Caterpillar Update

The newest study on early fetal loss and Eastern tent caterpillars began on May 21 and will continue for the next three weeks. The research is funded by the Grayson-Jockey Club Research Foundation and headed 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

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 book on the Eastern tent caterpillar, said he had spoken to LeBlanc about the Kentucky study. Fitzgerald said he thought it was important to use the last instar (last phase of the larval or caterpillar stage), which is what the researchers are using. The last instar caterpillars are the ones who leave the trees and wander across the fields in search of a place to spin a cocoon.

Caterpillars have five “molts” while in the nest. A molt is like a snake shedding its skin. The Eastern tent caterpillar sheds it’s external cuticle (exuviae) and leaves pieces in the tent. There are about 300 caterpillars in each tent, so with five molts that means 1,500 exuviae or fragments thereof. It is possible that these fragments of the shed cuticle of the caterpillar might play a part in the 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 cause irritation to other animals. The harmful effects of moths or butterflies (or their larvae) on people is called lepidopterism. (The term “erucism” refers to the deleterious effects of the caterpillars on humans). 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 is known that a person can develop sensitivity with prolonged exposure and have a stronger response to the irritant. For example, Fitzgerald said he and his lab assistants dissected 35,000 caterpillars for a study and by the end of the project, everyone had rashes on their hands and arms.

Fitzgerald said that it is interesting that research in Spain and the United States in the past couple of 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. Symptoms in humans from these caterpillars can include welts on skin, eye irritation, and even anaphylactic shock in some people.

“It’s a reasonable working hypothesis that contact with the setae (hairs) and cuticle of the caterpillar could cause an allergic reaction,” said Fitzgerald.

LeBlanc said researchers in Kentucky who come in contact with the caterpillars and their frass are taking precautions by wearing gloves and masks. There is still some 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 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. He said 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. Fitzgerald said some people are very sensitive to this substance.

“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 proved 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 walk 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 that previously thought.

“The essence of science lies in testing hypotheses,” said Fitzgerald.

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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