The University of Kentucky College of Agriculture has released preliminary results from a collaborative project involving the UK Livestock Disease Diagnostic Center, the UK Department of Veterinary Science, and the Hagyard Davidson McGee Equine Hospital. This project investigated the potential of Eastern tent caterpillars (ETC) to cause fetal loss when administered to mares in the late stages of pregnancy. This is a preliminary report and should not be interpreted as ruling out other possible causal factors related to Mare Reproductive Loss Syndrome (MRLS).The experiment had 11 pregnant mares in later term of pregnancy divided into two groups--six in the experimental group and five in control group--with treatments administered through a nasogastric tube. Mares in the treatment group were administered 50 grams of macerated (crushed) ETC mixed in 70 ml of normal saline once a day for nine days. The total volume of material given each mare in the experimental group was 120 ml. The control group of mares received 120 ml of normal saline for nine days. The mares were confined to stalls during the entire period of the experiment, fed hay, and had access at all times to clean drinking water. Mares were walked once daily to provide exercise. All six mares exposed to the ETC lost their pregnancies; five had late fetal loss during the exposure trial and the sixth mare lost her fetus six days after the trial ended. All mares in the control group maintained their pregnancies. The experiment began June 25 and continued through July 3. Post-experiment monitoring of pregnancy status of the control mares and general health observations of all mares will be made July 25. Specimens taken during the experiment are being analyzed at the Livestock Disease Diagnostic Center. The results of this experiment are similar to those reported by Rood and Riddle Equine Hospital and their collaborators using mares in early pregnancy, and they also confirm observations of UK researchers that exposure to caterpillars was associated with pregnancy loss.