It has been reported in a new study that lice have become more resistant to over-the-counter treatment in the 25 US states. The study was carried out at the Southern Illinois University. Dr Deon Canyon of the Office of Public Health Studies at the University of Hawaii said the study is not surprising.

The academic told that there is proof that it is becoming more difficult to kill lice even in Australia. He studied head lice when he was at Queensland's James Cook University, Dr Canyon said. According to Dr Canyon, "The more you use a treatment the less effective it becomes. You spray a bunch of lice and some die instantly, some take a while to die and everything in between”.

He added that if one continues spraying, the ones that are more resistant will breed up and if 98 out of 100 are killed, the left continue multiplying. Next time, when the product is used on the child’s head, it could just be 50% efficient, he said.

It has been found in a recent study carried out by James Cook Professor Rick Speare that head lice have grown resistant to the pesticide Malathion. Permerthrin, which is the important ingredient in several lice treatment products, has now become less effective.

It has been found by Professor Speare that as much as 80% have become resistant to permerthrin that is derived from the chrysanthemum flower. Kyong Yoon, Ph.D. carried out the US study. He found that lice were unaffected by pyrethroids, which is a type of insecticide that is used in many lice treatments.