|College of Agriculture|
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.