"Long before it's in the papers"
August 19, 2015

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Report: head lice are evolving tougher, harder to zap

Aug. 19, 2015
Courtesy of the American Chemical Society
and World Science staff

For stu­dents, the start of the school year means new clas­ses, new friends, home­work and sports. It al­so brings the threat of head lice. The itch-inducing pests lead to missed school days and frus­trat­ed par­ents, who could have even more rea­son to be wary of the bug this year. 

Sci­en­tists re­port that lice popula­t­ions in at least 25 states have de­vel­oped re­sist­ance to over-the-coun­ter treat­ments still widely rec­om­mended by doc­tors and schools.

Such a phe­nom­e­non rep­re­sents ev­o­lu­tion in ac­tion. Ev­o­lu­tion oc­curs when one or a few in­di­vid­u­als in a spe­cies have an ad­van­tage in a cer­tain en­vi­ron­ment. Lice who can re­sist the in­sec­ti­cides in chem­ic­ally treated hair are an ex­am­ple. Such in­di­vid­u­als end up re­pro­duc­ing more than their peer­s—so eventually, a trait can even spread through the whole spe­cies.

The lice re­search­ers pre­sented their work Aug. 18 in Bos­ton at the Na­t­ional Meet­ing and Ex­po­si­tion of the Amer­i­can Chem­i­cal So­ci­e­ty. 

“We are the first group to col­lect lice sam­ples from a large num­ber of popula­t­ions across the U.S.,” said Ky­ong Yoon of South­ern Il­li­nois Un­ivers­ity, Ed­wards­ville, one of the re­search­ers. 

“What we found was that 104 out of the 109 lice popula­t­ions we tested had high lev­els of gene muta­t­ions, which have been linked to re­sist­ance to pyrethroids.”

Pyrethroids are a family of in­sec­ti­cides used widely in­doors and out­doors to con­trol mosquitoes and oth­er in­sects. It in­cludes per­me­thrin, the ac­tive in­gre­di­ent in some of the most com­mon lice treat­ments sold at drug stores.

Yoon said the mo­men­tum to­ward wide­spread pyrethroid-re­sistant lice has been build­ing for years. The first re­port on this de­vel­op­ment came from Is­ra­el in the late 1990s. Yoon be­came one of the first to re­port the phe­nom­e­non in the U.S. in 2000 when he was a grad­u­ate stu­dent at the Un­ivers­ity of Mas­sa­chu­setts, Am­herst.

“I was work­ing on in­sec­ti­cide me­tab­o­lism in a po­ta­to bee­tle when my men­tor, John Clark, sug­gested I look in­to the re­sur­gence of head lice,” he said. “I asked him in what coun­try and was sur­prised when he said the U.S.”

In­trigued, Yoon con­tacted schools near the uni­vers­ity to col­lect sam­ples. He sus­pected that the lice had de­vel­oped re­sist­ance to the most com­mon anti-lice in­sec­ti­cides. So he tested the pests for a tri­o of ge­net­ic muta­t­ions known col­lectively as kdr, which stands for “knock-down re­sist­ance.” kdr muta­t­ions had in­i­tially been found in house flies in the late ‘70s af­ter farm­ers and oth­ers had shifted to pyrethroids from DDT and oth­er harsh in­sec­ti­cides.

Yoon found that many of the lice did in­deed have kdr muta­t­ions, which af­fect an in­sect’s nerv­ous sys­tem and de­sen­si­tize them to pyrethroids. Since then, he has ex­pand­ed his sur­vey.

In the most re­cent stu­dy, he cast the wid­est net yet, gath­er­ing lice from 30 states with the help of a broad net­work of pub­lic health work­ers. Popula­t­ion sam­ples with all three ge­net­ic muta­t­ions as­so­ci­at­ed with kdr came from 25 states, in­clud­ing Cal­i­for­nia, Tex­as, Flor­i­da and Maine. Hav­ing all the muta­t­ions means these popula­t­ions are the most re­sistant to pyrethroids. Sam­ples from four states—New York, New Jer­sey, New Mex­i­co and Ore­gon—had one, two or three muta­t­ions. The only state with a popula­t­ion of lice still largely sus­cep­ti­ble to the in­sec­ti­cide was Mich­i­gan. Why lice haven’t de­vel­oped re­sist­ance there is still un­der in­ves­ti­ga­t­ion, Yoon said.

The so­lu­tion? Yoon said lice can still be con­trolled with dif­fer­ent chem­icals, some of which are prescription-only.

But the situa­t­ion al­so of­fers a cau­tion­ary tale. “If you use a chem­ical over and over, these lit­tle crea­tures will eventually de­vel­op re­sist­ance,” Yoon said. “So we have to think be­fore we use a treat­ment. The good news is head lice don’t car­ry dis­ease. They’re more a nui­sance than an­y­thing else.”


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For students, the start of the school year means new classes, new friends, homework and sports. It also brings the threat of head lice. The itch-inducing pests lead to missed school days and frustrated parents, who could have even more reason to be wary of the bug this year. Scientists report that lice populations in at least 25 states have developed resistance to over-the-counter treatments still widely recommended by doctors and schools. Such a phenomenon represents evolution in action. Evolution occurs when one or a few individuals in a species have an advantage in a certain environment. Lice who can resist the insecticides in chemically treated hair are an example. Such individuals end up reproducing more than their peers—so eventually, a trait can even spread through the whole species. The lice researchers presented their work Aug. 18 in Boston at the National Meeting and Exposition of the American Chemical Society. “We are the first group to collect lice samples from a large number of populations across the U.S.,” said Kyong Yoon of Southern Illinois University, Edwardsville, one of the researchers. “What we found was that 104 out of the 109 lice populations we tested had high levels of gene mutations, which have been linked to resistance to pyrethroids.” Pyrethroids are a family of insecticides used widely indoors and outdoors to control mosquitoes and other insects. It includes permethrin, the active ingredient in some of the most common lice treatments sold at drug stores. Yoon said the momentum toward widespread pyrethroid-resistant lice has been building for years. The first report on this development came from Israel in the late 1990s. Yoon became one of the first to report the phenomenon in the U.S. in 2000 when he was a graduate student at the University of Massachusetts, Amherst. “I was working on insecticide metabolism in a potato beetle when my mentor, John Clark, suggested I look into the resurgence of head lice,” he said. “I asked him in what country and was surprised when he said the U.S.” Intrigued, Yoon followed up on the lead and contacted schools near the university to collect samples. He suspected that the lice had developed resistance to the most common insecticides people were using to combat the bugs. So he tested the pests for a trio of genetic mutations known collectively as kdr, which stands for “knock-down resistance.” kdr mutations had initially been found in house flies in the late ‘70s after farmers and others had shifted to pyrethroids from DDT and other harsh insecticides. Yoon found that many of the lice did indeed have kdr mutations, which affect an insect’s nervous system and desensitize them to pyrethroids. Since then, he has expanded his survey. In the most recent study, he cast the widest net yet, gathering lice from 30 states with the help of a broad network of public health workers. Population samples with all three genetic mutations associated with kdr came from 25 states, including California, Texas, Florida and Maine. Having all the mutations means these populations are the most resistant to pyrethroids. Samples from four states—New York, New Jersey, New Mexico and Oregon—had one, two or three mutations. The only state with a population of lice still largely susceptible to the insecticide was Michigan. Why lice haven’t developed resistance there is still under investigation, Yoon said. The solution? Yoon said lice can still be controlled with different chemicals, some of which are prescription-only. But the situation also offers a cautionary tale. “If you use a chemical over and over, these little creatures will eventually develop resistance,” Yoon said. “So we have to think before we use a treatment. The good news is head lice don’t carry disease. They’re more a nuisance than anything else.”