Some fear the tiny bloodsuckers are becoming resistant to commonly used over-the-counter chemicals.
"A lot of anecdotal information leads us to believe that there is that possibility," said Sue Partridge of the Centers for Disease Control and Prevention's Division of Parasitic Diseases. "It is becoming a great concern to CDC and researchers throughout the United States."
An estimated 10 million Americans are infested with lice each year. Pre-school and elementary age children are infested most often.
Neither school districts nor federal or local health officials track live infections or outbreaks because the creatures don't carry disease and are viewed mainly as a nuisance.
But a Harvard University study released last week found some head lice, taken from 75 infected children, were not susceptible to permethrin, the active ingredient in one of the most popular pediculicides.
"We confirmed that there are resistant lice," said Richard Pollack, an entomologist and lead author of the study who admits scientists don't know how widespread the resistance is.
Only a few scientific studies have found evidence of lice that are resistant to the chemicals used for years to wipe out infestations.
But parents like Randy Foster of Decatur, Ga., who used a lice-killing shampoo for six weeks on her 3-year-old daughter's hair with nothing but frustration to show for it, have little doubt about the insects' ability to survive.
Foster turned to folk remedies first mayonnaise, then Vaseline and said the petroleum jelly did the trick, though it took hours to wash out.
Makers of lice treatments say failures to eradicate the insects likely are caused by misapplication or forgetting follow-up treatments or misdiagnosis.
Head lice can be hard to distinguish from dandruff, other insects and even dirt, said Pollack, who said he still recommends over-the-counter products as the first line of defense.
Still, some schools are increasing their efforts to avoid outbreaks.
Jan Borelli, superintendent of schools in El Reno, Okla., hired a part-time nurse last year just to check for head lice. Her own two children became infested, and the medicated shampoos didn't work.
Borelli now sends a monthly newsletter encouraging parents to check their children for head lice, telling them what to look for and describing alternative treatments, such as olive oil, though most home-remedies have not been scientifically proven.