Lice Resistant to Common Treatments
April 6, 2000 (Washington) -- Anguished parents who pour head lice
treatments on their kids only to find live critters a few minutes or hours
later already have experienced what scientists in Massachusetts have proven:
Many lice have mutated and are now resistant to permethrin, the main chemical
ingredient in the most popular lice products.
And in the same way that super-bacteria are invincible to common
antibiotics, the lowly louse may ultimately outwit all efforts to kill it,
unless smarter -- and perhaps simpler -- strategies are found.
"We are just beginning to get a picture of how widespread [permethrin]
resistance is," J. Marshall Clark, PhD, tells WebMD. "Clearly the
resistance is there and it is established and it is a real problem." Clark
was the lead investigator of the study, published in a recent pesticide
journal. He is the director of the Massachusetts Pesticide Analysis Laboratory
and an entomology professor at the University of Massachusetts, both in
Clark and his team collected live head and body lice from individuals in
Israel and Panama, and from school children in England, Massachusetts, and
Florida. Lice were housed in incubators or kept free on volunteers, and were
fed four to five times daily on humans.
Nymphs and mature lice were exposed to concentrations of permethrin
equivalent to those found in over-the-counter lice products, such as Nix. They
measured the time it took for the lice to die, and used DNA testing to examine
the lice genes. They found two gene mutations in the lice that were not killed
"Resistance levels of the Florida head lice and the Massachusetts head
lice were 132 and 223 times higher than that of the Israel population and 41
and 68 times higher than that of the Panama head lice population," the
In response to a request for an interview, Warner-Lambert, the manufacturer
of Nix, issued a written statement to WebMD. The statement commends Clark and
the other authors for developing a technique "for examining resistance in
head louse populations." It also calls the findings of resistance
"interesting," but maintains the lice samples were not representative
of the entire nation.
"I do think there is a problem with permethrin-resistant lice, but no
one knows how large the problem is," Sharon Raimer, MD, tells WebMD. "
Until we know, permethrin is still a reasonable treatment with which to start
because it is safe and easy to use." Raimer is a professor in the
department of dermatology at the University of Texas Medical Branch in
However, other experts say that research convinced them that most permethrin
products are probably ineffective. Jerry F. Butler, PhD, an entomologist at the
University of Florida, who reviewed the study for WebMD, says the research
shows "massive failure" of permethrin and confirms what Florida
officials have long suspected.