Friday Parasite #3: Back to School Special

It’s the first full week of school, and we’ve been graced with the annual memo about lice. As parasites go, head lice (Pediculus humanus capitis)are pretty low-key. They don’t have complicated multi-host life cycles, and they don’t seem to carry any diseases (they leave that to their close relative the body louse. Typhus… brrrr). All head lice really do is scurry over human heads, drink blood, and lay eggs. But they can spread through a class of little kids like wildfire.


Head lice are tiny insects that spend their entire lives on human heads. Females glue eggs to the base of hairs. After about a week, nymphs hatch out and start biting the scalp to get blood meals. They molt three times over a week or two, becoming sexually mature adults at the last molt. Adults can live about a month, and the females lay over 100 eggs during that time.

But lice can’t survive very long away from their human hosts. If a louse falls off a person, it generally dies within a day or two. Their bodies are adapted to hanging on to hairs – their legs even have specialized curved claws that fit around hair shafts. And they don’t have wings. So they can only get from person to person by crawling. For that they need direct head-to-head contact. And little kids are all about big hugs, leaning close, and sharing things with their buddies. Sometimes they bring more home than good feelings.


SEM image of adult louse from the Sussex Center for Advanced Microscopy; other images from the CDC.