Caterpillars Cause Early Fetal Loss (Q & A)

A research project has identified an association between Eastern tent caterpillars and early fetal loss in mares (see Principle investigators in the study were Bill Bernard, DVM, Dipl. ACVIM, a specialist in internal medicine, and Michelle LeBlanc, DVM, Dipl. ACT, a specialist in equine reproduction, both at Rood and Riddle Equine Hospital in Lexington, Ky.; and Bruce Webb, PhD, a specialist in entomology at the University of Kentucky. (Others involved in the research are listed in the article linked above.)

Following are some questions and answers from Bernard and LeBlanc about the research.

Q: Why did you do this study?

Bernard: Because of the importance to the industry to show what is causing MRLS (mare reproductive loss syndrome). It was initiated by dr. Webb’s field trial that suggested caterpillars or frass (excrement) was associated with early fetal loss. But from that project, you couldn’t tell if it was the caterpillars, the frass, or maybe something in the grass. (Webb’s study was done in enclosed grass pens).

LeBlanc: There also was clinical data we wanted to evaluate. We did a physical exam on each mare every eight hours. We scanned them (performed reproductive ultrasound examinations) every day to see if there were problems with the pregnancy, and if so, were they happening quickly or over time. We did daily blood work to see if there are any parameters that can be checked on the farm to identify mares <I>before<I> they lose their pregnancies.

Q: Will those blood tests work?

LeBlanc: We still need to do stats on the CBC (complete blood counts) and the blood to see if anything is statistically significant.

Q: Did anything surprise you in the study?

Bernard: How quick they lost their pregnancies. That might have been because of the number of caterpillars we used.

LeBlanc: Maybe we did overload their systems.

Bernard: But we didn’t want to give one caterpillar and someone say that wasn’t enough to cause problems. I also thought it was the frass, not the caterpillars. We went into the project with the hypothesis that caterpillars or frass cause early fetal loss.

LeBlanc: I was the skeptic on the losses we would see. I thought the route of invasion was inhalation or through the mucous membranes.

Bernard: I learned you have to keep an open mind.

Q: What were you looking for when you did the ultrasound and other exams?

LeBlanc: With ultrasound, we looked for three primary changes we reported clinically. One was fetal death. We also looked at the allantoic and amnionic fluids, and we watched them (in mares which aborted) go from black (the normal echo for fluid) to being white from particulate matter. And it happened very quickly. It wasn’t a gradual change. That could be related to dose.
With the blood, we want to know if blood samples can be taken on the farm to give any indications of mares at risk of losing their pregnancies.

Bernard: The blood work was usually non-specific. For example, if the white cells count is elevated, it’s not diagnostic. We need to monitor the liver enzymes and renal functions that might give us an indication of the toxic components of the Eastern tent caterpillar. We need to know what the toxic compound is.
LeBlanc: We stored a number of samples each day from each horse so they can be used in future studies.

Q: What’s next?

Bernard: There still are questions as to whether we used starved caterpillars or whether they had any fresh frass. It’s possible they still had some frass in them.

LeBlanc: It’s a misnomer to call what we used in the study frass as it not only contained frass, but the molted skins of the caterpillars. We need to compare the contents from nests to fresh frass in the laboratory and test how they are different and what is lost in storage. (The frass used in the study was frozen at -80° C and 5° C.)

Bernard: We also need to take some things in the caterpillar and give it to pregnant mares, such as the calcium oxylate. This is a toxin in certain species, but there are no reports of association with early embryonic loss.

(Editor’s Note: the last instar stage of the caterpillar—the wandering caterpillars—are not the same as previous stages or instars in that they contain a toxic material found in much larger quantities than occurs in previous instars. The Malpighian tubules--which are excretory tubules that are analogs of kidneys and store waste—accumulate a yellowish substance that will be used in the creating the cocoon [the material is incorporated into the cocoon as crystals, which serves to stiffen the structure]. This substance is about 90% calcium oxylate monohydrate. Some people are very sensitive to this substance. For more see

LeBlanc: Mares that abort do it early, between 40 and 120 days, when the endometrial cups have formed and when the placenta is forming. We need to look at changes in cytokines in the blood because the toxic substances from the caterpillars may adversely affect the ability of the placenta to attach. It is interesting that the early losses occurred during placental formation and that the late losses involved early detachment of the placenta from the uterus. The placenta is the common denominator in both scenarios (She noted that this study only looked at early fetal losses, not late-term abortions.)

Four of the five mares given caterpillars aborted. Only one of those had Alpha streptococcus, and three others had Serratia, which also is an opportunistic bacteria. So, we ask, was that bacteria in the hairs of the caterpillar, was it picked up from the soil and did it get carried into the mare? I have only seen one Serratia abortion in 20 years before this study. I’m of the opinion it was opportunistic and was just carried in by the caterpillar.

Bernard: We need to find out what is in the caterpillar that is causing the problems.

LeBlanc: We need a large toxicology lab to get involved as well as the farms.

Bernard: We need to finalize proof that the caterpillar is the cause. We need to prove what it is in the caterpillar that causes early fetal loss and prove that.

LeBlanc: Can we cause early fetal loss with a mare ingesting caterpillars? Yes. Now we need to fulfill “Koch’s Postulate” to find out what chemical, protein, or compound is the problem and feed that to pregnant mares.

Bernard: We need the funding to do this. It was disappointing to me that last year I went to meeting after meeting and people said money was no object in solving this problem. This year we needed money, and it was hard to find. The funding needs to be regulated by a scientific panel, like Grayson-Jockey Club (Research Foundation).

LeBlanc: We need protocols and channels to follow.

Bernard: The funding needs to be made to solve what the toxic component of the caterpillar is, investigations into the caterpillar itself, calcium oxylate, and estrogenic properties found in caterpillars. I’d love to jump right into looking at the calcium oxylate question, but we need funding.

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.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners