On April 26, 2001 Dr. Tom Riddle, a Kentucky equine practitioner, was ultrasounding some 60 day fetuses for sex determination when he noted two without heartbeats and, on the next farm, two fetuses dead in utero. This was the beginning of the 1,500 Early Fetal Losses (EFL) recorded in Kentucky broodmares in 2001. Companion mini-epidemics were reported, all together now known as Mare Reproductive Loss Syndrome (MRLS) with economic losses estimated at US $500,000,000 for 2001 and 2002.
Along with the appearance of MRLS were unusual concentrations of Eastern Tent Caterpillars; ongoing investigation pointed to their involvement for the simple reason that areas lacking caterpillars also lacked MRLS. The only toxin clearly identified with the caterpillars was cyanide, from their black cherry tree leaf diet. As the epidemic wound down in May 2001, the official “working hypothesis” was that cyanide via the caterpillars was the culprit of MRLS.
Caterpillar control measures were recommended including the removal of Black cherry trees. Eastern Tent Caterpillars would not be available again until Spring, so researchers tested the ability of everything but caterpillars to produce abortions. However, equine fetuses proved extremely resistant to attempts to induce abortions, including liberal exposure to cyanide, so the 2002 caterpillar season started with the cyanide hypothesis largely discounted.
When the 2002 caterpillars arrived it was soon shown that exposure of pregnant mares to caterpillars readily produced early and late abortions. Numerous possibilities were evaluated and ruled out. Ultimately, after painstaking research, the barbed bristles on the caterpillars seemed to be the culprit, with experiments in pigs and rats offering strong support of this hypothesis. Research at the Gluck Equine Center had very rapidly linked MRLS to the caterpillars, allowing focused and effective control measures to be efficiently put into place.
Caterpillar related abortion syndromes are not restricted to Kentucky . This spring cases of MRLS were reported in Florida , and a similar syndrome called Equine Amnionitis and Fetal Loss occurred in Australia in '04. Based on the Kentucky work on MRLS, Processionary caterpillars were listed among the possible causes of EAFL in keeping with the MRLS model and experimental work in Australia has shown equine abortions can be induced by administration of Processionary caterpillars. Additionally, there are suggestions in field data from Florida that other caterpillars may be associated with equine abortions, especially if caterpillar concentrations are high.
It takes money to do fine research but in the long run, these investments are miniscule when compared to the equine industry's economic impact and the importance of the health of our horses, and TCA is proud to be a part of the funding. Grants, coupled with cooperative efforts between Gluck Equine Research Center, the Department of Entomology, the Lexington Animal Disease Diagnostic Laboratory, equine practitioners and especially Rood and Riddle Equine Hospital is what solved this terrible mystery so quickly and we can be forever grateful and hopefully, generous.