In the United States, many children are returning to school after a nearly three-month summer break from classes. With the start of school, health officials are warning children and their parents about a new kind of head lice.

Being infected with head lice can cause a child to miss classes. The insects are difficult to kill. They can also cause discomfort in the child, who may try to remove them from the skin.

A new study suggests that lice populations in at least half of the 50 states have become resistant to the chemical pyrethroid. The substance has been used for years in products to kill the insects.

Seventeen-year old Ben Kupferman of California just returned home from a summer camp. He brought some unwanted visitors with him.

"I was just scratching my head and one of the, the lice just came out. It was a, just on my finger, crawling around."

The Centers for Disease Control and Prevention says lice spread to anywhere between six to 12-million children in the United States every year. All these boys and girls are between three and 11-years old.

In the past, parents bought a chemical product at a store and put it on their child's head. It usually killed the lice within a few days. But experts say the product is not as effective as it once was because the insects have developed tolerance and even resistance to it.

Angela Baker is a doctor at the Children's Hospital of Wisconsin.

"Certain ones (lice) have the potential to mutate and protect themselves against whatever we're using to try to kill them."

David Gaines is a public health entomologist in the state of Virginia. He says lice have been developing resistance to chemical treatments for some time.

"This has probably been coming for many, many years. There's reports of insecticide-resistant lice going back decades."

When traditional treatments don't work, doctors prescribe stronger drugs. But some parents object to using strong chemicals on their children. They instead want to use a more natural treatment.

Beverly Mann joined with other parents to create a lice-treatment business. Their company uses products that do not have strong chemicals in them.

"Well, we use an all-natural treatment oil and we do give (a) guarantee with our services. So it, it is pesticide-free. We do the combing, we do the picking, so the parent at home doesn't have to."

Gerry Wolburg launched a lice-removal business three years ago when he found lice on his daughter's head. He uses heat to kill lice eggs.

"It desiccates the, the lice with hot air, and if you leave eggs in the hair they're desiccated -- that means they're, they're shriveled up. They're dried (up) and we like to point out that it's like leaving a boiled egg in a hen-house -- it's not gonna, it's not gonna hatch."

Lice do not carry diseases. But they are troubling, difficult to kill and can make the child feel bad. And they can keep children from attending school for a few days.

I'm Christopher Jones-Cruise.

Zlatica Hoke reported this story from Washington. Christopher Jones-Cruise adapted it for VOA Learning English. George Grow was the editor.

lice – n. a small insect that lives on the bodies of people or animals

crawl(ing) – v. to move slowly

tolerance – n. a body's ability to become adjusted to something (such as a drug) so that its effects are experienced less strongly

mutate – v. to change and cause an unusual characteristic or quality to develop in a plant or animal

entomologist – n. a scientist who studies insects

insecticide – n. a chemical substance that is used to kill insects

decade – n. a period of ten years

prescribe – v. to officially tell someone to use (a medicine, therapy or diet) as a treatment

comb – v. to smooth or separate (hair or fibers) with a comb

pick – v. to remove unwanted material from (something) by using your finger or a small tool

desiccate – v. very dry; having the water removed

shrivel – v. to cause (something) to become dry

What kinds of treatments are used on schoolchildren with head lice in your country? We want to hear from you. Write your thoughts in the comments section.