Update on the Eastern Tent Caterpillar

The newest study on early fetal loss and Eastern tent caterpillars (funded by the Grayson-Jockey Club Research Foundation) began on May 21 and was scheduled to continue for three weeks. The research is led by internal medicine specialist Bill Bernard, DVM, Dipl. ACVIM, president of the Kentucky Association of Equine Practitioners; and reproductive specialist Michelle LeBlanc, DVM, Dipl. ACT, both of Rood and Riddle Equine Hospital in Lexington, Ky. (see Article Quick Find #3591 at www.TheHorse.com).

In researching more information about Eastern tent caterpillars, we again turned to Terry Fitzgerald, PhD, a distinguished university professor of biological sciences at the State University of New York College at Cortland. Fitzgerald, who literally wrote the book on the Eastern tent caterpillar, spoke to LeBlanc about using the last instar (last phase of the larval or caterpillar stage) in the Kentucky study, which is what the researchers are using. The last instar caterpillars are the ones who leave the trees and wander across fields to find a place to spin a cocoon.

These caterpillars have five "molts" (shed their skin) while in the nest. The Eastern tent caterpillar sheds its external cuticle (exuviae) and leaves pieces in the tent. There are about 300 caterpillars in each tent, so five molts results in 1,500 exuviae or fragments thereof. It's possible that these fragments of the shed cuticle might play a part in mare reproductive loss syndrome (MRLS) since nests that have frass (excrement) used in the studies also have exuviae, including hairs.

It is known that the setae or hairs of caterpillars, including the Eastern tent caterpillar, can irritate other animals. Fitzgerald said there are about 150 types of caterpillars worldwide with irritating hairs, and that the Eastern tent caterpillar's hairs are usually not particularly irritating.

However, he added, it's known that a person can develop sensitivity with prolonged exposure and have a stronger response to the irritant.

Fitzgerald finds it interesting that research in Spain and the United States in the past few years has pointed to immune responses in humans caused by caterpillars (not Eastern tent caterpillars). He said immune responses are different from toxins or irritants because proteins in the hairs or on the cuticle serve as allergens. Human symptoms of caterpillar irritation can include welts on the skin, eye irritation, and even anaphylactic shock (severe, possibly fatal systemic reaction upon exposure to a specific antigen) in some people.

"It's a reasonable working hypothesis that contact with the setae and cuticle of the caterpillar could cause an allergic reaction," said Fitzgerald.

LeBlanc said Kentucky researchers who come in contact with caterpillars and frass are taking precautions with gloves and masks. There is still concern that the route of the irritant or toxin isn't known, so it could be by ingestion, inhalation, contact, or a combination.

Fitzgerald also said that the last-instar caterpillars--the wanderers--aren't the same as previous stages or instars in that they contain a toxin in much larger quantities than occurs in previous instars. He said the Malpighian tubules (excretory tubules similar to kidneys that store waste) accumulate a yellowish substance that will be used to create the cocoon (the material is about 90% calcium oxylate monohydrate and incorporated into the cocoon as crystals, which helps stiffen the structure). "This is a substance that has been shown to be irritating to people, causing skin or respiratory irritation," said Fitzgerald. "This last instar is significantly different from earlier instars."

Fitzgerald said he doesn't think it has been definitively proven that Eastern tent caterpillars are causing MRLS, but the Kentucky study could go a long way in doing so.

Fitzgerald's previous work on caterpillars points to the necessity of basic research. For example, research on how far Eastern tent caterpillars can go once they leave the nest could have proven valuable in looking at infested trees and occurrence of MRLS. He also did basic research last year into the cyanide content of Eastern tent caterpillars and their frass, helping prove that cyanide is a less likely candidate for causing MRLS than previously thought.

It's important that the horse industry remember that funding basic research is important to understanding future problems.

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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