The study, published recently in the Journal of Medical Entomology, found insecticides such as permethrin – one of the most common treatments for head lice – used to be up to 100 per cent effective in killing the parasites when it was introduced in 1984.

However, the chemical is now almost 100 per cent ineffective, after the lice developed a widespread "knockdown resistance" mutation, which can be passed on through generations.

The study examined more than 14,000 lice, which were collected from 479 people during nearly two years to mid-2015. Lice were taken from people in rural and urban areas at 138 locations throughout the US.

It found lice from 132 of the collection sites had an average resistance of 100 per cent to the treatment. Only a single site had lice that had no mutations, meaning the treatment would be effective, while others had varying degrees of mutations.

Overall, 98.3 per cent of the lice had the mutation, indicating it was "almost uniformly present at high levels in lice collected recently in the United States", the study found.

"Resistance levels were not decreasing with time," it said. "This finding is expected, as the OTC [over-the-counter] products containing the pyrethrins or pyrethroids are still being used."

Dr Cameron Webb, a medical entomologist at the University of Sydney, said it was well known that when insecticides were used for a long period, they would decline in effectiveness.

"It's not surprising at all, because we know that insect pests of agriculture or public health concern, when we have a reliance on pesticides to control them, there's always a chance they will develop a resistance," he said.

"We know it's happening with mosquitoes, we know it's happening in bedbugs, and it's no surprise at all that it would be happening in head lice."

For other pests such as mosquitoes, there was talk of using genetic modification to make them vulnerable once more, Dr Webb said. However, this wasn't on the cards for head lice since they don't pose a serious risk to their hosts – so parents will now be left with using conditioner and combs to address any outbreaks.

"There's no evidence that head lice pose a significant threat to the health of our children," Dr Webb said. "A bit of time with our kids, getting them to sit still and work through their hair, is going to be just as effective as some of these insecticide treatments."

with Melanie Kembrey