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