No longer does the mysterious Mare Reproductive Loss Syndrome appear to be the result of incredibly high mycotoxin levels in pasture grasses; now the most likely cause appears to be cyanide brought onto pastures from wild black cherry trees, carried by Eastern tent caterpillars. At first glance, this seems a bit incredible-how could a few measly caterpillars bring enough of anything into pastures to cause so many foal deaths and other problems across a whole state (and parts of others)?
Conditions have to be exactly right, according to Dr. Jimmy Henning, professor of agronomy at the University of Kentucky, a speaker at tonight's information meeting at the Keeneland racetrack. This spring in Kentucky and some neighboring areas, there was a hard frost in April followed by a drought. This combination wilted the leaves of the wild black cherry trees, significantly increasing their toxic cyanide content. Some of the leaves would have then dropped onto the ground in some pastures, but the major route for cyanide into pastures appears to be via caterpillar.
The Eastern tent caterpillar is highly adapted to life on and near wild black cherry trees; Henning estimated that 95% of caterpillars begin life on them. They're immune to the cyanide of their favorite habitat, carrying it on and in their bodies as a deterrent to predators. When these caterpillars leave the trees to feed elsewhere, they carry cyanide residues on their bodies, spit it at predators when threatened, and excrete it onto the ground, fences, etc. This spring brought an abnormally large Eastern tent caterpillar crop, thus we saw an exponentially larger number of cyanide deliverymen than usual. Horses would have picked up the cyanide from fences and/or grass the caterpillars crept over, spat on, and on which they deposited wastes. Cyanide appears to account for the effects veterinarians have seen in mares, aborted and stillborn foals, and other cases.
Though cyanide breaks down quickly and is therefore difficult to test for, it seems to be the most logical cause after white clover, ergot alkaloids, fescue toxicosis, Fusarium mycotoxins, and hay contamination were conclusively ruled out by testing. Henning suggests that one of the problems we had during the drought that we don't have now is that the dry pastures were in poorer shape then, with poor growth that the horses were eating down to the ground. This would make it more likely that the horses would pick up cyanide from the ground. Henning theorizes that the reason some farms had fewer problems was because they were mowing the pastures more during that critical time, which might have helped spread out and break down the cyanide and/or sift it down through the grass to the ground where horses were less likely to eat it than if it were on the top of the grass.
The researchers and veterinarians speaking at the meeting agreed that with the caterpillars mostly cocooned now, recent rains improving the growth of pastures, and rain diluting what cyanide is left on the fields, they feel it's reasonably safe to turn horses back out in the pastures. Fencing off the areas near cherry trees is a possible precaution, or removing the trees altogether. Though more testing is needed to verify that cyanide is truly the culprit of MRLS, today's experts felt fairly confident that they have found the cause of the syndromes. "You have to consider the wild black cherry tree/Eastern tent caterpillar complex as a strong suspect in the cause of MRLS," Henning concluded.