It's on the list of parenting nightmares: Your kid comes home from school with lice, leaving you literally and figuratively scratching your head about what to do.
The back-to-school months are a common time for news of lice outbreaks to spread. Yet, parenting websites and message boards are filled with conflicting and often frightening information.
The good news, experts say, is that head lice are treatable (including the ones that have developed resistance to common shampoos) and the parasites, at least for now, don't cause diseases.
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And even though the scalp bugs are a hassle to deal with, scientists say that they represent a classic example of how evolution works. Their DNA also harbors secrets about the earliest history of humankind, stretching back millions of years.
"The head louse has been around us since we last shared a common ancestor with chimpanzees, and through all of our history, that thing has come along with us," said David Reed, an evolutionary biologist at the University of Florida's Museum of Natural History in Gainesville. "It's interesting to me to think that this absolutely reviled parasite has something useful to say about where we came from and how we came to be the way we are."
Head lice (Pediculus humanus capitis), according to the Centers for Disease Control and Prevention, are parasitic insects that live on the human head, eyebrows and eyelashes. One of thousands of species of related lice, human head lice eat our blood, live close to the scalp, and lay their eggs at the base of hair shafts.
Because they are insects, the presence of lice in our hair grosses out a lot of people. But they don't cause fevers or illnesses, making them one of the more benign parasitic afflictions to plague people.
"They're a social disease and of all the social diseases a person can acquire, they are the one you want to stand in line for," said Richard Pollack, a public health entomologist who teaches at the Harvard School of Public Health and Boston University and is president of Identify, an independent company that offers pest identification and guidance. "They may not be fun to acquire, but head lice are so incredibly trivial."
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No one can say for sure how common lice are in the United States. Head lice are not a reportable condition, so public health departments don't track numbers. And even though many websites, including the CDC's, claim that between six and 12 million kids between the ages of 3 and 11 get infested each year, those numbers are based on inaccurate statistical extrapolations from sales of over-the-counter lice treatments more than a decade ago. True numbers, Pollack said, are likely much smaller.
After screening thousands of kids in schools around the country, Pollack found an average prevalence of about one percent, though that number can range from zero to five percent or more from one school to another down the road. If one percent of all 55 million American kids Kindergarten through 12th grade had lice, total number of cases would number 550,000, not millions.
Cases peak in the second through fourth grades, then drop off sharply. Babysitters, siblings, parents and grandparents experience a bump in risk, too. The main reason that young kids are most vulnerable to cases of head lice is that the insects seem to spread mostly through head-to-head contact.
In research he has yet to publish, Pollack and colleagues recruited kids who each had hundreds of lice on their heads and attempted to get the parasites to transfer to everyday objects, such as combs, hats and headphones. The researchers mimicked daily use, but were unable to get the insects to move.
"Not a single louse or louse egg transferred to any of the things we tested," he said. "We really tried to get it to happen."
The first step when confronting news that there might be a louse outbreak at your kid's school is to check his or her head carefully. Lice and their eggs are tiny and hard to see. And the vast majority of suspected cases turn out to be something else, like dandruff, cookie crumbs or other kinds of insects that have landed on the head, said Pollack who used to hop in cars and airplanes to check out reported "epidemics," only to discover that misdiagnosis was rampant.
Findings like those have led the American Academy of Pediatrics to urge schools to relax their lice policies.
In confirmed cases, studies show that daily combing with a fine-toothed louse comb, works alone about half of the time, Pollack said. For kids with lots of lice or hard-to-comb hair, over-the-counter treatment shampoos work well and are safe.
Some lice have developed resistance to those shampoos, though, making a trip to the pediatrician for prescription treatments necessary. Unlike the "natural" treatments that many parents prefer, including olive oil and mayonnaise, Pollack said that these new treatments have been tested and approved for safety and effectiveness.
The prevalence of lice infestations doesn't seem to have changed much for decades, Pollack said. That shows just how good the insects are at adapting to live on us.
In fact, Reed and other researchers have been using the history of mutations in louse DNA to track human evolution and migrations out of Africa.
As people moved around the globe, lost most of their body hair, and developed ways to chemically attack lice, the insects have changed and thrived along with us. Yet, lice still carry genes that have been preserved since the time that they infested Neanderthals.
"I see them as a window into our evolutionary past that can tell us a lot about human evolution that we can't just learn from human genes and fossils," Reed said. "They have a written story in their DNA about our past, and it's up to us to figure out what that is."
"There is this sliver lining that these horrible nasty parasites that drive parents absolutely crazy have some redeeming qualities, at least to some louse researchers," he added, acknowledging that if his 10-year old ever comes home with head lice, he might feel differently.