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Does Gestational Diabetes Occur in Pregnant Mares?By Dr. Bryan Waldridge · March 12, 2012

Gestational diabetes in humans is defined as high blood sugar that starts or is first diagnosed during pregnancy. Progesterone is a hormone that helps to maintain pregnancy and it can antagonize the effects of insulin. Clinically important changes in blood glucose concentration in healthy pregnant mares are not frequently encountered by veterinarians. A study was recently conducted to investigate insulin and glucose interactions in pregnant mares.

Twenty-two pregnant and ten non-pregnant Thoroughbred mares were used in the study. Mares were kept at pasture and supplemented with hay as needed. A frequently sampled intravenous glucose tolerance test (FSIGTT) was performed when mares were at 25-31 weeks of gestation and again at 47 weeks of gestation. After the first FSIGTT, mares were grouped by age and body condition score, and randomly assigned to either a high-starch or high-fat, high-fiber feed.

Pregnant mares at 28 weeks of gestation had slower clearance of glucose from their blood and greater insulin release than open mares. Pregnant mares fed a high-starch feed had greater increases in blood glucose and insulin than open mares and pregnant mares that were fed a high-fat, high-fiber feed. Pregnant mares also had slower clearance of glucose from the blood, with or without insulin. Pregnant mares fed a high-starch feed had prolonged hyperglycemia and elevations in blood insulin concentration.

Gestational diabetes, as it occurs in women, is not commonly diagnosed in mares. Elevated blood glucose concentration and decreased insulin sensitivity during pregnancy may be an adaption that allows more glucose to be available to the fetus and mammary gland, which depend on glucose for energy. The results of this study showed that mares in late gestation react differently to high-starch meals than open mares and mares fed high-fat, high-fiber meals. Pregnant mares that are already predisposed to insulin resistance (pituitary pars intermedia dysfunction, equine metabolic syndrome, etc.) or treated with drugs that interfere with insulin may have even more pronounced abnormalities in their blood glucose and insulin concentrations. The starch content of the pregnant mare’s meal, time from feeding, and stage of pregnancy should affect how veterinarians interpret blood glucose and insulin concentrations in laboratory data from mares in late gestation.

George, L.A., W.B Staniar, T.A. Cubitt, K.H. Treiber, P.A. Harris, and R.J. Geor. 2011. Evaluation of the effects of pregnancy on insulin sensitivity, insulin secretion, and glucose dynamics in Thoroughbred mares. American Journal of Veterinary Research 72:666-674.