Kentucky MRLS 2003 Update

There was some apprehension in Central Kentucky as to whether the elimination of cherry trees and spraying for caterpillars would be enough to decrease the numbers of mare reproductive loss (MRLS) abortions that occurred in 2001, and to a lesser degree in 2002. The removal of the cherry trees to eliminate the most suitable habitat for the Eastern tent caterpillar (ETC) moth to lay her eggs, spraying trees to kill the caterpillars when the trees could not be removed, and the natural cycle of the ETC to die off after peak concentration years have all affected the concentration of caterpillars in 2003.

In 2001, a very high percentage of February and early March bred mares resulted in MRLS abortions (greater than 50%). In 2002, some farm management controls were put in place which resulted in a much lower percentage of abortions in the February and early March bred mares (less than 10%).

Research trials completed in spring of 2002 indicated that the ETC was indeed involved in the MRLS abortions. In the breeding season of 2003, strict precautions to remove pregnant mare exposure to the ETC in the entire Central Kentucky area resulted in a miniscule MRLS loss in the February- and March-bred mares. Any loss that was a MRLS suspect abortion was related to difficulty in completely eliminating exposure to the ETC.

In a recent poll of 2,037 mares examined at 60 days of gestation by area veterinarians, only 16 mares were not carrying a viable fetus. Six of these were suspect MRLS abortions.

  • 2001--February and early March bred mares greater than 50 % aborted from MRLS;
  • 2002--February and early March bred mares less than 10% aborted from MRLS;
  • 2003--February and March bred mares less than 0.3% suspect abortions from MRLS.

Even though MRLS abortions, pericarditis, and uveitis have been almost completely eliminated through removal of contact with the caterpillar, more work needs to be done to determine the exact causative agent within the caterpillars. Ongoing research (Phase I Trials) suggests that the causative agent is in the exoskeleton (skin) of the caterpillars. Trial proposals are now being reviewed by the MRLS Think Tank. This is a group formed in 2002 to review proposals, prioritize and coordinate research efforts between different research groups to prevent duplication, remove competition for funding, share information discovered, and add consultant expertise from experienced toxicologists and entomologists from throughout the country. The Think Tank is composed of representatives of the Kentucky Association of Equine Practitioners, Kentucky Thoroughbred Owners and Breeders, Kentucky Thoroughbred Association, University of Kentucky College of Agriculture (Gluck Equine Research Center, Livestock Disease Diagnostic Center, Department of Entomology, Department of Agronomy), and a panel of seven nationally noted toxicologists and entomologists.

Presently, funding is being sought for the Phase II Trials that are approved by the Think Tank. Phase II Trials will focus on fractionating the exoskeleton of the ETC to determine which part carries the causative agent. The results of the Phase II Trials will determine the direction of the Phase III Trials. Anyone interested in inquiring about the fundraising effort should contact: Dr. Richard Holder, Hagyard-Davidson-McGee Associates, 859/255-8741;, or Dr. Nancy Cole at CORNERWOOD@AOL.COM

Even though the MRLS syndrome has all but been eliminated, careful surveillance needs to be continued for caterpillar control not only for Kentucky, but for other parts of the country as well.

About the Author

Richard D. Holder, DVM

Richard D. Holder, DVM, graduated from the University of Texas in 1969 and Texas A&M College of Veterinary Medicine in 1972. He has been an equine practitioner at Hagyard-Davidson-McGee Associates, PSC, in Lexington, Ky., since 1974 with emphasis on reproduction, and he currently holds the office of treasurer for the firm. In the past several years, he has been instrumental in developing the technique for equine ultrasonic fetal sex determination and has given numerous presentations on this subject. Holder is the resident owner of Offutt-Cole Farm in Midway, Ky.

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