Lice don’t spread the way you probably think they do. Here’s what to know.
As school starts back up, so does the number of head lice cases. But humans have been dealing with the parasites for millennia—and that may continue indefinitely.
Pencils are sharpened, lunch bags are packed, carpools have been arranged. As kids head back to school, an invisible population is in for the buffet of a lifetime—head lice.
That’s right—much to the chagrin of parents, the start of school also brings a peak in head lice cases. Every year, the sesame seed-sized, blood-sucking insects affect an estimated 6 to 12 million American children between the ages of 3 and 11. Though relatively benign as parasites go, lice infestations can cause discomfort, itching, infection, and in extreme cases, bug-related paranoia and delusions.
Despite years of data and millions of cases, we haven’t been able to eradicate head lice. From public health officials to evolutionary biologists, experts described our storied relationship with lice and what to do if you’re feeling lousy.
What are lice, and how do they spread?
Lice have been with us since the beginning. One of our oldest known parasites, lice have coevolved with humans for millennia, refining their behavior to match ours.
“They’ve been on this journey for millions of years, the hosts and the parasites,” says David Reed, associate provost at the University of Florida and an expert on the genetics of mammals and their parasites. He says this is evident in the size of their claws, which correlate with the diameter of human hairs—so they’re perfectly suited to grab and climb strands. “They’re tied to us, and they’re evolving in ways that allow them to be more successful on humans.”
As creepy as that may sound, lice are hardly the supervillains they’re made out to be. “They don’t have a tiny fraction of the attributes that people seem to think they do,” says Richard Pollack, senior environmental public health officer at Harvard University. Lice are physically incapable of jumping and don’t fly or develop wings. In reality, Pollack says, they spread almost exclusively through direct head-to-head contact—and even that is slow and limited. “The only way a louse could be airborne is if you pick it up and throw it across the room.”
So, what of the warnings that you can catch lice from shared hair brushes, hats, helmets, and headphones? It’s theoretically possible but exceedingly rare, according to Reed. Studies have shown that lice don’t transfer well via fomites—inanimate objects like combs that can transfer infectious agents. Even if a shared hairbrush is used almost immediately, he adds, “that doesn’t look like it has high potential for aiding in transmission.” The same logic applies to furniture like movie theater seats, carpets, and school desks, Pollack says. Still, the U.S. Centers for Disease Control and Prevention recommends washing or quarantining beds, couches, linens, hairbrushes, and clothing that have been exposed to lice.
Though anyone, regardless of age or hygiene, can be infected with lice, the bugs are most common among school-aged children who spend time in close contact with each other. And unlike body lice, head lice are more nuisance than health threat, according to the California Department of Public Health. Head lice can cause localized infection if excessive scratching introduces bacteria into an open wound, but the bugs themselves don’t spread disease or carry human pathogens, the CDPH says. In contrast, body lice, which are typically found among people living in crowded, unsanitary conditions with limited access to showers and clean clothing, are common vectors for bacterial diseases, including typhus, relapsing fever, and trench fever.
Still, experts agree the dangers of transmission aren’t worth keeping kids out of the classroom. Reversing decades of “no-nit policies,” the U.S. Centers for Disease Control, along with the American Academy of Pediatrics and National Association of School Nurses, no longer recommend sending students with lice home from school.
The lowdown on nit-picking, mayonnaise, and other treatments
The only sure-fire way to ward off lice, Pollack says, is to shave your head. “Without hair, there’s no way that a head louse could stay on the scalp. It will slide off much like a rabbit would on a bowling ball.”
Dramatic hairstyles aside, the best prevention is avoiding direct head-to-head contact, though Pollack says that “the vast majority of people will never get head lice.” Thankfully, for the unlucky ones, there are plenty of effective treatments.
The first step is to confirm that there are live crawling lice, and not simply eggs. If so, Pollack recommends lubricating hair with conditioner and combing thoroughly with a louse comb to remove the bugs and nits. While the first round should remove a good number of parasites, he advises combing every few days until no live lice are found. You’ll be sure they’re gone if you haven’t seen any for two weeks. For hair that’s not easily combed, or a persistent infestation, over-the-counter, FDA-approved shampoos or lotions should do the trick.
Pediculicides, which are lice-specific insecticides administered topically to the scalp, work by paralyzing and killing both juvenile and adult lice. Pyrethroid-based products were once the favored insecticides, but over time some head lice have begun developing resistance to the drug, according to John Clark, an emeritus professor of toxicology at the University of Massachusetts Amherst. Since then, new options have come to market. Spinosad, for instance, is the active ingredient in several common pediculicides and is considered by the U.S. Environmental Protection Agency to be a safe and effective treatment—for now. As Clark says, the human-lice arms race will continue indefinitely.
For those who prefer to avoid insecticides, Clark recommends products containing dimethicone compounds, which suffocate the bugs by interfering with their trachea. Dimethicone has been shown to be highly safe and effective, killing upwards of 90 percent of both live lice and eggs in some studies. Heat is another non-toxic option, though Clark urges caution with personal hair dryers, which “tend to burn the scalp before lice die,” he says.
As for those at-home remedies, experts urge caution. While mayonnaise and Vaseline might kill some lice, they’re not nearly as efficient or long-lasting as approved drugs, Clark says. And more extreme substances—like kerosene or gasoline—are a recipe for disaster.
The good news is, the odds of a persistent lice infestation is fairly low. “A lot of these infestations never go anywhere,” Pollack says. “Even if they’re not discovered, if they’re not treated, a lot of them just peter out.”
