But they say they don't have enough scientific evidence yet to confirm the theory. We have not yet met reasonable standards of scientific proof. Only 24 deaths have been reported since Sunday and only four in the past three days. The strongest theory into the outbreak's cause suggests mares somehow ingested cyanide produced from the leaves of the trees after a prolonged winter.
Researchers believe Eastern tent caterpillars also played a role.
The caterpillars nest in the cherry trees and are immune to the poisonous chemicals in the leaves. That theory, however, has holes. Henning said tests have failed to support other hypotheses as to the cause of the outbreak, which began in late April. Whatever the cause, researchers are confident that the worst of the crisis is over. David Powell, equine epidemiologist for the University of Kentucky's Gluck Equine Research Center, said he feels safe advising owners and farmers to put their horses back out to pasture with no fear of the illness reappearing.