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Mare Reproductive Loss Syndrome

Following is a statement sent to State Veterinarians May 18, 2001
TO:  State Veterinarians

DATE:  May 18, 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.

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 17, a total of 9 additional aborted/stillborn equine fetuses/foals (5 late-term and 4 early-term) have been submitted for diagnostic testing/evaluation. The total as of this count since April 28, 2001 is 486.
Information on early fetal losses and daily figures on submissions to the UK Livestock Disease Diagnostic Center (LDDC) indicate incidents have fallen over the last seven days. A summary of each day's submissions can be found on the Internet at

The coordinating group has been studying details of mare breeding records for 2001. This identified more precisely the time of the critical insult giving rise to the syndrome, between April 17 and 23. Based on information provided, late abortions, the birth of weak foals, and early fetal losses appear to be linked to the same causal event.

Studies undertaken at the LDDC on equine samples over the previous 3 weeks have found no evidence whatsoever of an infectious or contagious disease.

The list of possible candidates including mycotoxins, fungal endophytes, phyto-estrogens and other compounds in pasture are under intensive investigation. Fusarium mycotoxins remain a candidate with many pasture samples proving positive for zearalenone, an estrogenic mycotoxin that has been shown to cause reproductive problems, cardiac and eye problems in horses and other species. With the notable exception of Moldy Corn Poisoning (equine leukoencephalomalacia), little is known with respect to the toxicological effects of mycotoxins in horses.

Examination of tent caterpillars has revealed high levels of zearalenone. The significance of this finding is not clear. Caterpillars and their excrement have tested negative for cyanide.

We have been receiving calls from individuals in other states inquiring about equine events being canceled or state-ordered quarantines.  To our knowledge, no event has been canceled that is attributable to the MRLS occurrence.  Certainly no event has been canceled at the direction of the Kentucky Department of Agriculture's Office of State Veterinarian.  We are communicating with the sponsors and managers of equine events scheduled to
be held in Kentucky.  Our advice to out-of-state participants is to consult with their State Veterinarian's Office and attending veterinarian regarding any precautions that should be taken.

Regarding information reported that other states are experiencing a similar occurrence of MRLS:  When inquiries are received from the news media, our standard response is that we have received calls from practitioners and horse owners in other states suggesting that they too may be experiencing a similar occurrence. We provide to the State Veterinarian from where the call originated, as much factual information as possible concerning the call.  In addition, we caution the reporter making the inquiry that with the attention this syndrome is receiving, individuals are being much more cautious and are reporting any and all abnormalities being observed in their horses.

We will continue to keep you apprised, as new information becomes available.

E.S. Rusty Ford
Equine Programs Manager

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