The epidemic of Mare Reproductive Loss Syndrome (MRLS) that hit central Kentucky last spring will have an estimated economic impact of $336 million through 2003. The results of a recent study (a 43-page report, prepared for the Kentucky Governor’s Task Force on MRLS) may shed light on factors that predisposed mares to MRLS.
Researchers involved in the project include Dr. Noah Cohen of the College of Veterinary Medicine, Texas A&M University; Dr. Vincent Carey, a biostatistician at the Harvard Medical School, Harvard University; Dr. James Donahue, a veterinary epidemiologist in Madison, Wisconsin; and Dr. Janyce Seahorn, University of Kentucky Livestock Disease Diagnostic Center (UKLDDC). Vital assistance was also provided by veterinarians from Hagyard-Davidson-McGee Associates and the Rood & Riddle Equine Hospital in Lexington, KY. The study was supported by the Commonwealth of Kentucky and the Grayson-Jockey Club Research Foundation. Partial support also was provided by the Link Equine Research Endowment.
The study compared over one hundred various characteristics of mares that experienced fetal losses attributed to MRLS in 2001 against mares bred successfully during the previous year. Factors studied included age, parity, feeding practices, pasture characteristics, pasture management and preventative health care practices along with putative risk factors, such as exposure to cherry trees, caterpillars, hemlock, and trace minerals.
Conclusions drawn from the study indicate the following factors increased the risk of MRLS associated early-term abortion:
Feeding hay in pasture during the 4-week period prior to abortion;
A greater than usual amount of white clover in pasture during the 4-week period prior to abortion (and during the spring of 2001 relative to the spring of 2000);
A heavier burden of caterpillars in pastures during 2001;
Prior abortion during the previous 5 years; and
Elk or deer being seen at the premises during the preceding 12 months.
“The most significant result of this study is that some factor in pasture predisposed to the problem and that there were unusual conditions influencing the pastures during 2001,” said Cohen. “Our findings suggest that methods for limiting exposure to pasture during similar extraordinary conditions may be used to prevent or control recurrence of the condition.
“As for ruling out other putative risk factors for MRLS such as cherry trees and hemlock, we had adequate statistical power to detect at least a fifty percent increase in risk for each element, indicating that these were not important risk factors for MRLS-associated early-term abortions and stillbirths.”
The study cites an unusual amount of white clover and heavier than usual number of caterpillars in pastures, indicating that the pasture conditions were atypical during 2001. The results, however, are unable to determine whether white clover or caterpillars directly caused MRLS or were merely indicators of unusual environmental conditions that occurred during the spring of 2001 that resulted in exposure to some toxin(s) or infectious agent(s) in pasture. Because these findings were particular to the unusual environmental conditions that occurred during 2001, restricting access of mares to pastures may be unnecessary under more normal environmental conditions. Restricting access of mares to pasture during environmental conditions similar to those during the spring of 2001, however, would be expected to prevent or limit development of MRLS.
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