Transport Stress and Insulin Levels in Horses

Virtually all horses experience some level of stress when they are transported, even if they have been trailered many times and appear to stay totally calm throughout the experience. Motion, traffic noise, separation from their friends, and the muscular effort of maintaining balance all combine to increase production of cortisol, a hormone released in stressful circumstances. Horses that have been transported to an equine hospital are therefore likely to have somewhat higher cortisol levels than they would show if this hormone were measured in a familiar, quiet setting.

Cortisol may influence levels of glucose and insulin, parameters that are evaluated when a veterinarian makes a diagnosis of insulin resistance or equine metabolic syndrome. If insulin and glucose levels are different from normal, could this be an effect of transport and not an underlying health condition? Could this situation complicate a diagnosis? Researchers at North Carolina State University designed a study* to answer these questions.

To investigate how insulin concentrations differed before and after transport, blood samples were collected from 41 horses, some of which were healthy and others that had been diagnosed with equine metabolic syndrome (EMS). Samples were taken before transport when the horses had no grain and only limited access to hay or pasture. Horses were given hay during transport, and a second sample was taken from each horse after transport. Healthy horses were transported for brief (1 to 3 hours) or long (8 hours) periods of time. Horses with EMS were transported for 1 to 3 hours.

Transport raised insulin levels in both groups of horses. It is possible that the rise in insulin was related to consumption of hay, and therefore a rise in blood glucose, during transport. In healthy horses, there was a significant effect of transport on insulin concentrations, but the length of transport time had no significant effect. EMS horses had significantly higher insulin concentrations than healthy horses before transport, but insulin concentrations in some healthy horses were as high following transport as the pretransport levels in EMS horses. This could lead to a healthy horse being diagnosed as insulin-resistant if its blood was tested immediately after arrival at a veterinary clinic.

The researchers warned that diagnosis of insulin resistance based on blood sampling could be most reliable when the sample is taken from a rested, unstressed horse that has not been transported and has consumed only low-carbohydrate forage prior to the sample being drawn.

*Pratt-Phillips, S., J. Gill, R. Mansmann, and K. Tillotson. 2014. Effect of transport on serum insulin concentrations in normal and equine metabolic syndrome-affected horses. In: Macro and trace elements in equine nutrition: An update. Proc. 7th European Workshop on Equine Nutrition, Leipzig, Germany, p. 112-113.