Foal Loss Possible in Caterpillar Areas

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 Thomas Tobin, MVB, MSc, PhD, MRCVS, Dipl. ABT, of the Gluck Equine Research Center:

  • Since live ETC were shipped in from Michigan for the study, it was proven that this isn't a Kentucky problem, but rather a caterpillar-associated problem.
  • Evidence suggests that late-term abortions are associated with ETC; a previous study focused on the association with early fetal losses.
  • The losses during this study of late-term abortions came more quickly than losses during the early fetal loss study done in conjunction with Rood and Riddle Equine Hospital.
  • This study has changed opinions on how 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 that 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.

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

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 Neil Williams, a veterinarian and pathologist at the Kentucky Livestock Disease Diagnostic Center. The first five mares which aborted did not have fetuses with symptoms consistent with those seen in naturally occurring mare reproductive loss syndrome (MRLS). The mare which aborted late in the study had a fetus that had some changes consistent with MRLS seen in the field. Williams also said that researchers did not observe cloudy amniotic or allantoic 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 Deborah Williams, a veterinarian and research specialist hired last fall to help with the university's MRLS studies. She said these were nurse mares of various breeds and crosses brought in 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; the first mare aborted Thursday night; the second mare aborted on Friday; and 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.

Caterpillars for this study were shipped in from northern Michigan by Dana Richter, PhD, of Michigan Technological University. Tobin credits Richter for this research project going forward.

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

What's next on the research front? What is the connection between the ETC and MRLS? What is the "dose/response" where a certain 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. However, Williams and Tobin emphasized that within a year of a catastrophic problem taking place, many answers have been forthcoming due to diligent management and scientific research.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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