Facts Brought to Light in MRLS Study

Four interesting facts were brought to light because of the late-term abortion and Eastern tent caterpillar (ETC) study that was recently completed at the University of Kentucky, according to Dr. Thomas Tobin of the Gluck Equine Research Center:

- Since live Eastern tent caterpillars were shipped in from Michigan for the study, it was proven this isn't a Kentucky problem, but rather a caterpillar-associated problem.
- Evidence suggests late-term abortions are associated with Eastern tent caterpillars; a previous study focused on the association with early fetal losses.
- The losses during this study of late-term abortions came quicker than losses during the early fetal loss study done in conjunction with Rood and Riddle Equine Hospital.
- This study has changed people's minds on how the bacteria might play a role in mare reproductive loss associated with the caterpillars.

On this last point, Tobin said researchers and veterinarians used to think something happened to the mare, possibly the ingestion of caterpillars; the fetus became distressed and/or died; then bacteria took advantage of the situation and thus were found in high numbers on post-mortem examination. He said it is possible that the bacteria are somehow involved after the mare takes in something from the caterpillar but before the fetus becomes distressed and/or dies.

One thing needed to be cleared up from earlier reports. Two bacteria that are commonly found in the horse's environment--Actinobacillus equuli and Streptococcus species, both of which grow best in low-oxygen situations--were often found in naturally occurring late-term abortions. A hodge-podge of bacteria were found in naturally occurring early fetal losses. That was true in the study done with Rood and Riddle Equine Hospital on early fetal loss.

However, in this study the late-term abortions were not associated with a high number of Actinobacillus equuli and Streptococcus species. There were bacteria found, but it was more of a mixture of what is normally found in the horse's environment. Why?

It's possible that the high rate of administration of the caterpillars in this study contributed to quicker abortion than would happen with normal exposure in the field, said Dr. Neil Williams, a veterinarian and pathologist at the Livestock Disease Diagnostic Center. He added that the first five mares which aborted did not have fetuses with symptoms consistent with those seen in naturally occurring MRLS. He thinks that also was due to the quicker than normal abortions in the study. The mare which aborted late in the study did have a fetus that had some changes consistent with MRLS seen in the field.

"That makes sense because we think that it (the loss process) naturally occurs over several days," said Williams.

Symptoms that were consistent in the last fetus that aborted in the study and naturally occurring MRLS were infection of the placenta, inflammation of the umbilical cord, and apparent early detachment of the placenta. He said researchers did not observe cloudy amniotic or alantoic fluids with these late-term abortions as was seen with the early fetal losses.

All of the mares in the study were in their last trimester of pregnancy, said Dr. Deborah Williams, a veterinarian and research specialist hired last fall to help with MRLS studies. She was responsible for the daily examinations on the mares and administration of the caterpillars. She said these were nurse mares of various breeds and crosses brought in from Eastern Kentucky for the experiment. The administration of the caterpillars was modeled after what was done in the Rood and Riddle study, where mares were kept stalled and were given a set amount of caterpillar slurry (experimental) or pure saline (controls) via nasogastric tube.

She noted that the study began on Tuesday, June 25, and the first mare aborted Thursday night. The second mare aborted on Friday. The final mare aborted six days after the end of administration of the caterpillar slurry.

The mares were given basic physical examinations and had blood drawn and sera stored for future study. None of the mares showed any signs of distress or disease during the study (including no pericarditis, uveitis, or laminitis).

Caterpillars for this study were shipped in from northern Michigan by Dr. Dana Richter of Michigan Technical University. Tobin credits Richter for this research project going forward, and for shedding new light on the problem of Eastern tent caterpillars and pregnant mares.

"My sense is that if you put these caterpillars around pregnant horses anywhere, the mares would be at risk," said Tobin.

Neil Williams said that it is possible, and even probable, that this problem has been around in previous years in different parts of North America, but that veterinarians didn't have the technology to detect it. He said while it's not known if reproductive losses are occurring in other species, there have been a couple of cattle operations that reported calf losses. "But calving season is different from foaling season, so they aren't at risk during the time of heavy caterpillar populations," said Williams.

While there is no documented evidence that Eastern tent caterpillars are any more than irritating to humans, researchers are being careful when handling them. They are wearing gloves and masks in the laboratory.

What's next on the research front? What actually is the connection between the ETC and MRLS? What is the "dose/response" where a certain amount of exposure to caterpillars causes problems and lower than that does not? Is there a way to prevent the losses aside from eradicating caterpillars? These are all questions to be answered with future research.