Sep. 13, 2002 AMHERST, Mass. – Head lice, which crop up on the heads of six-to-12 million people in the U.S. every year, are becoming increasingly resistant to the special shampoos used to treat them, says University of Massachusetts Amherst toxicologist John Clark. He and researchers in his lab are trying to solve the problem by studying head lice reared on an artificial scalp designed in conjunction with researchers at the University of California-Davis. The project, funded by the National Institutes of Health, was presented last month at the meeting of the American Chemical Society in Boston.
"I get phone calls from parents and school nurses who are very stressed out, saying that the over-the-counter treatments just aren't working," said Clark. Since most schools will not allow children back in the classroom until all of the lice and their eggs, or nits, are eradicated, kids are missing out on school, and parents often must stay home from work to care for them until the infestation is cleared up.
Most commercial anti-louse shampoos are based on the pyrethrins, an extract that is derived from chrysanthemums which has low toxicity to humans. The problem, according to Clark, is that similarly acting compounds, such as DDT, have been used for more than 60 years in getting rid of lice, and more and more of the insects, which live only on human heads, are surviving the treatments. Essentially, lice vulnerable to the treatments are being slowly killed off, and those that can withstand it are surviving and reproducing, creating a strain of hardy survivors. That translates into a best-case scenario for the lice, and a worst-case scenario for humans, said Clark, who points to studies finding that 50 to 98 percent of the parasites remain after being exposed to these insecticides.
"We need to look at alternative treatments and alternative compounds, because we may reach a point at which the treatments we're using now just won't work at all," he said. The products have relied on one group of compounds that act on one function: the insect's ability to use sodium to produce a nerve impulse. This has resulted in a common resistance mechanism for the natural pyrethrins and synthetic pyrethroids, such as permethrin. If alternative compounds were identified, people would be better able to treat infestations effectively, he says.
But studying the human head louse has presented a problem: the louse doesn't live anywhere but on a human scalp. "If you remove the louse, it dies,"said Clark. Scientists, no matter how dedicated, were understandably unwilling to allow lice to live on them, or to ask human subjects to do the same. Thus, an artificial habitat for head lice now enables scientists to breed and study the pests on a genetic and molecular level.
The "artificial feeding system" looks like a mini-refrigerator, but it is in fact an incubator kept at 31 degrees Celsius and 75-percent humidity. Inside sits a small plastic container, through which human blood is pumped, at body-temperature. The lice reside on an artificial membrane in test tubes dangling in the blood. Researchers recently added small, upright thatches of human hair to the apparatus, which the lice responded to with enthusiasm. Clark notes that researchers are still perfecting the "scalp" but preliminary results are very promising and its use eliminates direct human involvement.
Clark points out that there are other options in treating persistent head louse infestations by prescription medications, including lindane, malathion and ivermectin. All of those options have their own suite of drawbacks but are all still effective, he says. Also, there are people who are trained to remove lice and nits from hair using specially-designed combs, but the process can take up to four hours.
Nevertheless, the quickest way to deal with an infestation is by using commercial, over-the-counter insecticidal shampoos, "and there's a lot of unhappy parents telling school nurses, and telling me, that these shampoos aren't working very well any longer," said Clark. "What we're finding in the scientific community is agreement that effective control by permethrin and by the pyrethrins is in serious jeopardy unless scientists monitor resistance and look for new treatments."
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