Lice has been an unpleasant part of many families with school-aged children, but this year, the threat has spiked with the appearance of super-lice.

In 25 states today, there have been reported cases of lice that has developed an extremely high resistance to the usually effective over-the-counter treatments, according to the American Chemical Society after conducting tests, according to the Huffington Post.

“If you overuse a product, over time, the selection pressure will cause insects to develop resistance to it,” Kyong Yoon, an assistant professor of biological and environmental science at Southern Illinois University-Edwardsville and lead author of this ongoing research said.

Normally, head lice are treated with pyrethroids which is an indoor-outdoor insecticide, also commonly used for mosquito control. But through research, Yoon and his team realized that there were some lice that had developed a gene mutation, usually referred to as knockdown resistance (kdr) against the pyrethroids. This means that the past treatment for head lice no longer works.

“Lice don’t have wings, and they don’t jump, so they move where people move,” Yoon said.

At this time, across the nation, it is an ultimate vulnerable time for head lice transmission.

“It’s a really, really serious problem right now in the U.S.,” Yoon told Time. “Though head lice aren’t known to transmit any diseases, they can be an itchy nuisance — and now, they’re harder to kill.”

The resistant lice were originally discovered in Israel in the 1990’s, then Yoon was one of the very first researchers to report on pyrethroid-resistant lice in the U.S. in 2000.

“At this particular time, we have five states that are not completely saturated with the kdr mutations,” said co-author, J. Marshall Clark, a professor of veterinary and animal sciences at the University of Massachusetts.

Although not gentle or even very safe, there are still some chemicals at this time that can kill this type of super-lice such as asivermectin or spinosad, both available by prescription.

“For the past 20 to 25 years, we only used the products containing over-the-counter pyrethroids,” Clark said. “One, because they were very available. Two, because they are incredibly safe.”