Lousy news: Pesticide resistance.

Head lice plague children the world over. These parasitic bugs, however, are not equally vulnerable to some modern delousing shampoos. U.S. lice are more likely to survive a dousing than are the parasites in Sabah, Malaysia, a new study finds. This suggests the Western bugs are becoming resistant to at least one of the most popular delousing pesticides.

Richard J. Pollack of the Harvard School of Public Health in Boston and his colleagues collected lice from the heads of 75 U.S. children, most of whom had been unsuccessfully treated, and another 58 kids in Malaysia. The researchers then housed the critters in dishes lined with permethrin-impregnated paper. Shampoos containing this relatively nontoxic pesticide have become popular delousing treatments in the United States but are not used in Sabah.

While virtually all Malaysian lice quickly succumbed to both small and large doses of the pesticide, almost none of the lice from the U.S. children did. Indeed, Pollack says, for the U.S. lice, "if a little permethrin wasn't effective, neither was a larger dose." The message for parents, he and his colleagues report in the Sept. 15 ARCHIVES OF PEDIATRIC AND ADOLESCENT MEDICINE, is to be ready to switch shampoos. If one doesn't rout the bugs, change to a treatment with a different active ingredient. Also, when possible, parents should religiously groom infested hair with louse-catching combs, the researchers say.