Bacterial Involvement in MRLS

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. The question was and still is: Are these bacteria primary or secondary players in the abortions of fetuses and the occurrence of eye and heart problems?

Research released last week showed that the setae (small hair-like projections on caterpillars) cause microgranulomas in the alimentary tract of pigs fed Eastern tent caterpillars (ETC). That raises the possibility that if the setae cause the same problems in horses, the bacteria could invade through the sores made by the setae in the wall of the alimentary tract. If those bacteria get into the circulatory system, they would be able to travel throughout the body and cause problems in susceptible areas such as the heart, eye, and uterus of the pregnant mare. The thought behind this is that these tissues are susceptible because the immune system is somewhat surpressed or less effective in these locations, therefore the bacteria establish there and cause problems.

In experiments when caterpillars were gavaged (ground up and lavaged into the stomachs of mares), they did induce abortion similar to MRLS. When the caterpillars were irradiated to kill living organisms on the surface of the skin--such as bacteria--the caterpillars still could cause abortions. The same bacteria found in field cases of MRLS were recovered (Streptococcus and Actinobacillus). Bruce Webb, PhD, an entomologist with the University of Kentucky’s Department of Agriculture, said these observations lead his research team to theorize that MRLS occurs because of the interactions of ETC, bacteria, and placental fluid infection by the bacteria.

“This looks like a great story of biologic interactions,” said Webb. “You might not recognize certain data is important initially, but eventually it will fall into place, like the bacteria/ETC relationship.”

It took the realization that the setae might be causing thousands--if not millions--of microscopic wounds throughout the alimentary tract that allow bacteria normally found in the horse’s gut access to the circulatory system. The bacteria then establish an infection, causing various clinical problems with pregnancy, eyes, heart, and even within the brain.
This also might help explain some of the observations of veterinarians and horse owners who, in 2001, said they were seeing foot problems during the MRLS outbreak. The foot is highly vascular, and thus would be a site that could become infected.

Research to find out if horses get the microgranuloma lesions in the alimentary tract is being done and results should be out soon.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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