Mare Reproductive Loss Syndrome

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Mare Reproductive Loss Syndrome (MRLS) was observed in Kentucky in 2001.

MRLS was first identified on April 26, 2001 by Dr. Thomas Little, who on that day observed an unusual number of equine in utero early fetal deaths in 60 day-old fetuses that he was ultrasound examining for sex determination. These early fetal losses were followed by a sequence of early and late fetal losses and, somewhat later, the coincident pericarditis, uveitis and encephalitis syndromes were identified.

During the three weeks around the first of May, 2001, about 20% to 30% of Kentucky's pregnant mares suffered abortions. Of foals conceived in spring 2001, about 2000 were lost, the so-called Early Fetal Losses (EFLs). Of foals conceived during the spring of 2000, and then close to term, at least 600 were lost, the so-called Late Fetal Losses (LFLs). Based on these overwhelming reproductive losses, the syndrome was named the Mare Reproductive Loss Syndrome (MRLS). The total economic loss to Kentucky and the racing industry for the 2001 MRLS season has been estimated at $500 million.

During the spring of 2001 Kentucky experienced an unusually heavy infestation of Eastern tent caterpillars. As part of an extensive and multifaceted investigation spearheaded by the University of Kentucky, a rigorous epidemiological survey by Dr. Roberta Dwyer and her associates confirmed an association of MRLS with presence of the caterpillar. The exact mechanism by which the caterpillar caused MRLS is still under investigation.

For more information see A Toxicological Investigation of Mare Reproductive Loss Syndrome