Irritating Caterpillars

While we all were irritated to some extent by the massive amounts of caterpillars in 2001 and 2002--stepping on them, having them climb our fences, houses, cars, gates, and everything else around our homes and farms--there is new evidence that the setae (hair-like projections) on the caterpillars can become embedded in the lining of the alimentary tract (GI tract) of animals and cause inflammation, discovered Neil Williams, DVM, PhD, of the University of Kentucky’s Livestock Disease Diagnostic Center.

Williams performed the examinations and histological examinations of the pigs used in an experiment where the animals were fed Eastern tent caterpillars (ETC) in an effort to cause abortions similar to MRLS (mare reproductive loss syndrome) in broodmares. The abortions did occur as a result of feeding ETC to the pregnant pigs, and the pigs were studied at the Diagnostic Center.

“As we examine tissues, we learn to look for ‘unusual’ things,” said Williams. While examining samples from the alimentary tract (which includes the pharynx, esophagus, stomach, small intestine, and colon), Williams noticed multiple small, localized areas of inflammation in some of the pigs. These were not visible with the naked eye, and he noticed them only in some of the pigs. Williams was “blinded” and didn’t know which pigs had been fed caterpillars. So, he noted in which pigs he found the lesions.

He discovered that all five pigs which had been fed caterpillars had these microgranulomas (small, localized areas of inflammation only able to be seen under a microscope), and none of the pigs that had a regular diet free of ETC had the lesions.

Further study showed that many of the microgranuloma contained a small, hair-like fragment that didn’t look like a mammalian hair. He obtained some ETC and confirmed that the hair-like fragment in the microgranulomas were identical to ETC setae in appearance.

“The setae are passing through the alimentary tract and embedding in the submucosa and causing an inflammatory reaction,” explained Williams. “I saw this multiple times in various sites throughout the alimentary tract.”

He said the setae penetrated the protective mucosal layer and embedded in the submucosa, where the reaction was intense. Williams didn’t find the setae in the muscle layer, which is the next layer beyond the submucosa.

He said the reaction that caused the microgranuloma is like when a splinter gets into your finger. “It’s a foreign body-type of reaction,” he explained. “The body tries to wall it off.”

Williams said he only found these microgranulomas in the submucosal layer of the alimentary tract, and he didn’t see them in any other organs or in placentas or fetuses of the pigs.

See microgranulomas and other caterpillar images here

He noted that the mucosal layer, among other things, functions as a protective barrier to the highly vascular submucosal layer. The bulk of the blood supply to the alimentary tract lies in the submucosal layer, as does the lymphatic supply to the digestive tract. This breech of the protective barrier might offer a means of entry for bacteria into the bloodstream.

“We don’t know if this is leading to abortions, or if the reaction is just there because the pigs were consuming caterpillars,” said Williams. “There are plans in the immediate future to look for these types of lesions in the mare.”

It should be remembered that in most experiments with ETC and mares, a “slush” of caterpillars was tubed directly into the stomach of the mares. Those experiments revealed that it is something in the integument (outside) of the caterpillar that could cause abortions in mares. (For more information on this research see article #2690.)

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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