Caterpillar not off hook as MRLS bait

June 1, 2002

By: Stephanie Davis

DVM Newsmagazine

In May, the University of Kentucky (UK) released preliminary test results suggesting the eastern tent caterpillar and its waste remain chiefly responsible for increased foal losses in 2001, but the picture is "far from fully developed."

The results are "an encouraging sign," says Dr. Nancy Cox, associate dean of research at UK's College of Agriculture.

"Certainly the results do point to the caterpillars' role in the cause of Mare Reproductive Loss Syndrome (MRLS), because the mares that did lose their pregnancies in our (latest) experiment had symptoms similar to MRLS observed last year," she says.

The UK College of Agriculture's Departments of Entomology, Veterinary Science and the Livestock Disease Diagnostic Center collaborated on this project to officially rule "in" or "out" the caterpillar.

Last year, the UK Gluck Equine Research Center conducted a survey of farms, which demonstrated a link between the presence of eastern tent caterpillars and the incidence of MRLS. As a follow-up, the new project is designed to definitively say whether the association is a correlation or if caterpillars were causally associated. At press time, the trial was near completion.

Three test groups

The experiment divided 29 pregnant mares into three groups. The first group was exposed for six hours daily to increasingly high levels of eastern tent caterpillars and their frass. The group was moved to a plot where 10,000 caterpillars were dispersed.

"There's no place in the world that would naturally have that many caterpillars concentrated in one area with those kind of exposure levels," explains Bruce Webb, Ph.D., entomologist and lead investigator of the latest trial.

The second group was exposed only to the insect's frass. Lastly the third set, the control group, received as little exposure to caterpillar larvae and frass as possible.

In the treatment with caterpillars and frass, six of 10 mares lost foals. The second treatment with only frass resulted in six of nine mares losing foals. In the control group, three of 10 lost foals.

Interpretation of results

Researchers at UK say such numbers are not "statistically significant" and the project is only another step forward.

"We will have to repeat that experiment and get results that are statistically significant," says Cox. "In the research we did, three in our control group lost their pregnancies. We would have expected that none of those would. The reason we think they did is because some caterpillars escaped from our other treatments."

Although the results are not statistically sound, they are moving scientists in a more conclusive direction.

"There's a strong tendency that leads us to expect they are causally associated, but we would not be able to say that unequivocally at this time," says Cox.

Will the case of MRLS ever be definitively solved?

"I don't know if scientists will ever say anything is final, or else they'd be out of a job," says Webb. "I will say that the results show that the data weighs more heavily that caterpillars were a primary factor. More trials will most likely be conducted."

In the meantime, the university does not plan to use these preliminary results in any way to be a predictor of the extent of MRLS that may be on farms this year.

Lingering theories

While the latest caterpillar results show promise, other theories are still not entirely fleshed out, says Cox.

"Certainly caterpillars are the frontrunner of our causes right now. But we're still looking at cyanide and cyanide-like compounds, some anti-estrogen causing agents.

"Scientists at UK are still considering that the caterpillars' frass or excrement or grass or fungi might be producing an anti-estrogen compound."

The caterpillars or the insects in combination with the soil, grass, weather and fungi may yield a substance that caused termination of the pregnancy.

As for the theory that horses ingested toxic cyanide by eating caterpillars, the theory has yet to be proven in the laboratory, according to Cox. The cyanide, apparently, is depleted shortly after caterpillars ingest the substance.

"But, laboratories are different from the great outdoors, we'd have to confirm our evidence in an outdoor experiment," she says.

Hemlock also didn't make the UK list of pet theories, although Cox says it continues to be investigated at Clemson University.

For now, the goal: to be able to answer by year-end, "Caterpillars? Yes or no," says Cox. That would be the beginning of a new chapter of MRLS investigation.