Three concerned equine practitioners visited the Gluck Equine Research Center the first part of May 2001 to discuss what they had discerned as unusual activity in their broodmare practices. The rest of May and early June encompassed our initial exhausting experiences with mare reproductive loss syndrome (MRLS). Through it all, we learned a great deal not only about the horses we care for, but also about ourselves. Mostly, we learned that we need one another--owners, managers, practitioners, researchers, and consultants--because only a strong cooperative effort would allow us to meet the challenge of MRLS.

May and June could be characterized as surveillance and damage control on a 24-hour basis. Following the initial release of MRLS descriptions, we spent long hours examining and rechecking early-bred mares for signs of change on ultrasound examination as managers worked tirelessly. This was all too often an exercise in frustration; we watched helplessly as mares aborted in clear sight on an ultrasound screen. This frustration was only multiplied by long nights of complicated foalings. As practitioners, we stood alongside managers and dedicated foaling staffs disgusted that all too often our best efforts only produced a compromised neonate and a distressed mare.

There were two other clinical entities associated with MRLS. Some young horses and adults showed signs of lethargy and depression that resulted from copious amounts of fibrinous fluid around their hearts (pericarditis). Simultaneously, veterinarians saw young horses with an unusual eye condition occurring in only one eye, with diffuse yellow discoloration consistent with a profound and non-responsive uveitis.

As we dealt with MRLS, countless people who support our industry toiled at the Livestock Disease Diagnostic Center and the Gluck Center. Dedicated pathologists and their assistants processed unimaginable numbers of specimens to rule out known causes for these events and to help shed light on the pathologic findings. Specialists from the Gluck Center processed farm surveys to gain epidemiological information. UK's Agronomy Department looked at pastures, forage samples, and soil analyses to characterize the environmental settings of these unusual events.

We left 2001 with about 2,500 early-term abortions, over 500 late-term abortions, about 60 pericarditis cases, and about 40 confirmed unilateral uveitis cases. MRLS touched us all, and some more than others; it broke not only the spirit, but the purse strings of many farms. We accumulated information on risk factors--from weather patterns, to host plants, to unusual insect populations, to time allowed exposed to the environment--all of which had exceptions to refute their validity. We entered 2002 armed with new recommendations and a monitoring program by the UK College of Agriculture hoping to never need any of them, and wondering if something so devastating could return.

Unfortunately, by the last week of April 2002, several practitioners were convinced that there were early fetal losses due to MRLS based on ultrasound findings. We feverishly met with colleagues, consultants, and farm management staffs to compare affected versus unaffected premises. This time, we were able to define certain locations on farms and between farms with higher loss rates. At this time, we received our best "compass heading" as a clinical trial by Drs. Bruce Webb and Karen McDowell at UK indicated that exposure of pregnant mares to Eastern tent caterpillars (ETC) and their frass (excrement) reproduced the ultrasound findings consistent with our field cases. With this evidence, many of us could correlate our higher loss rates in mares to locations where we noted higher numbers of caterpillars.

Over the following months, numerous trials were undertaken to reproduce the clinical signs of MRLS in both early- and late-term mares with direct exposure to caterpillars. We have yet to successfully replicate any of the non-reproductive syndromes with these experiments. In 2002, we experienced fewer clinical numbers than in 2001, which held up well in our comparisons to the countless number of ETCs on which we trod upon in the previous year compared to the number of insects seen in 2002 (although certainly plenty for many farms!). While a piece of the puzzle finally seemed to fall into place, several very important questions remain. There is still concern about the precise route of insult by the ETC. Although we feel confident that ingestion is supported, how many insects would have to be eaten? Could these ETCs amplify toxic factors in the environment? What toxic factor(s) could be associated with the ETC? What is needed to economically and efficiently control their levels on farms?

The contributions of many will be necessary to completely understand the MRLS mystery. MRLS touches a vast layer of the industry, including horse owners, managers, maintenance staff, foaling attendants, pathologists, agronomists, entomologists, researchers, and veterinarians. We all hold keys to understanding this syndrome through observations in field and laboratory. We must communicate effectively with one another to understand and prevent future losses to MRLS that our industry can ill afford.

About the Author

Stuart E. Brown II, DVM

Stuart E. Brown II, DVM, is a veterinarian with the Hagyard-Davidson-McGee veterinary firm in Lexington, Ky., who limits his practice to equine reproduction. Besides being a partner at HDM, Dr. Brown also specialize in public sales auction work as part of his thoroughbred practice.

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