By Jill Griffiths, Dr. Allison Stewart, and Dr. François-Rene Bertin

Very ill horses—those with severe colic, for example—often have very high blood glucose levels and very low insulin levels. Researchers know that, in such situations, death is a common result. What they’re still trying to figure out is the cause and effect of this complex interaction.

Researchers from The University of Queensland, in Australia, and Auburn University, in Alabama, recently took a closer look at the links between insulin and glucose in acutely ill horses.

They studied 58 horses aged 5 and older admitted to Auburn’s J.T. Vaughan Large Animal Teaching Hospital as emergency cases and diagnosed with systemic inflammatory response syndrome (SIRS). A SIRS diagnosis required horses to have two or more of the following: high heart or respiratory rates, high or low body temperature, or abnormal white blood cell counts. The team evaluated horses’ hormone and glucose concentrations.

University of Queensland equine internal medicine and emergency and critical care specialist Allison Stewart, BVSc (Hons), MS, Dipl. ACVIM, ACVECC, said when some horses become severely ill, the pancreas stops working, so insulin levels fall.

“No one has published insulin concentrations in sick adult horses or any results of improved survival from administering an insulin infusion to regulate glucose concentrations in critically ill horses,” she said. “In humans, this therapy has been shown to reduce patent mortality.

“When very sick people suffer from insulin dysregulation, they are administered intravenous insulin and their glucose concentrations are monitored very closely to keep the concentrations within normal levels. This has been shown to increase survival rates.”

Insulin is produced in the pancreas and is the hormone that regulates glucose metabolism. In healthy people and horses, an increase in blood glucose levels leads to an increase in insulin levels. In sick people and horses insulin dysregulation sometimes occurs, which means the insulin concentration does not rise in response to the increase in glucose, so blood glucose continues to rise. The lack of insulin is likely related to the pancreas not working adequately.

But high insulin levels are also a problem.

“In horses, high concentrations of insulin cause laminitis both experimentally and in horses with equine metabolic syndrome,” Stewart said. “This is what occurs when a horse eats sugar-rich feed such as oats or rich grass or hay—their blood glucose goes up, then their insulin goes up in response to the glucose, and if they have insulin dysregulation and failure of the cells to respond to insulin, the insulin increases further and predisposes the horse to laminitis.”

In terms of treating extremely sick horses with SIRS, this laminitis link makes giving insulin infusions risky. This is one of the things the researchers wanted to unravel in their research.

“We found that when really sick horses have high glucose levels, high insulin levels were protective; horses with high glucose and high insulin were actually more likely to survive,” Stewart said. “When the sick horses had high glucose but low insulin, they were more likely to die. These horses were so sick that their pancreas wasn’t working properly.

“In horses where the pancreas was still working—that is, their insulin levels were still high—they had better prospects of survival,” she said. “This means there is an argument for using medicine-grade insulin to treat sick horses with high glucose levels. In these cases, careful infusion with medical grade insulin may be worthwhile.”

But Stewart cautioned that, while these early results appear to show a potentially worthwhile intervention, it would take specialist intensive care to implement, and it’s not clear-cut.

“The sample size we have worked off is relatively small,” she said. “In humans, this work has been tried and tested using thousands and thousands of patients. We don’t have the funding to do that in horses.

“The best option to lead to good outcomes for very sick horses is still early referral to specialist care,” she said.

The study, “Insulin dysregulation in horses with systemic inflammatory response syndrome,” was published in the Journal of Veterinary Internal Medicine.

Allison Stewart and François-Rene Bertin are based at the School of Veterinary Science, The University of Queensland, Gatton, Queensland, Australia. Jill Griffiths is a freelance writer based in Western Australia.