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Mare Reproductive Loss Syndrome: Update

In 2001, 30 percent of the pregnant mares in Kentucky aborted their foals, resulting in an estimated $300 million blow to the thoroughbred industry. The mysterious illness, Mare Reproductive Loss Syndrome (MRLS), was initially linked to a peak population of Eastern tent caterpillars with a predilection for cherry leaves, which carry a precursor to cyanide. Because of UK research results in 2001 and 2002, many horse farms took preventative steps, chopping down cherry trees and muzzling mares to keep them from eating caterpillars.

The caterpillar-cyanide theory didn't hold up in early lab tests, but the caterpillar's role as the causative agent in the epidemic is clear. Bruce Webb (entomology), Karen McDowell (veterinary science), Neil Williams (Livestock Disease Diagnostic Center), and Merlin Lindeman (animal sciences) have conducted recent experiments that seem to indicate small hairs on the caterpillar's exoskeleton can partially penetrate the intestine, potentially resulting in abortion.

While MRLS losses have declined in the past two years, down to 12 percent in 2002 and much less than 1 percent in 2003, two teams of 15 scientists at UK are continuing work to reveal the mechanism by which ingested caterpillars cause mares to abort. For more, visit