of the flexor tendons to keep the coffin bone in place within the foot. When lamellar integrity is compromised, the coffin bone can rotate or sink within the foot, sometimes even penetrating the sole.
- Vascular compromise Circulation in the equine foot is quite intricate; compromise it even a little and tissue death occurs pretty quickly.
2. Physiologic stress. Chronic pain causes cortisol release, which makes the animal more susceptible to infection, among other things. Also, horses with severe laminitis spend a lot of time lying down. However, even humans can get bedsores and muscle damage from lying in place for too long, and most of us don’t
weigh half a ton or more.
If your horse has EMS, is obese, has laminitis, cut back on the grain and consider switching from alfalfa to grass hay. Give your horse more exercise. But what if the horse isn't just pleasantly plump and still has EMS?
First, only your veterinarian can help you determine the cause of your horse’s weight or health issues. Talk to her about your horse’s body condition. Ask her to teach you how to measure your horse’s body condition score so that you can better monitor his weight. Also ask for tips on nutrition and feeding. If you have an overweight horse with a history of laminitis, ask about followup care. Chances are your veterinarian may recommend restricting pasture access and feeding a low non-structural carbohydrate diet.
Remember that prevention is most of the battle when dealing with EMS and EMS-associated laminitis. Once a horse develops laminitis, it is too late; damage is permanent. Your veterinarian can help you work with your horse to lose weight.