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June 6, 2003

Deductive Investigation of the Causes of Mare Reproductive Loss Syndrome: Experiment 4

Karen McDowell, Department of Veterinary Science
Bruce Webb, Department of Entomology
College of Agriculture
University of Kentucky

The University of Kentucky College of Agriculture releases the results of a recently completed experiment conducted by the Departments of Veterinary Science (Drs. Karen McDowell) and Entomology (Dr. Bruce Webb and Mr. Walter Barney) and the Livestock Disease Diagnostic Center (Drs. Neil Williams and Mike Donahue). This is the fourth in a series of ongoing experiments designed to identify the factor or agent responsible for fetal losses due to MRLS (see MRLS reports of June 12, 2002 and October 25, 2002, at this WEB site;

Eastern Tent Caterpillars (ETC) were collected in central Kentucky in the Spring of 2003 and stored at -80oC until use. Thirty-five early pregnant mares were divided into 7 groups of 5 mares each, with individual treatments added to each mare’s feed for a 10 day period. Mares in Group 1 were fed Eastern Tent Caterpillars, and served as positive controls. Mares in Group 2 were fed saline, and served as negative controls. Three additional groups of mares were fed ETC that had been carefully dissected into three portions, the exoskeleton (skin and associated structures; Group 3), the gut (Group 4), or the remainder of the internal insect tissues (Group 5). The final two groups of mares were fed ETC that had been homogenized in saline and then separated by size (greater than 0.45 microns, Group 6; or smaller than 0.45 microns, Group 7). Each treatment fed to each mare represented the equivalent of 50 grams of ETC larvae.

Fetal losses occurred in all 5 mares fed ETC and in 3 of 5 mares fed ETC exoskeleton (statistically significant at p=0.05). No losses occurred in the negative control (saline) group, in mares fed other ETC tissues (gut or internal tissues), or in mares fed homogenized insects (either the large or small size fraction).

All fetuses were recovered between 3 and 14 days after the first day of treatment. Increased echogenicity of fetal fluids prior to fetal death and bacteriologic findings in fetal tissues were consistent with MRLS as the syndrome is recognized in the field.

The results of this study indicate that the factor from ETC that causes equine abortions is present in or on the insect exoskeleton. This study also confirms earlier observations that frozen ETC cause abortion when given in feed. That no losses occurred when ETC were homogenized suggests that homogenization of the insects inactivated the factor or agent, regardless of its size. The collaboration between the Departments of Veterinary Sciences and Entomology and the Livestock Disease and Diagnostic Center is continuing with additional experiments to be performed in the coming weeks. These continue to focus on identifying the factor or agent that causes MRLS.

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