An abortigenic disease known as mare reproductive loss syndrome (MRLS ) significantly impacted the horse industry in the Ohio Valley in late April and early May, 2001 and 2002. In 2001, approximately 25% of all pregnant mares aborted within several weeks (over 3,000 mares lost pregnancies), and abortion rates exceeded 60% on some farms. MRLS was a newly recognized disease, and it cost the state of Kentucky approximately $330 million in 2001 alone (Thalheimer et al., 2001).

An epidemiologic survey conducted in the summer of 2001 revealed a temporal correlation between MRLS and presence of eastern tent caterpillars (ETC; Malacosoma americanum) on horse farms (Dwyer et al., 2003). However, a statistical correlation does not necessarily mean that the caterpillars caused the abortions, so several groups of scientists from around the country designed experiments to determine the role ETC, as well as other agents, played in MRLS . Summaries of those studies are available online.

Controlled research experiments performed as collaborative efforts by Drs. McDowell, Williams, Donahue, Webb (University of Kentucky) and Newman (Venture Laboratories, Lexington, Ky.) demonstrated that horses will inadvertently eat ETC when the insects are present in the pasture or in other feedstuffs; MRLS -type abortions can be induced in experimental animals (mares or sows) by feeding them ETC; and the only part of the caterpillar that causes MRLS abortions is the exoskeleton (skin or cuticle).

Eastern tent caterpillars are hirsute (hairy) caterpillars, and the experiments revealed that the hairs (setae) embed into the submucosa of the alimentary tract, creating microgranulomatous lesions. We hypothesized that those lesions allow bacteria from the mare’s alimentary tract, principally streptococci and actinobacilli, to invade the mare’s circulatory system. The bacteria then establish infections in tissues where the mare’s immune surveillance is reduced, such as the fetus and placenta. Fetal/ placental fluid bacterial infections lead to fetal death and abortion characteristic of MRLS (McDowell et al., 2004; Webb et al., 2004). Movie clips of horses consuming ETC can be viewed