Between May 5 and June 15, the Livestock Disease Diagnostic Center (LDDC) diagnosed 13 foal loss cases caused by mare reproductive loss syndrome (MRLS) in Central Kentucky. This number was up slightly from 2003-2008 when only three to five cases were reported each breeding season.

Research in the past five years concluded the stiff hairs on Eastern tent caterpillars consumed by pregnant mares pierce the horses' digestive tracts, possibly allowing bacteria to enter the bloodstream. The bacteria can then attack the placenta and spread to the fetus. MRLS can cause late-term foal losses, early-term fetal losses, and weak foals. Fetal death from infection by these alimentary tract bacteria is the hallmark of MRLS.

"This year, before we saw the first case of MRLS, entomology researchers at UK predicted an increase in caterpillar numbers this spring," said Neil Williams, DVM, PhD, Dipl. ACVP (see extension report "With our ability to predict increases in caterpillars and diagnose abortions, we at the university try to help the industry by putting out information to alert horsemen."

This spring, seven of the cases were late-term foal losses and six were early fetal losses, according to Williams. Of the 13 cases, seven were Thoroughbreds. Other breeds, which included one case each, were American Saddlebred, Standardbred, Rocky Mountain, Oldenburg, Gotland, and one mixed breed fetus. The numbers of cases by county were: Woodford County, four; Scott and Jessamine Counties, two apiece; and Bourbon, Boyle, Fayette, Oldham, and Rockcastle Counties, one apiece.

"Although an investigation was not conducted, several of the involved farms reported high numbers of caterpillars," Williams said. "Compared to recent years, this represents a slight increase in number of cases and corresponds to the increase in Eastern tent caterpillar population this spring."

A brief history of MRLS in Kentucky

During 2001, when MRLS hit Central Kentucky hard, an estimated 30% of the 2001-2002 Thoroughbred foal crop was lost, and the state suffered an economic cost of approximately $336 million due to losses suffered in all breeds of horses.

"It was observed there was a huge increase of Eastern tent caterpillars (in 2001 and 2002) that coincided with the unusual foal losses," Williams said. "At first, caterpillars were not considered to be the cause."

Epidemiological and field studies conducted by UK researchers demonstrated that MRLS was associated with unprecedented populations of Eastern tent caterpillars, wild black cherry trees, and waterfowl on horse farms in Kentucky. Later research established the link between consumption of caterpillars and abortion. Williams said as a result of MRLS, a lot of farms in Central Kentucky have taken action and implemented procedures to reduce the number of caterpillars on the farms each spring.

Now, the LDDC compiles a yearly surveillance of the number of MRLS cases each breeding season.

Neil Williams, DVM, PhD, Dipl. ACVP, is the associate director at the LDDC. Jenny Blandford is the Gluck Foundation Assistant at the Gluck Center.

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