Eastern tent caterpillars (ETC), plus some bacteria, plus a mechanism to deliver bacteria to blood, equals MRLS (mare reproductive loss syndrome). This summary of a hypothesis based on accumulating data was presented at a regular weekly meeting of the entomology group at the University of Kentucky to graduate students, researchers, and a few industry visitors.

Early in the foal loss syndrome in 2001, researchers at the Kentucky Livestock Disease Diagnostic Center noticed an unusual abundance of a couple of families of bacteria in aborted fetuses (Streptococcus and Actinobacillus). These bacteria are normally found throughout the alimentary tract of horses–from their mouths to their large colons–and naturally occur in the environment of horses