(Edited press release)Recent research conducted by the University of Kentucky indicates that there is something in or on the exoskeleton (skin and associated structures) of the Eastern tent caterpillar that causes horses to abort. The study, completed this spring, is the fourth in a series of experiments designed to determine what the factor or agent is responsible for fetal losses that result from mare reproductive loss syndrome (MRLS).In the experiment, 35 pregnant mares were divided into seven groups of five mares each, with individual treatments added to each mare's feed for a 10-day period. Mares in group 1 were fed Eastern tent caterpillars (ETC), and served as positive controls. Mares in group 2 were fed saline, and served as negative controls. Three additional groups of mares were fed ETC that had been carefully dissected into three portions: the exoskeleton (group 3), the gut (group 4), or the remainder of the internal insect tissues (group 5). The final two groups of mares were fed ETC that had been homogenized in saline and then separated by size (greater than 0.45 microns, group 6; or smaller than 0.45 microns, group 7). Each treatment fed to each mare represented the equivalent of 50 grams of ETC larvae.Fetal losses occurred in all five mares fed ETC and in three of five mares fed ETC exoskeleton. No losses occurred in the negative control (saline) group, in mares fed other ETC tissues (gut or internal tissues), or in mares fed homogenized insects (either the large or small size fraction).All fetuses were recovered between 3 and 14 days after the first day of treatment. Increased echogenicity of fetal fluids prior to fetal death and bacteriologic findings in fetal tissues were consistent with MRLS as the syndrome is recognized in the field.