North American head lice are developing a resistance to over-the-counter treatments. 

After years of being killed off with a signal treatment the lice have developed a "knockdown resistance," or TI genetic mutation. The mutation allows them to withstand today's non-prescription treatments, WebMD reported. 

"This isn't really controversial," study co-author John Clark, a professor of environmental toxicology and chemistry at the University of Massachusetts, Amherst. "This is a problem we've been showing in development over a period of about 20 years. But our new work now shows that head lice are now almost 100 percent [knockdown resistant]. That means there's an awful lot of resistant insects out there in the U.S. and elsewhere."

Researchers tested the lice in 32 mostly urban areas of the U.S. and Canada, the team found more than 84 percent of lice found in both countries between 1999 and 2009 carried the TI mutation; Between 2007 and 2009 that number started to grow to 100 percent. Most of today's treatments are made up of pyrethroid compounds like permethrin. 

"Europe and South America actually stopped using these pyrethroid compounds years ago," Clark said. "It's not that these compounds aren't in themselves good formulations. But the very last thing you want to do is treat a single pest population with one compound that has one mode of action for years and years -- and that's exactly what we did. The result is that these compounds are just not effective anymore."

"But the good news is that over the last three years we have seen four or five new compounds entering the market," Clark said. 

Doctor David Pariser, a professor of dermatology at Eastern Virginia Medical School agreed about the pressing need to update head lice treatments, despite being worried about how it would affect healthcare costs. 

"I completely agree with this study's assessment regarding resistance, and with the thinking about the new drugs that we now have available," Pariser said. "I've had personal experience with one of them, as I was involved in the clinical trials, so I know they're very effective."

"While these new agents are great, they are prescription drugs, which means they are more costly. It may not be the actual insured consumer who will feel it if he's only paying a co-pay for the new meds, but certainly it's going to be a much bigger cost to the health care system as a whole," he said.