Eastern Tent Caterpillars Confirmed to Cause Abortion in Pregnant Mares

A recent study funded by Grayson-Jockey Club Research Foundation, Rood and Riddle Equine Hospital, and several other entities, identified an association between the administration of starved Eastern Tent Caterpillars (ETC) and early pregnancy loss in the mare. The study had support from Taylor Made Farm, Ernie Paragallo, Fasig-Tipton Co., and the Kentucky Thoroughbred Farm Managers Club, and was conducted by Dr. Bill Bernard, Dr. Michelle LeBlanc, and Dr. Bruce Webb.

The study involved three groups of mares. One group received starved caterpillars, one group received caterpillar frass (waste), and one group served as control. Four of five mares administered caterpillars lost pregnancies within 8 to 13 days subsequent to the first dose of ETC. No control mares or mares that received frass aborted.

The project design consisted of 3 groups of 5 mares each. Mares were administered their respective treatment by nasogastric tube for 10 days. Group one mares served as controls and received 50 ml of water, group 2 mares received 2.5 grams of stored frass diluted in 50 ml of water, and group 3 mares received 50 grams of crushed fresh ETC mixed in 50 ml of water. Mares were housed in stalls with no exposure to grass beginning 12 days before they were given any treatment and remained in stalls for the entire experimental period. Mares were walked twice daily to provide exercise. Mares were between 38 and 88 days of gestation on day one of treatment. The mares that aborted were 49, 64, 70, and 96 days of gestation. Alpha streptococcus was cultured from one aborted fetus while Serratia was cultured from the remaining 3 fetuses. Neither pericarditis nor ophthalmologic disease was observed.

The investigators concluded from this study that ETC can cause early fetal loss in the pregnant mares and that “stored” frass does not cause early fetal loss. The study did not define the toxic component of the caterpillar responsible for the early fetal losses.

Primary participants were: Bill Bernard DVM, Dipl ACVIM, Rood and Riddle Equine Hospital, Lexington, Ky., and president of the Kentucky Association of Equine Practitioners; Bruce Webb Ph.D., University of Kentucky, Lexington, Ky.; and Michelle LeBlanc DVM, Dipl ACT, Rood and Riddle Equine Hospital, Lexington, Ky.

Also involved were: Dr. Richard Holder DVM, Hagyard-Davidson-McGee; Dr. Stuart Brown DVM, Hagyard-Davidson-McGee; Dr. Bart Barber DVM, Rood and Riddle Equine Hospital; Dr. Johanna M. Reimer VMD, Dipl ACVIM, ACVC, Rood and Riddle Equine Hospital; and Dr. Claire Latimer DVM, Dipl ACVO, Lexington, Ky.