If you suspect your horse might have a metabolic disease, organizing timely and accurate diagnostic testing is key for developing a management plan and identifying future risk of hyperinsulinemia-associated laminitis (HAL). Insulin dysregulation (ID) is a condition in which horses have abnormal insulin responses to sugars and starches in forage and concentrate diets. These horses usually demonstrate clinical signs such as regional fat deposits and generalized obesity, and possible bouts of laminitis. Some horses with ID also have a “lean” body type and co–existing PPID. It is most important to recognize that horses with ID are often at greater risk of developing HAL, a painful condition partially driven by ID and characterized by inflammation of the laminae, which contributes to the failure of this tissue to suspend the coffin bone in the hoof capsule.

“Testing for ID can at times be confusing and challenging,” said Amanda Adams, PhD, associate professor at the University of Kentucky’s Gluck Equine Research Center, in Lexington, during her presentation at the 2023 American College of Veterinary Internal Medicine Forum, held June 15-17, in Philadelphia, Pennsylvania. “Veterinarians should remember that the available tests are not a ‘one-size-fits-all’ model and should look at the complete picture, including clinical signs of disease and the individual horse’s responses to testing when evaluating results.”

Knowing When to Test

Veterinarians might choose to test a horse for ID based on:

• Signalment.
• Breed (pony breeds, Spanish breeds, gaited breeds, and Warmbloods are generally at higher risk of metabolic problems).
• Clinical signs such as regional adiposity and generalized obesity.
• Divergent hoof rings (indicate a history of laminitic episodes).
• Prior diagnosis of pituitary pars intermedia dysfunction (PPID, previously called Cushing’s disease).
• The potential use of intra-articular or systemic steroids.

The two most common ways to test for ID are measuring the horse’s resting blood insulin levels or testing these levels after an oral sugar test (OST). In addition to these diagnostic tests, more recently, veterinarians have started measuring horses’ insulin responses to their individual diets, whether it be pasture, hay, concentrate, or a mixture of all three. When testing horses for resting insulin levels, it is no longer recommended to use complete overnight fasting, but it is still necessary to withhold grain or concentrates to test the horse’s resting insulin levels from hay or pasture alone.

“The OST is a better and more sensitive way to test for ID, as it reflects that horse’s postprandial insulin response after a meal consumption,” said Adams. While practitioners still recommend fasting horses for three to six hours prior to an OST, recent research suggests fasting might not be necessary if your horse is maintained on low-nonstructural-carbohydrate forage, she added. If you are unsure if they can be maintained on a low-NSC forage, use a short fast three hours prior to the OST.

“If when using a low dose of corn syrup at 0.15 mL/kg body weight, the insulin results show the horse is suspect ID, it could be useful to use the higher corn syrup dose of 0.45 mL/kg body weight dose to help confirm the diagnosis (for more accurate results),” said Adams.

Horse owners should work with their veterinarians to monitor their animals’ metabolic status. “A horse’s metabolic type can change over time,” she added. Therefore, it is important to stay consistent with the testing method when performing future testing to increase the likelihood of an accurate comparison.