Confirming the suspicions of many frustrated parents, Harvard University public health experts have proven that pesky head lice have grown resistant to chemical treatment.
A new study shows about a 90 percent survival rate among a population of 209 lice plucked from the heads of U.S. kids and placed inside petri dishes containing the chemical permethrin, the active ingredient in Nix, the leading treatment for louse infestations.
All children in the study had been treated previously with the chemical.
By comparison, the chemical killed 66 of 67 lice harvested from the heads of kids living in the remote Malaysian town of Sabah, Borneo, who were never before exposed to the chemical.
One louse apparently escaped.
"It is evident that lice infesting [chemically exposed] U.S. children are less susceptible than are lice infesting untreated Sabahan children," according to Richard J. Pollack and his research team, who publish their findings in the September issue of the Archives of Pediatric and Adolescent Medicine.
"The frequency of resistance seems to have increased."
Every year, an estimated 6 million American children are infested with the wingless, six-legged crawlers on their scalps.
The general prevalence of resistance to permethrin has not been determined, the Harvard team cautioned.
The resistant lice were found in two random samples of children from Cambridge, Mass., and Boise, Idaho -- but this does not mean that all, or even most, lice in the United States are resistant.
For that reason, permethrin and related chemicals called pyrethrins remain the treatment of choice for newly identified infestations, Pollack said.
"If live lice persist following such treatments, then one may consider that these lice may be resistant," Pollack wrote, "and further treatment may be warranted with other insecticides."
Increasingly, parents swap horror stories of "super lice" that refuse to respond to treatment, despite diligent shampooing, nit-picking and cleaning.
It can cost a family $60 or more to treat a single infestation.
In desperation, some turn to such home remedies as mayonnaise, olive oil or margarine.
An Iowa girl suffered severe burns when her mother doused her head with gasoline. An Oklahoma girl became severely ill after her hair was soaked in agricultural-strength insecticide.
But experts at the Centers for Disease Control and Prevention in Atlanta, as well as most pediatricians and the manufacturers of de-lousing chemicals, have insisted that there is no definitive evidence of "super lice" in the United States -- although such bugs have been identified in other nations.
Insecticides do not cause mutations that lead to resistance, according to the Harvard scientists.
But in Darwinian fashion, bugs that survive exposure to the chemical because of some ability to avoid, detoxify or eliminate the toxin are able to endure -- while the weaklings succumb.
This leads to the emergence of a more resilient strain.