MRLS Study Reaches Task Force

A new study has pinpointed several factors that predisposed mares to early fetal loss in Central Kentucky and nearby areas in 2001, but was unable to identify a specific cause. Feeding hay in pasture prior to abortion, a history of abortion in mares, the presence of deer or elk, and an abundance of white clover in pastures were associated with higher incidences of early fetal loss (EFL). The presence of Eastern tent caterpillars and exposure to hemlock or wild cherry trees were not associated with an increased risk for EFL.

According to Noah Cohen, VMD, PhD, MDH, Dipl. ACVIM, of the College of Veterinary Medicine at Texas A&M University, the study showed most importantly that whatever predisposed mares to problems was in the pasture. He also stated, "The conditions that existed in the pastures during 2001 were quite unusual--so keeping mares out of the pasture during 2001 could have helped prevent exposure to the cause of the problem. It doesn't necessarily mean that mares should be kept off pasture in 2002, because it's unlikely that the extraordinary climatic and environmental events of 2001 will recur this year."

Cohen and the other researchers compared more than 100 characteristics of mares which experienced abortions associated with mare reproductive loss syndrome (MRLS) in 2001. The contribution of hemlock and wild cherry trees to MRLS has been debated, but the current study showed that neither was a risk factor for MRLS-associated EFL. "We had enough statistical power to detect even a small elevation in odds of disease," explained Cohen, "but there was no significant difference."

Feeding hay in the pasture during the four-week period prior to abortion was one risk factor associated with EFL. "We believe that it wasn't the hay or the practice of feeding hay, per se," explained Cohen. "Horses that were fed hay in their stalls were at less risk than horses fed hay in pasture. Presumably, hay fed in pasture was fed on the ground, possibly increasing exposure to something in or on the pasture." Ninety-two percent of affected mares were fed hay in the pasture, compared to 48% of the control group (not affected by MRLS-associated EFL).

An abundance of white clover and Eastern tent caterpillars has also been considered as a possible cause of, or contributor to, EFL. However, abundant white clover might be merely an indicator of pasture conditions. And, while there were more caterpillars on farms with EFL, after adjusting for the effects of feeding hay in pasture and the higher amount of white clover, the association with caterpillars didn't remain statistically significant.

The study also showed that mares which had abortions during the previous five years were more likely to have EFL, but the clinical significance of this is undetermined. The presence of deer or elk on the premises during the previous 12 months was associated with an increased risk of EFL, possibly due to environmental factors favorable for such animals (e.g, more heavily wooded areas), but the significance of this also remains undetermined.

Feeding mineralized or non-mineralized salts wasn't shown to be protective against MRLS-associated EFL.

A study will be released soon on the late-term abortions associated with MRLS. That will conclude the three-tier study (the first was on pericarditis; see Article Quick Find #3211 at, and researchers will review the three studies together.

"Despite considerable effort, it is quite possible that the causal agent(s) will remain unknown," said Cohen.

The 43-page report was done for the Kentucky Governor's Task Force on MRLS by Cohen; biostatistician Vincent Carey, PhD, of the Harvard Medical School; veterinary epidemiologist James Donahue, DVM, of Madison, Wis.; and Janyce Seahorn, DVM, Dipl. ACVA, Dipl. ACVIM, of the University of Kentucky Livestock Disease Diagnostic Center. Veterinarians from the Lexington-based equine veterinary firms Hagyard-Davidson-McGee Associates and Rood and Riddle Equine Hospital assisted significantly. The Commonwealth of Kentucky, Grayson-Jockey Club Research Foundation, and Link Equine Research Endowment provided funding and support.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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