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Mare Reproductive Loss Syndrome

Following is a statement sent to State Veterinarians May 23, 2001
TO:  State Veterinarians

DATE:  May 23, 2001

SUBJECT:  Mare Reproductive Loss Syndrome - Update

Dr. Don Notter, Kentucky State Veterinarian, has asked that I provide to you the latest information regarding the Mare Reproductive Loss Syndrome (MRLS) occurring in Kentucky's equine population.  The following is a summary of information concerning the ongoing investigation.  You can find links to the most current information regarding the investigation at

Dr. Lenn Harrison, director of the Livestock Disease Diagnostic Center in Lexington, reports:

As of noon May 22, one early-term aborted fetus was submitted for diagnostic testing/evaluation. This brings the total count since April 28, 2001 to 529. A summary of each day's submissions can be found on the Internet at
As was previously indicated, the coordinating group has identified the time of the critical insult giving rise to the syndrome as having occurred between April 17 and April 23. In addition, you were advised that based on information provided, late abortions, the birth of weak foals, and early fetal losses appear to be linked to the same causal event. There continues to be no evidence found of an infectious or contagious disease.

Discussions of results obtained have caused the investigation to shift to a more thorough examination of the potential role of the Eastern Tent Caterpillar (ETC) in MRLS. Although several potential causal agents have not been totally eliminated, mycotoxin test results obtained to date have been mostly negative. Investigations into the role of ergot alkaloids are also yielding results that do not fit the profile of the causal agent. The potential role of both mycotoxins and ergot alkaloids have not been totally ruled out at this point in the investigation and either may still play a
contributing role to the cause of MRLS. Examinations of caterpillars and their excrement have initially tested negative for cyanide, but have revealed high levels of the mycotoxin, zearalenone.

Visual correlations on several farms that have experienced MRLS show the presence of wild cherry trees (ETCs prefer wild cherry trees as their primary host) in or near fields where affected mares were grazing.  It remains unknown if there is a one-to-one correlation between the presence of wild cherry trees and incidences of MRLS. A very in-depth look is now under way to determine if and how ETCs or wild cherry trees have roles in the cause of MRLS.

Heavy infestations of ETC were reported last year (2000) as well as this year.  In review of past UK publications of Kentucky Cooperative Plant Pest News and Kentucky Pest News, the last heavy infestations of ETCs occurred in
Central Kentucky during 1979, 1980 and to a lesser extent in 1981. Early fetal loss in mares had reportedly occurred in the area with an undefined causal agent in both the 1980 and 1981 foaling seasons. You can review similarities in weather conditions of the three years (2001, 1981 and 1980) over the Internet at the following web-site:

Again, the coordinating group believes the critical insult that gave rise to MRLS occurred between April 17 and April 23. Central Kentucky experienced a very cold March, followed by a very warm April, with frost/ freeze occurring
on April 17-18 (refer to the weather patterns). It is believed that ETCs were moving during this time period as they had defoliated trees and were not yet full grown.

The coordinating group wants to emphasize that the cause for Mare Reproductive Loss Syndrome has not yet been determined and many options are still under consideration and investigation as to its cause. There will be an information sharing session on Thursday, May 24 similar to the session held previously on May 10. The session will begin at 5 p.m. at the Keeneland Sales Pavilion. The entire session can be viewed live over the Internet from
the web-site:

We will continue to keep you apprised as new information becomes available. For more detailed information about ETCs, go to the web site:

E.S. Rusty Ford
Equine Programs Manager

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