Insulin Resistance and Layup Time

Q.  My 18-year-old Percheron/ Morgan gelding was diagnosed with insulin resistance. Just prior to being confined to stall rest for a connective tissue injury, his insulin was 67 microIU/mL. The vet did a dexamethasone suppression test to rule out pituitary pars intermedia dysfunction (PPID), and it was well within normal bounds. After a six-month layup and a very controlled diet low in nonstructural carbohydrates (NSC, hay soaked to reduce sugar, chopped forage, and concentrated feed with 11% NSC), his insulin was 110. The vet prescribed pergolide at 1 mL/day, fearing that PPID had developed during the layup. The horse lost his appetite after a week or two, as I understand horses may do when first put on pergolide, but he also began to exhibit signs of discomfort that made me think he'd developed ulcers. After two months of this behavior, I stopped giving the pergolide, and he began to eat and act normally in just a few days. I have several questions: Could the high insulin be a result of confinement rather than PPID, despite the controlled diet? (He does not have any PPID symptoms beyond high insulin and the resulting cresty neck.) Is it safe to leave him off pergolide until he can be retested for PPID this spring? What are the effects of pergolide on a horse who doesn't have PPID?

To compound things, he reinjured his leg after getting away from his handler, and we're looking at another six-month layup. What else can I do to keep his insulin down without putting stress on the healing leg?

Agata, via e-mail

A. It would be difficult to say if the confinement was causing the high insulin; rather, it could be the stress of the injury and the confinement making laboratory testing for metabolic syndrome (insulin resistance) skewed and erratic. It does not sound like leaving the horse off the pergolide will have any consequence, and a more complete evaluation should be performed when the horse is sound and not stressed. Depending on the injury, the greater concern might be the development of laminitis in the good leg since the horse is already predisposed to laminitis because of insulin resistance.

Check with your veterinarian about thyroxine at 44 mg/day to control weight issues and cyproheptadine as an alternate control for PPID. Be cautious of products that purport to decrease imbalances due to metabolic syndrome, as most are not proven. If weight is controlled and the horse is reasonably sound, the risk of laminitis might be lowered.

About the Author

Doug Byars, DVM, Dipl. ACVIM

Doug Byars, DVM, Dipl. ACVIM, is Director of the medicine clinic at Hagyard-Davidson-McGee equine practice in Lexington, Ky.

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