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Super lice on verge of conquering Canadian scalps as mutant parasites develop immunity to insecticides

A new generation of mutant lice, immune to the poisons of decades past, is on the verge of conquering Canadian scalps, says a new report from the University of Massachusetts. With super lice now constituting 97.1% of Canadian head lice cases, the study, published this month in the Journal of Medical Entomology, warns a new method of killing the mutated parasites is “critically needed.” The super lice have a “TI mutation,” a gene that makes them resistant to pyrethrins and pyrethroids, the cheap and safe insecticides that constitute the key ingredients in most anti-lice shampoos, creams and sprays.

In the United States, more than 3,500 consumer products use one of these two insecticides. In Canada, they dominate the market for over-the-counter delousing products. But while these chemicals were once a miracle lice-killing cure, only 2.9% of Canadian head lice cases can now expect to use these products with any success.

For everyone else, they can often make the problem worse. If used in high-enough concentrations, traditional anti-lice products can kill some mutated lice, “but usually not all,” J. Marshall Clark, the study’s author, told the National Post. “You will kill some lice, leaving the more resistant lice to breed and create more resistant lice.”

Researchers collected lice samples from 16 sites in Quebec, Ontario and British Columbia, using test subjects ranging in age from four to 64. In 13 of those areas, mutated lice had supplanted their unmutated predecessors. Only in the Ontario communities of Sudbury, Oakville and Central Toronto could researchers find some lingering pockets of easily killed “susceptible” lice. However, they suspect holdout populations of traditional lice may continue to exist “in more rural areas.”

For this, Canada is still among the lucky ones. The University of Massachusetts study found U.S. super lice rates standing at 99.6%. Only a remote Texas town and a Navajo Indian reservation were spared. In other countries, benign varieties of head lice are almost extinct. In 2010, a survey of TI mutated head lice found they had rates of 100% in Britain, Australia and Uruguay, and were making steady gains elsewhere, from Denmark to Israel to Argentina.
This global plague is no accident, of course. Just as the widespread use of antibiotics produced deadly strains of drug-resistant “superbugs,” mutated lice were similarly brought into being by the rampant availability of anti-lice shampoos.

Photomicrograph of Pediculus humanus lice. Isolated.

As recently as the mid-1990s, pyrethrin and pyrethroid-based treatments were killing lice with almost 100% effectiveness.

But as traditional lice were progressively wiped out, super lice moved in to take their place. By 2001, pyrethrin and pyrethroid were only killing about 80% of North American head lice. A few years later, effectiveness had plummeted to 55% and even as low as 28%.

“The marked reduction in effectiveness reflects the development of resistance in [head lice],” reads a 2012 paper in Canadian Family Physician. Insecticide resistance is “now the most common cause of treatment failure.”

Most tellingly, in countries that never adopted pyrethrin and pyrethroid treatments, super lice are unknown. South Korea and Thailand treat lice using a harsher chemical known as lindane. As a result, recent surveys in both countries did not find a single case of a T1 mutated head louse.

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