"We can prevent mare reproductive loss syndrome (MRLS) as we experienced in 2001 and 2002 by keeping horses away from caterpillars," said Bruce Webb, PhD, a University of Kentucky researcher who has been studying the condition that terminated thousands of mare pregnancies in Central Kentucky and nearby states. The acute problems of MRLS might be behind us, but MRLS is present tense for Webb and other researchers who spend their laboratory hours studying the condition. The pervading theme at a Nov. 30 MRLS update meeting at Keeneland sale pavilion in Lexington, Ky., was that researchers have learned much about MRLS, but more needs to be understood before they can intervene in the MRLS disease process.

The scientists presented 14 months of Kentucky Thoroughbred Owners and Breeders Foundation-sponsored research material on novel ways to control Eastern Tent Caterpillars (ETC, which have been linked to MRLS) with species-specific viruses, laboratory models for MRLS, and other MRLS-related material. Possible future study avenues were also presented with the suggestion that we might better understand the abortion-causing mechanism of MRLS and other "natural" causes of foal loss if we are able to monitor mare and fetal health by using catheters and probes placed in a fetus and the dam’s uterine artery to monitor blood flow between a mare and the fetus, among other things.

Tent Caterpillar Research

Eastern Tent Caterpillars have long been the major focus in MRLS studies, and researchers now know that these caterpillars cause tiny lesions in the mare’s gastrointestinal (GI) tract and suspect that these lesions allow bacteria in the GI tract to enter and circulate in the mare’s body to somehow reach the fetus, and ultimately cause abortion. Karen McDowell, PhD, David Horohov, PhD, and Neil Williams, DVM, PhD, of the University of Kentucky performed skin tests, much like human allergy tests, to find out if external ETC hairs could modify the mare’s immunological response. Suppression of her natural immune response could prevent a mare from attacking bacteria that invade from her GI tract via those lesions. McDowell injected an irritant commonly used in allergy testing, PWM (Poke Weed Mitogen, which produces a predictable reaction) and setae alone and together, intradermally. They expected the welt reaction to the PWM/setae to be less than with the PWM alone, indicating immunosuppression by the setae. That wasn’t the case, however, both in mares previously exposed to ETC and those with no prior exposure to ETC