Caterpillars that Cause Abortion in Mares

Learn how researchers determined that hairy, bristly caterpillars can cause abortions and how to protect mares.

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Caterpillars that Cause Abortion in Mares
Photo: Courtesy Dr. Joan Carrick
By CJ (Kate) Savage, BVSc(Hons), MS, PhD, Dipl. ACVIM, WEVA Junior Vice-President and Oceanic Board Member

The horse industry was shaken in 2001 and again in 2004 when abortion storms occurred in two Thoroughbred breeding strongholds with none of the usual reasons implicated. Central Kentucky was hit with this mare reproductive loss syndrome (MRLS) in 2001, while Scone, Australia, was impacted in 2004. Both locations lost a substantial portion of their foal crops.

Scientists quickly started looking into the cause of these outbreaks. Researchers in Kentucky, using an epidemiological study to ascertain what factors could be considered risk factors, developed a few interesting theories. One of the most surprising to many horse people was exposure to Eastern tent caterpillars (ETC)—one of the many types of hairy, bristly caterpillars.

How Is This Possible?

During the 2001 MRLS outbreak, more than 20% of Kentucky’s pregnant mares suffered abortions. However, it is likely that prior abortion storms in the 1980s were also due to the ETC, but technology (e.g., ultrasonography) at the time was not sufficient to detect problems with the fetus until abortion occurred

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