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