In an all-out effort to learn how to defeat resistance in lice, scientists at the University of Massachusetts say they are the first to come up with a way to actually grow colonies of mutated lice in their laboratory for closer study. Kyong Yoon, a Ph.D. student at the University of Massachusetts at Amherst, described the work to scientists attending the 224th national meeting of the American Chemical Society, the world's largest scientific society.
The most common ingredient in various products used to treat head lice is permethrin, an insecticide derived from natural compounds in chrysanthemum flowers, according to John Clark, Ph.D., a professor in the university's department of entomology and Yoon's faculty advisor.
The researchers hope their work will lead to new ways to control head lice.
In limited sampling studies of people in various parts of the United States, resistance to permethrin has ranged from 50 percent in some Los Angeles school children to about 98 percent in a migrant workers' camp in Florida, Clark noted during a telephone interview. Clark's research group also did sampling in Texas where resistance was found to be 75-80 percent. His group is aware of resistance in Boston and western Massachusetts although they have not yet measured the level of resistance, he added.
Clark and Yoon are the first researchers to develop an artificial feeding system that allows them to raise their own mutant lice for study.
"Our work is the first report of artificially rearing human head lice and carrying out a molecular linkage study in U.S. populations," according to Clark.
"We have a membrane and automatic blood feeding system that allows us to rear these things on little hair tufts," Clark said. "We just cut little hair tufts and make teepees out of them and the head lice kind of sit on this membrane and they think they're on a scalp and they wander down and take a blood meal and then go back up."
The controlled environment allows the scientists to examine the biochemical and molecular mechanisms of resistance and monitor the frequency of the mutations that cause resistance, Clark noted.
"We don't have an effective control process out there. That means they're becoming harder to control and the chances of increased infestation are certainly higher," Clark said. "The longevity of effective control provided by permethrin is in serious jeopardy unless effective monitoring of this resistance is begun and coupled with the use of new and novel acting pediculicides."
Permethrin is a neurotoxin that kills nonresistant lice by attacking the nervous system, similar to the mode of action of DDT, an insecticide once in common use in the United States. But unlike DDT, "permethrin is one of the least toxic and environmentally safe insecticides available," Clark added.
DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane) was banned in this country in 1973 because of concerns about human health and insect resistance but it is still used in some parts of the world. At one time, DDT was used as a treatment for body lice, according to Clark.
Permethrin and DDT work by causing a channel -- specifically, the voltage-sensitive sodium channel -- in the lice's nervous system to stay open too long. This apparently paralyzes the insect so it is unable to eat, although no one is sure of the actual cause of death, Clark noted. But in permethrin-resistant lice, mutations in the channel prevent it from staying open long enough to kill the lice.
Clark's lab was the first to identify these mutations about two years ago.
Because of the similarities in how permethrin and DDT kill insects, Clark thinks the huge amounts of DDT that were used in the past may be a contributing factor in the development of permethrin resistance in lice. But overuse of permethrin products is probably the bigger contributor to resistance, he added.
"I believe, and a lot of other people that study resistance believe, that it's the misuse of these over-the-counter products that have been the real problem in terms of selecting resistance."
Clark says the ability to raise human head lice in a laboratory setting also allows his group to investigate what role lice may have in disease transmission, even including agents of bioterrorism, such as anthrax.
"We're really interested in this whole aspect of bioterrorism, particularly cutaneous anthrax," Clark pointed out. "You'd have a lot of problems in cities and that's probably where these bioterrorism agents are going to be disseminated."
"If you had like eight percent of your children in the cities with open head wounds [caused by feeding lice], and somebody comes and dumps a lot of cutaneous anthrax on everybody, there's a real chance that these kids can be at high risk for infection," Clark explained.
Another area of interest to Clark's research group is finding effective alternative treatments. "We have identified a number of natural products, such as terpinen-4-ol from tea tree oil, that are safe and novel alternatives to permethrin," he said.
The next compound that is likely to come on the market for the treatment of head lice is ivermectin, according to Clark. "It's the major compound we use to worm horses and cows and sheep and things like that." When the compound was used in Africa by the World Health Organization to kill parasitic worms that cause river blindness, it was discovered that children given the product had no fleas or lice on them.
Clark says there are some physicians who are already giving ivermectin to people to control hard-to-treat cases of head lice.
The poster on this research, AGRO 47, will be presented at 1:00 p.m., Tuesday, Aug. 20, at the Marriott Copley Place, University Hall, during the "Agrochemical Educational Awards for Graduate Student Travel" poster session.
Kyong Yoon, is a Ph.D. student at the University of Massachusetts, Department of Entomology, in Amherst, Mass.
John Clark, Ph.D., is a professor at the University of Massachusetts, Department of Entomology, in Amherst, Mass.
-- Marvin Coyner