MRLS Probably Not Caused by Virus or Bacteria

A research project has determined 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, an internal medicine specialist at 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 (LDDC), conducted the experiment in late May/early June. This project was funded by KTOB, creating what researchers are calling the "triad" approach--university, private practice, and industry involvement--to solving this problem.

The experiment showed that irradiated Eastern tent caterpillars can induce fetal loss in late-term pregnant mares. This suggests MRLS is caused by a non-infectious agent in caterpillars (irradiation at sufficient levels has been shown to kill infectious agents such as viruses and bacteria). The study was the first experiment in late-term pregnancies to reproduce pathological findings similar to naturally occurring late-term MRLS.

"The big question people wanted answered was is it a biologic agent or a chemical?" noted Bernard. "Most people think it's something in the caterpillar, whether it's something they leave on the grass or something in the cater itself is unknown."

After gathering caterpillars from Kentucky (the numbers were much smaller this year than in the past two years), those caterpillars were irradiated and given to mares to see if it caused abortion. Three out of six mares did abort.

"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 that reproduced abortions with the same findings upon necropsy as were seen during the MRLS crises of 2001-2002.

Bernard says there's not a consensus on what the next step should be in solving the MRLS puzzle.

"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. "So, the next step is to do chemical extractions and take chemicals--such as water soluable/lipid compounds--out of tissues and test them. This project is like finding a needle in a haystack. But we've reduce it down to integument, and if we then can find what kind of compound, we can take that hay stack and divide into hay bales.

"I believe it's a chemical," said Bernard. "Some still think it's a combination of chemicals and bacteria. Those are theories. That question won't be answered until find out what the chemical is and reproduce the abortions."

Bernard and Sebastian have proposed another experiment to do extractions and try and determine what type of chemical compound might be involved in ETC-induced MRLS abortions. Bernard said the University of Georgia is interested in seeing if ETC from that state can cause abortions.

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

"We don't think this phenomena is limited to caterpillars of this state," said Sebastian.

"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. "Sooner or later some farm is going to have an abortion problem and have lots of caterpillars. Then we can be proactive and answer the questions rather than being reactive and trying to find the answers."

Bernard and Sebastian are aware of only two cases of pericarditis (heart problems) associated with the syndrome 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 Kentucky Thoroughbred Owners and Breeders (KTOB) Foundation was able to fund this project and others, and we're looking forward to sending these results out to our Think Tank group composed of local researchers, practicing veterinarians, and seven scientific researchers in the United States outside of Kentucky. We will be awaiting their responses and thoughts," said David Switzer, executive director of the KTOB and Kentucky Thoroughbred Association.