Mare Reproductive Loss Syndrome Update

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.  

 Photo of a seta (hair) attached to an eastern tent caterpillar

Figure 3. Photo of a seta (hair) attached to an eastern tent caterpillar.

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 (Figure 3). 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 online.  

Abortions in Australia have been linked to hairy processionary caterpillars (Ochragaster lunifer), and experimental dosing of pregnant mares with processionary caterpillars via nasogastric tubing resulted in abortions with pathologic and bacteriologic findings similar to those characteristic of MRLS (Cawdell-Smith et al., 2007).

Since 2002, only a few abortions have been diagnosed as MRLS at the University of Kentucky Livestock Disease Diagnostic Laboratory. These have only occurred sporadically and have consistently been temporally associated with the seasonal occurrence of ETC. Investigation of some of the cases has demonstrated potential exposure to ETC in the mares' environment.


Cawdell-Smith A.J., K.H. Todhunter, S. Shini, N.R. Perkins, W.L Bryden. 2007. Establishment of a link between processional caterpillars and pregnancy loss in Australian Mare. Proceedings of the Twentieth Symposium of the Equine Science Society, pp 161-162.

Dwyer, R. M., L. P. Garber, J. L. Traub-Dargatz, B. J. Meade, D. Powell, M. P. Pavlick, and A. J. Kane. 2003. Case-control study of factors associated with excessive proportions of early fetal losses associated with mare reproductive loss syndrome in central Kentucky during 2001. J. Am. Vet. Med. Assoc. 222:613-619.

Thalheimer, R. and R. G. Lawrence. 2001. The Economic Loss to the Kentucky Equine Breeding Industry from Mare Reproductive Loss Syndrome (MRLS) of 2001. Department of Equine Business, College of Business and Public Administration, University of Louisville, Louisville, KY. A Report Commissioned by the Office of the Governor, Commonwealth of Kentucky.

McDowell KJ, NM Williams, JM Donahue, L Poole, WE Barney, B Coe, S Deborde, L Ennis, KE Newman, M Lindemann, B Lynn, BA Webb. 2004. Deductive Investigations of the Role of Eastern Tent Caterpillars in Mare Reproductive Loss Syndrome. Proceedings of a Workshop on the Equine Placenta, pp 99-103.

Webb, B. A., W. E. Barney, D. L. Dahlman, S. N. DeBorde, C. Weer, N. M. Williams, J. M. Donahue, and K. J. McDowell. 2004. Eastern tent caterpillars (Malacosoma americanum) cause mare reproductive loss syndrome. Journal of Insect Physiology. 50:185-193.

Contact: Dr. Karen McDowell, 859/257-4757, Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Ky.

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

Equine Disease Quarterly is a quarterly equine disease research newsletter published by the University of Kentucky's Gluck Equine Research Center, and funded by underwriters at Lloyd's of London, brokers, and their agents.

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