The examination of mycotoxins, wild cherry trees, molds, cyanide, odd climatic conditions, and unusually heavy caterpillar infestation has led to one more possible theory to how mare reproductive loss syndrome was triggered this spring. The “hybrid” theory suggests a blend of caterpillars, molds, and the right moment during gestation might have caused the late-term abortions and early fetal losses.

Dr. Bruce Webb, an entomologist at the University of Kentucky, has been examining the contributions of “frass” to the concoction of trigger factors for the losses. Frass is the technical term for caterpillar excrement, a mixture of urine, and feces. Caterpillars feed on wild cherry tree leaves containing levels of cyanide; an earlier theory suggests horses reacted to cyanide found in the frass.

“The way I look at this is that you need to look at every link in the chain,” Webb said. “When I start looking at the cyanide hypothesis--there’s no doubt that there’s cyanide in the leaves. The caterpillars eat those leaves, but cyanide is detoxified in the foregut of the caterpillar.”

That would suggest frass deposited on the ground would not be toxic to horses.

“The cyanide theory didn’t really fit what was known in insect physiology,” he said. “In early June, we did some experiments that suggested that caterpillar frass did not have lots of cyanide in it.”

But what Webb and Dr. Kyle Newman, a nutritional microbiologist of Venture Laboratories, discovered was that mycotoxins, previously suspected as MRLS culprits, might be involved in the caterpillar theory. Scientists have said mycotoxins are released by molds under stressful climatic conditions like the hard freeze that hit in April.

Webb and Newman have shown that frass supports the growth of molds which may produce mycotoxins known to cause reproductive problems, and are collecting caterpillar egg masses to hatch in the laboratory this winter.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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