They’re creepy. They’re crawly. And now, they’re harder to kill.

Head lice across the continent have grown into chemical-resistant “super lice,” according to recent research, meaning the blood-suckers are tougher to kill with traditional shampoos and ointments.

The research, published in the Journal of Medical Entomology, suggests that 97 per cent of North American lice have evolved into the powerful strain of bugs.

“Lice do what all mother nature’s creatures do, and that is react when there is something in the environment, like a pesticide, that wants to kill them,” Caroline Bonham of the London, Ont.-based Lice Squad told CTV News.

As kids across Canada gear up to head back to school, health officials are urging parents to keep a close watch on their children’s scalps and opt for shorter haircuts. Otherwise, they could be left with an itch that’s hard to scratch.

“Keep children’s hair up close and tight to the head,” recommends Dale-Ann Erneweine, owner of The Cootie Catcher Head Lice Removal in Kitchener, Ont. “Little boys, if you can keep their hair short, is always the best bet.”

With traditional shampoos out of the picture, the last line of defence against lice, experts say, is the trusty metal comb.

“I always say, the bug will never become resistant to a comb like the fly won’t become resistant to a fly swatter,” Bonham said.

Lice are passed through head-to-head contact and are quite common among children. According to health experts, the dawn of the selfie helped spread the pests among teenagers, who are known to butt scalps in the all-too-common group photos.

“The cardinal rule of lice prevention is: you don’t do head-to-head contact. And with selfies, that’s what it is,” Dawn Mucci-Gooch, president and founder of Lice Squad Canada, told CTV.ca in 2014.

But a case could be made for the traditional comb approach. In 2015, the World Health Organization announced that lindane, an insecticide used in some head lice treatments sold in Canada, was found to cause cancer. The agency referenced several studies conducted in the U.S. and Canada that showed a 60-per-cent increased risk for non-Hodgkin lymphoma in people who used the treatment.

The Canadian Pediatric Society followed up by advising parents against the use of lindane for infants, young children and pregnant and nursing mothers.