Eastern Tent Caterpillars Cause Early Fetal Loss, Too

The case against Eastern tent caterpillars (ETC) causing mare reproductive loss syndrome (MRLS) is getting stronger, say researchers. Gavage (tubing into the stomach) of caterpillar larvae has been shown to cause late-term abortions that are characteristic of MRLS, which hit Kentucky and surrounding states in 2001 and 2002. Researchers recently determined that gavage of ETC also can cause early fetal loss, and that frass (caterpillar excrement) does not cause early fetal loss.

The study was completed by William Bernard, DVM, Dipl. ACVIM, and Michelle LeBlanc, DVM, Dipl. ACT, both of Rood and Riddle Equine Hospital in Lexington, Ky.; and Bruce Webb, PhD, and Arnold J. Stromberg, PhD, both of the University of Kentucky. It was published in the Sept. 1 Journal of the American Veterinary Medical Association.

Since early fetal losses during the MRLS outbreak years happened only in mares that were past 35 days gestation, the researchers used 15 pregnant mares with fetuses from 40-89 days of gestation, assigned randomly to one of three groups. One group received frass, another group received starved ETC, and the control group received water. Mares were kept in stalls, were not allowed access to pasture for 12 days before or during the 21-day trial, and were walked twice daily.

Four of five mares which received the caterpillars aborted, and differences between this group and the other two were statistically significant. Mares from the control group and the group receiving the frass did not abort. Upon ultrasound, the appearance of allantoic fluid (fluid in a fetal membrane that helps form the placenta), the day before or the day of fetal death, was consistent with what was seen on ultrasound of affected mares during the MRLS outbreak.

It appears that frass has been cleared as an MRLS cause. "The toxin or whatever is inducing the abortion is not in the frass," said Bernard. "Some thought that it might be the contents of the (caterpillar's) gastrointestinal tract or something they might be leaving on the grass. A lot of people still have problems believing the mare actually eats the caterpillar."

According to Bernard, some have criticized the study because natural consumption was not used, but Bernard said this was the only way to truly standardize and monitor what the horses received. "This was the first study that showed a statistically significant difference (between affected and control horses) and had a control group that zero of the mares aborted," he said. "We wanted to make sure the caterpillars got to the stomach."

A Question of Bacteria and its Origin
Researchers noted an important difference between the fetuses collected during the MRLS crisis and those examined during this study. While Streptococcus species of bacteria were found in fetuses during the 2001 and 2002 abortions, a Streptococcus species was only found in one of the fetuses aborted in this study. However, Serratia marcescens (a bacterium that occurs naturally in the ground, water, and the intestines) was recovered from three fetuses.

Bernard thinks this is significant and might give a clue as to how the abortions are triggered.

"No one can tell us for sure what is in the caterpillar that's causing abortions. Some people think that it's the hairs of the caterpillar (called setae, where bacteria could be harbored). Some think it's a toxin or chemical, and some think it's a bacteria--not particularly a bacteria in a caterpillar, but a bacteria that's being allowed to invade the horse because of the hairs of the caterpillar (which could penetrate the GI tract lining, permitting the bacteria to enter the lymph or circulatory system).

"There are also suggestions that an ascending placentitis (inflammation of the placenta that starts at the cervix) may be the cause," he said. A bacterium could gain entry into the uterus through a dilated cervix (because of a disease process initiated by the caterpillars), he explained.

"We didn't define whether it's a toxin or a bacteria that caused the abortions, but we did confirm it was the caterpillar," he said.  "There's still a lot of research to be done to determine a definitive cause,"

What's Next?
A study in which Bernard and others irradiated caterpillars will be published soon. "Probably the next thing is for someone to give the (caterpillar) hairs alone (to the horses) and determine whether the hairs are truly causing it," he said, suggesting that one group of mares receive "naked" or hairless caterpillars, and another group receive the setae. He says there is a method to remove caterpillar hairs for research experiments.

From there, researchers could potentially determine the physical part of the ETC associated with abortion and determine the causative component.


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.

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