MRLS Experiment Rundown

A meeting at Keeneland racetrack for a select group from local Thoroughbred breeding farms on Jan. 6 offered a wrap-up of the six experiments done by researchers at the University of Kentucky pertaining to mare reproductive loss syndrome (MRLS). The synopses of those experiments, as presented by Karen McDowell, PhD (equine reproduction), are as follows:

Experiments 1 and 2, April-June 2002
Design: To place Eastern tent caterpillars (ETC) and/or ETC materials (nest materials for experiment 1 or excrement/frass for experiment 2) on pasture where pregnant mares grazed.

Result: Abortions were induced in mares exposed to ETC for six hours a day for 10 days.
Conclusion: These studies provided the first experimental evidence that ETC can induce pregnancy loss in horses, and were the first to  reproduce the syndrome under experimental conditions. Preliminary results of Experiment 1 were released April 26, 2002, allowing farm managers to reduce exposure of mares to ETC in 2002.

Experiment 3, July-September 2002
Purpose: To mix ETC with sweet feed fed to pregnant mares, and to determine if the abortigenic activity was stable to freezing or autoclaving (exposure to high heat, approximately 120°C).

Result: Frozen ETC caused abortions, but autoclaved ones did not. Mares fed the same feed without ETC and kept in the same pastures with ETC-fed mares did not abort.

Conclusion: By mixing the caterpillars with feed in a bucket we removed the effects of pasture or hay. We also demonstrated that the abortigenic activity is destroyed by exposing the caterpillars to high heat.

Experiment 4, April-June 2003
Purpose: To determine which portion of the ETC larva contains the abortigenic activity. Caterpillars were carefully dissected under a microscope into three portions--the cuticle or exoskeleton, the gut, and the remainder of the insect. Each portion was mixed in sweet feed and fed individually to pregnant mares.

Result: Only the cuticle or exoskeleton of the caterpillar caused abortions.

Conclusion: That the cuticle alone is capable of inducing abortions lets us narrow our focus to this structure of the insect.

Experiment 5, June-July 2003
Purpose: To determine if the ETC exoskeleton abortigenic activity is extractable with aqueous or organic solutions, and if disrupting the exoskeleton by pulverizing it reduces abortigenic activity.

Result: The ETC abortigenic activity was not extracted with the aqueous or organic solutions. Pulverizing the exoskeleton to disrupt its integrity greatly reduced the abortigenic activity.

Conclusion: The fact that ETC cuticle induced abortions, but aqueous or organic extracts did not, indicates that the abortigenic activity is not an agent or substance that can be easily removed from the cuticle.

Experiment 6, June-August 2003
(Conducted with Merlin Lindemann of the Department of Animal Science at the University of Kentucky.)
Purpose: To determine if ETC causes fetal loss in animals other than horses, i.e., domestic pigs, and to carefully examine the alimentary and reproductive tissues of the pigs for signs of ETC ingestion.

Result: ETC can cause fetal loss in domestic pigs in a manner consistent with MRLS, and ETC setae (hair) were identified microscopically throughout the alimentary tract of gilts fed ETC, but not control gilts.

Conclusion: That ETC setae induced microgranulomas in the alimentary tract might implicate their involvement in the fetal loss syndrome. They might allow bacteria from the mare’s (or gilt’s) gut to travel to the fetus, where they proliferate and eventually kill the fetus.

McDowell noted that research now has identified setae embedded in the alimentary tract of mares fed ETC.

She noted that, “MRLS struck the equine industry hard and without warning in April and May of 2001. It could not be attributed to any known abortigenic agent or disease. It was caused by something in the environment, but was not transmitted between horses. Dr. (Roberta) Dwyer’s epidemiological survey revealed an association between ETC and MRLS. In the fall of 2001, we designed the first in a series of experiments to determine if the association was causative or simply correlative, and if it were causative, what is the mechanism by which ETC cause fetal loss in horses.”

She listed the unknowns at this point as:
• How do bacteria get to the fetus?
• Can other caterpillar larvae or other insects cause MRLS?
• Why primarily Streptococci and Actinobacilli (found in aborted fetal tissues)?
• What is the connection with pericarditis and uveitis?
• What is the involvement of the mare’s immune system?

Closing the Loop
Based on these experiments, McDowell said, the researchers have developed the following hypothesis:

ETC cuticle (setae) penetrate the intestinal lining and allow bacteria from the mare’s alimentary tract to invade the circulatory system. Bacteria establish infections in tissues where the mare’s immune surveillance is reduced. Fetal/placental fluid infections lead to fetal death and abortion, characteristic of MRLS.

Questions/Goals: Do bacteria which are normally resident in the mare’s alimentary tract move to the fetus via the mare’s bloodstream? Our goal is to trace the movement of bacteria from the alimentary tract to the fetus, and show that they induce fetal loss.


About the Author

Kimberly S. Brown



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