Mare reproductive loss syndrome (MRLS) has been a primary research project for many veterinarians and scientists since it began causing early and late term abortions, sickness in foals, pericarditis (heart problems), and uveitis (eye problems) in horses in the spring of 2001. Manu Sebastian, DVM, MS, a resident in Veterinary Pathology, and a PhD student at the University of Kentucky's Gluck Equine Research Center, recently presented a history of MRLS and resulting research, and he also described two studies that had not yet been presented to the veterinary community.

After reviewing clinical signs of MRLS, Sebastian listed the many different hypotheses that have been suggested as the cause of MRLS, including mycotoxins, calici virus (a virus causing lesions in sea lions and pigs), cyanide, poison hemlock, tall fescue, ammonia, and Eastern tent caterpillar (ETC) setae (hairs) and toxic molecules. The ETC have been implicated as the probable cause for the syndromes.

Sebastian described a number of studies performed that involved experimental induction of MRLS, and he described a new study. Sebastian and Lenn Harrison, VMD, the head of the University of Kentucky's Livestock Disease and Diagnostic Center (LDDC), posed the question, "Is there any agent that can get into the system of the mare by inhalation?"

In the study, four pregnant mares wore feeding masks with special double-layered wire sieves, behind which live ETC were kept (the caterpillars could not be eaten by accident due to the double layer). The mares wore the masks every day for six hours. All mares were examined daily via ultrasound to monitor fetal status. On day four, one of the mares dunked her muzzle in a water source, drowning the caterpillars, and shortly thereafter, this mare aborted, with pathology of the aborted fetal tissue being similar to that of MRLS cases.

In a follow-up study, ETCs were submerged in water for an hour and the resulting extract, or fluid after the organic matter, was strained and administered via nasogastric tube to four pregnant mares. They were checked for nine days, and on day six, one mare aborted and had to be euthanized due to a dystocia. There were no significant ultrasound findings in the other three mares on day nine. Using these small numbers of horses, it can be difficult to draw conclusions as to whether or not MRLS was definitively caused by the extract; however, inhalation is probably not the route of exposure to the ETC factor/agent.

While MRLS was present in 2002, the 2003 breeding/foaling season was much less eventful for MRLS. Sebastian said, "There were less than 20 abortions consistent with MRLS (in 2003), there were two cases of pericarditis in northern Kentucky, and there was no uveitis reported. The ETC population crashed, because most egg masses were destroyed in the winter and early spring."

This re-emphasizes that MRLS is directly correlated with the ETC population, he said.

"Basically as far as MRLS is concerned, it's over for clinicians and horse owners," Sebastian said.

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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