An obese horse is often—though not always—an insulin-resistant one, and detection methods for insulin resistance can be tricky to time, not to mention labor-intensive. François R. Bertin, DVM, a resident at the Purdue University College of Veterinary Medicine Teaching Hospital described a new testing technique that he has found useful for identifying insulin-resistant horses at the 2011 American Association of Equine Practitioners convention, held Nov. 18-22 in San Antonio, Texas.
The pancreas' beta-cells are responsible for secreting insulin to stimulate glucose uptake by glucose receptors. The glucose is moved into muscle cells where it is either used immediately or stored as glycogen for later access. An insulin-resistant (IR) horse has a decreased sensitivity to insulin (as a result, higher amounts of insulin are released than normal in response to ingestion of starch and/or sugar) along with a decreased maximal response to this hormone. High levels of circulating insulin in the bloodstream could put such a horse at risk for development of laminitis.
The recommended insulin-response test requires a veterinarian to take several blood samples over four hours. Bertin described an alternative testing procedure able to produce information on suspect IR cases, especially those horses with equine metabolic syndrome (EMS). While a cresty neck and fat pads at the base of the tail can be signs of EMS and indicative of a probably IR case, he emphasized that a definitive test is necessary for an accurate IR diagnosis.
In a research trial Bertin tested 12 horses around 16 years of age with body condition scores ranging from 3-9 (5 being optimal). The horses were fed only free choice hay and water. After measuring the baseline blood glucose of each horse, insulin was administered and another blood sample was taken 30 minutes later. Glucose readings were obtained stall-side with a hand-held glucometer, much like one used by human diabetics. If signs of hypoglycemia (sweating, shaking, or lying down) were noted following insulin treatment, Bertin recommended that the horse be given a dose of intravenous dextrose.
The conventional IR test and the two-step test were performed on all 12 horses. Bertin explained that the results of the conventional test were used to confirm whether or not the horse was considered insulin-resistant. He then compared the results of two-step test with those of the conventional test to determine the former's accuracy.
He and his colleagues saw no significant differences between the insulin-resistant and insulin-sensitive horses at the baseline glucose measurement, or in body condition score. Most of the blood glucose decrease occurred within 30 minutes of insulin treatment. Bertin reported that a decrease in glucose levels by 50% or more before 30 minutes indicated that a horse was insulin-sensitive. With this test, all insulin-sensitive horses reached the 50% threshold of glucose drop within 30 minutes of insulin treatment. The IR horses, however, took longer than 30 minutes to reach this point.
Bertin concluded that this test correlated well with the conventional insulin-sensitivity test, it was repeatable, and he observed no adverse side effects. He noted, “This two-step insulin-response test had the same accuracy, sensitivity, and specificity as the complete conventional insulin-sensitivity test.” The equine practitioner now has a useful and reliable low-cost test to administer in the field to detect IR horses, he concluded.
Bertin also noted that it's important to remember that while overweight horses are more likely to be insulin resistant than their slimmer counterparts, "it is important to say that there are obese healthy horses and insulin-resistant slim horses."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.