Warning of a new strain of head lice that is resistant to conventional chemical treatment has come from health authorities, greatly increasing worries of US parents as children are soon returning to school.

Going by results of a new study presented at the annual meeting of the American Chemical Society last week in Boston, at least 25 populations have lice populations that are resistant to pyrethroid, an insecticide contained in standard lice-treating products.

Ben Kupferman, 17, from California had no idea that his summer camp will end up with lice coming home with him. He said while recalling that he was scratching his head and one of the lice just came out.

Some six to 12 million lice infestations occur each year in the US in children 3 to 11 years old, as per the estimate of Centers for Disease Control and Prevention.

Parents previously relied on some over-the-counter product to get rid of lice within days. But these are increasingly becoming risible due to developing tolerance and even resistance to chemicals.

"Certain ones have the potential to mutate and protect themselves against whatever we are using to try to kill them", said Angela Baker, a pediatrician at the Children's Hospital of Wisconsin.

Lice have been developing resistance for some time, says David Gaines, a public health entomologist in Virginia.

Due to standard treatments becoming futile, many parents are resorting to prescription drugs from doctors. However, some parents don't like using strong chemicals on their kids, and want to give a try to more natural methods.