09/28/2014 05:00 EDT | Updated 11/27/2014 05:59 EST

Most-used head lice treatments no longer very effective, scientists say

With elementary school students back in class, more kids may be going home with itchy heads. That's due to the lice that get spread by the head-to-head interactions that happen frequently enough among kids that age.

While the official recommendation for treatment will likely include using a product on their hair that contains pyrethrins or permethrin, scientists are finding that head lice have acquired resistance to those compounds.

Over the past 35 years or so, head lice have built up considerable resistance to the pyrethroids via genetic mutations, says lice expert John Clark.

"The efficacy of all those products has gone way down. It started out at 100 per cent, now we're down to 20 to 30 per cent in recent clinical studies."

Natural pyrethrin formulations, made from chrysanthemums, were introduced in 1945 for the control of head lice. Permethrin, a synthetic pyrethroid, dates back to the 1980s.

Products containing those compounds - Nix, Rid, Kwellada, R&C Shampoo - dominate the head lice treatment market in North America.

"When these products came on the market, they were very efficacious and very good louse control agents," says Clark, who directs the Massachusetts Pesticide Analysis Laboratory and is a professor of environmental toxicology and chemistry at the University of Massachusetts Amherst.

He has co-authored a numbered of lice studies, including one published this year that looks at what scientists call "knockdown resistance" in head louse populations.

"Treatment failures"

The Clark paper documents how lice studies over the years have found the frequency of the mutations steadily and rapidly increasing, so that now the mutations are almost always present "within North American head louse populations and likely a major reason for the treatment failures encountered with pyrethrins- and pyrethroid-based pediculicides in both Canada and the United States."

Nevertheless. public health officials and organizations such as the Canadian Paediatric Society (CPS) and the U.S. Centres for Disease Control continue to recommend products containing those compounds.

Clark says we're "trapped in this scenario where we're being forced to use compounds that clearly aren't near as efficacious as they were 30 years ago."​

Bayer Healthcare, which claims its product Rid is "the #1 head lice treatment brand" in the U.S. — which they don't sell in Canada — told CBC News that while "lice can develop resistance to pediculicides," the insecticides used to kill them, but "When used according to directions, RID is effective in killing lice."

Bayer also notes that products like Rid which contain pyrethrin, or permethrin are approved by the FDA for the treatment of head lice.

Clark says it's hard to find financing for a rigorous clinical study that looks at whether a product still works. "Unfortunately, until we actually do something like that, there's nothing driving any of these organizations to change their position."

"The safest method"

A 2014 U.K. study that compared treatment with a mousse or a lotion containing a permethrin to  just wet-combing the hair found that, "None of the treatments was significantly more effective than any other."

This month, Consumers Reports published their advice on how to treat lice, saying "the safest method of getting rid of lice is to physically remove the insects and their eggs by combing with a lubricant such as a hair conditioner."

Michael Hansen, senior staff scientist with Consumers Union, says the conditioner, or olive oil, especially helps with the removal of nits, the louse eggshell.

Earlier understanding was that a louse cemented her nits to a hair shaft, usually near the base, but he says the nit can actually be slid up and down the shaft.

He pointed to a 2014 study in the Journal of Medical Entomology that says ordinary hair conditioner is just as effective at removing nits as special nit-removal shampoos and conditioners.

Consumers Reports has a step-by-step guide for lice and nit removal from an infested scalp.

Infestation usually involves less than 10 live lice, according to the CPS.

But an adult female louse can produce up to six eggs per day, which hatch seven to 12 days later.

Best practices

When removing nits, it's important to use a metal — not plastic — nit comb, Hansen says, "because the tines of those metal combs, they're small enough so that an individual hair can go through, but not the egg itself."

"Concentrate around the ears and the nape of the neck, those are the areas where you'll see the bulk of the eggs being laid."

Clark agrees with the Consumer Reports statement on nit-picking. "There's no question that it can be effective, it's just very time consuming." There's also professional salons for lice treatment, he adds.

New products available

For parents without the time and the energy for tedious nitpicking, there are new products on the market.

A number of products contain dimethicone, which suffocates lice. One Canadian manufacturer, Pediapharm, claims the insects "cannot develop resistance."

Clark says that's debatable.

"Insects have been around a long time and to say that we're going to come up with the silver bullet that's going to eliminate a louse that's been around for a million years and they're not going to find a way around it, is sort of naive.

"It may take a little longer to develop resistance, but I'm sure they can develop resistance even to the physical acting type of compound."

Hansen adds that the products sold in North America with lower dimethicone concentrations don't kill the nits, so "you have to continue to use it every few days."

A lousy strategy

Clark says over-reliance on one insecticide is a lousy strategy because of insects' ability to acquire resistance. He recommends, on a community level, using a variety of products "that have novel modes of action."

In addition to dimethicone compounds, he mentions products containing ivermectin, spinosad and benzyl alcohol, which kill lice in different ways. Those three usually require a prescription.

When asked about those compounds, Hansen doesn't dispute their effectiveness, but he notes they're "incredibly expensive" and questions their worth.

"Just do the simple combing every couple days, it works," he recommends.

Clark calls their prices "unbelievable," although they're often covered under insurance plans.

He says that in the U.S., a person has to twice use an over-the-counter product relying on pyrethrin or permethrin, and then show they're ineffective, before a doctor can write a prescription for one of the new products.

Hansen and his co-authors call for "an approach to management of head lice infestations that balances effectiveness and safety with treatment expense and the need to use treatments that have novel modes of action."