Heavy Horses (and Ponies)
In one study researchers found that 40% of owners considerably underestimated their horses’ body condition. “There is this sort of body dysmorphism thing,” said van Eps. “We need to change our definition in our minds of what constitutes normal body condition in horses.”
That means not allowing horses and ponies to reach body condition scores as high as industry culture and some competition arenas have deemed acceptable—even fashionable.
The insulin dysregulation (ID)—basically, the body’s inability to respond normally to insulin release—often lurking in these obese horses commonly leads to laminitis. Van Eps said the best way to manage ID in horses and ponies is to control diet and reduce obesity. “Exercise is difficult in some of these patients,” he said, whether due to extreme lack of fitness or because the horse already has laminitis, making it painful to work. “Diet makes much more of a difference; it’s much more important.”
He recommends weighing the horse to get an accurate measurement, then feeding hay at a rate of 1.5% of the horse’s body weight per day as a reliable and simple weight loss strategy.
“(Hay) should be either tested (for nonstructural carbohydrate [NSC] levels) or soaked,” he said, because these sugars can exacerbate the condition. Also consider testing NSC after soaking (he suggested Cumberland Valley Analytical Services for testing). Supplement the hay with a ration balancer to ensure vitamin and mineral needs are met.
Keep in mind not all horses with ID are obese (or even overweight) and, also, that horses with ID might have laminitis without obvious lameness. Van Eps said that in one study researchers found radiographic evidence of laminitis in 76% of horses presented for pituitary pars intermedia dysfunction (PPID, aka equine Cushing’s disease), but half of them had not shown signs of foot pain, and laminitis wasn’t recognized by the owners. Pituitary pars intermedia dysfunction leads to an overproduction of hormones such as adrenocorticotropic hormone (ACTH) and cortisol, and affected horses can have regional fat deposits and experience muscle wasting, among other signs, but they aren’t generally obese.
“Our goal for now (to prevent laminitis in endocrinopathic horses) is to target insulin dysregulation,” he said. “Diagnose them early, because this type of laminitis is often well-advanced before we see any outward clinical signs.”
Van Eps prefers to test plasma ACTH concentration for diagnosing PPID: either resting (baseline) or after thyrotropin-releasing hormone (TRH) stimulation (a little more sensitive). With this test veterinarians look for overproduction of ACTH, which the pituitary gland—which is enlarged or affected by a tumor in PPID horses—normally makes in relatively small amounts to control the function of the adrenal glands. Veterinarians must keep in mind seasonal reference ranges when they assess results.
He says that after starting the confirmed-PPID horse on Prascend (the FDA-approved pergolide drug), “you need to test them a month later and then at least once a year to see if their ACTH is under control, because you can’t control their laminitis if there’s an underlying uncontrolled endocrinopathy.”
Van Eps recommends using the oral sugar test to check for equine metabolic syndrome, another form of insulin dysregulation. This involves collecting a blood sample, followed by administering light corn syrup orally, and collecting another blood sample 60 to 90 minutes later. In a new study van Eps’ group showed that the oral sugar test can be performed immediately after the TRH stimulation test as a combined protocol without affecting the results, and this is best way to test for ID and PPID in one visit.
He welcomes equine veterinarians to contact him with any questions about interpreting results.
Indeed, metabolically, there’s a “type” of horse or pony that is prone to developing laminitis, but they’re not always visibly obvious. Researchers are working hard, though, to identify predictors in the genes, said van Eps.
“There’s a good chance in the future that we’re going to have some genetic markers and some genetic tests,” he said, “and potentially breed this out (of the horse population).”
In terms of medical treatments for ID, van Eps said there are now options that might suit different horses and ponies, and owners should always consult their veterinarian. “Levothyroxine may be a useful adjunct to weight loss and can help to improve ID,” he told The Horse. “Supplements containing polyphenols such as resveratrol may help to improve ID; however, there is very limited evidence for this in horses at this stage.
“Metformin can be useful in horses and ponies to reduce the insulin response to feeding if given in a targeted way shortly before meals/turnout,” he added. “There are other options on the horizon for targeting ID, including a recent paper with some exciting preliminary evidence on a drug class that helps to control insulin by enhancing the clearance of glucose from the blood by the kidneys. We are likely to see more medication options become available for horses and ponies with ID in the coming years.”.