Chemical Implicated in Causing MRLS

Research has shown that it probably isn't a virus or bacteria (a biological agent) that links the Eastern tent caterpillar (ETC) to mare reproductive loss syndrome (MRLS). Bill Bernard, DVM, Dipl. ACVIM, of Rood and Riddle Equine Hospital in Lexington, Ky.; and Manu Sebastian, DVM, MS, a pathologist with the University of Kentucky's Livestock Disease Diagnostic Center, conducted the experiment in late May/early June. This project was funded by the Kentucky Thoroughbred Owners and Breeders Foundation (KTOB).

The experiment showed that irradiated ETC can induce fetal loss in late-term pregnant mares. This suggests that MRLS is caused by a non-infectious agent in caterpillars, since irradiation at sufficient levels kills infectious agents such as viruses and bacteria.

After gathering ETC from Kentucky (numbers were much smaller this year than the past two years), the researchers irradiated them and gave them to mares to see if they caused abortion. Three out of six mares aborted. "That suggests that it is a chemical," said Bernard. "Two of them (aborted fetuses) showed funisitis (inflammation of the umbilical cord of the fetus) and characteristic pathologic signs of MRLS."

This was the first experiment to reproduce abortions with the same pathologic findings seen during the MRLS crises of 2001-2002.

"We did a project last year that showed that the toxin--or whatever it is--could be found in the integument (outer portion of the caterpillar), and Dr. (Bruce) Webb (of the University of Kentucky) showed that was true," said Bernard. "The next step is to do chemical extractions and take chemicals--such as water soluble/lipid compounds--out of tissues and test them."

Bernard said the University of Georgia wants to see if ETC from that state can cause abortions. He also has been working with the University of Washington because Western tent caterpillars are abundant there this year. He said it would be interesting to see if Western tent caterpillars cause the same problems as ETC. In earlier experiments, it was found that ETC from Michigan caused abortions.

"If we can show that Eastern tent caterpillars from several states--Kentucky, Michigan, Georgia, New York--can cause abortions, then we can assume any Eastern tent caterpillars can cause problems," said Bernard.

Bernard and Sebastian are aware of only two cases of pericarditis (heart problems) associated with the syndromes that occurred during the MRLS outbreaks. Both cases were from Northern Kentucky. Bernard said he has not seen any abortions, problem foals, or uveitis (eye) problems attributed to MRLS this year.

"We're glad the KTOB Foundation was able to fund this project and others," said David Switzer, executive director of the KTOB and Kentucky Thoroughbred Association. (For more on MRLS see .)

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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