Studies to determine the cause of mare reproductive loss syndrome (MRLS) have led investigators to associate the syndrome with exposure to the Eastern tent caterpillar. Although the exact cause of MRLS remains unknown, some component of the outer covering of the caterpillar (its “exoskeleton” or “integument”) appears to be involved.

In the first study to implicate the integument as a causal agent in MRLS, investigators fed one group of pregnant mares integuments that had been separated from caterpillars and another group the insides of the caterpillars. Half of the mares fed integuments aborted their foals while none of those fed only the insides aborted. A second study conducted by another group of researchers produced similar results–68% of horses fed integuments aborted while none of those fed the insides of the caterpillars aborted