LEXINGTON — Scientists are convinced that cyanide naturally present in cherry-tree leaves was responsible for the mysterious illness that killed hundreds of foals and fetuses in Kentucky thoroughbred country this spring.

        Researchers have not figured out exactly how the cyanide got into the horses' systems.

        One theory is that Eastern tent caterpillars, which infested the state in large numbers this spring, ate the leaves and deposited the poison in grazing areas or drinking water.

        To test that theory, researchers are feeding the leaves to the caterpillars in the lab and studying the creatures and their excrement.

        “I have no doubt that cyanide poisoning from cherry trees, or that combined with some other natural toxin, was behind this syndrome. Hopefully, these tests will be able to clear up some lingering questions,” said Dr. Thomas Tobin of the University of Kentucky's Gluck Equine Research Center.

        “We've found that the caterpillars neutralize some of the poison before it exits the body. But there's still plenty of poison there. Enough to affect the horses if they eat excrement-laced grass or drink from a trough where a number of caterpillars have drowned? That's what we're trying to establish.”

        The mystery illness killed more than 500 foals and caused hundreds of mares to miscarry early in their pregnancies.

        Scientists had theorized early on that fungus was responsible, but tests for the toxins were negative.

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