Toxicology: Beware the Woolly Worm

Most adults dislike caterpillars for vague, undefinable reasons, while most children like to stroke their cute, fuzzy backs. The adults are right. At least 50 species, among the hundreds of caterpillars in the U.S., are a hazard to health simply because some of the long and often colorful hair on their backs is irritating or even poisonous to the touch.

The worst offender, say Dr. Campbell W. McMillan and Dr. William R. Purcell in the New England Journal of Medicine, is the caterpillar that grows into one of the flannel moths, Megalopyge opercularis. Country folk use so many other names that they have confused the issue. In North Carolina it is usually the “woolly slug,” in Texas it is often “woolly worm,” and in between it may be the puss caterpillar, possum bug, or Italian asp. In Mexico it becomes el perrito, or little dog. By any name, it stings.

Houston doctors report that there seem to be epidemics of woolly-worm stings every four or five years, when the moths, and therefore their caterpillars, are especially numerous. In one recent year, Houston area doctors reported 2,130 cases; almost every one involved severe local pain and local swelling. One patient out of three had swelling of the lymph glands and a headache too severe to be relieved by aspirin. One in 20 went into shock, and eight patients had to be hospitalized, mainly for convulsions. Children are not the only victims: a Houston man was stung by a woolly worm’s long back hairs when he picked up his golf bag; soon his whole left arm was throbbing with pain up to the armpit. Even with Demerol and Benadryl, he was still in pain and had a headache the next day.

The woolly slug is concentrated in eleven states from Maryland to Missouri and Texas, but it has close kin in the Northeast: the caterpillar of the white moth, Lagoa crispata. Other common stingers are the range and saddleback caterpillars, and those of the buck, lo, tussock and brown-tail moths. Where the caterpillars are especially abundant, their hairs may fly through the air in such numbers as to bring on asthma attacks in children who never even touch the beast directly.