Head lice – the unwanted gift that keeps on giving

Human head louse in close-up. [Photo by Flickr user Giles San Martin under a CC BY-SA 2.0 license].

There are two telltale signs that your kid’s class is infested with head lice (or in scientific prose Pediculus humanus subspecies capitis). First, and foremost, scout for kids that obsessively scratch the back of their heads – chances are they have a little colony of lice brewing. Second, as many treatment formulas contain oily substances, beware the greasy appearance of post-treatment hair. But most of the time you don’t need to be in doubt, as there is usually a sign on the school door saying ‘there are head lice around’. In my little town, and in Sweden in general, head lice are now a normal occurrence in the younger kids, and an unwelcome gift when the children gets back to school in autumn.

And once a school has them, it seems they are impossible to weed out. Head lice infestations still carry a social stigma, especially among the older generation. Head lice were more or less exterminated in Sweden in the 1950-1980s, and were thought to be restricted to poor people with limited access to sanitation. However, that’s not true anymore (or perhaps never was) – there are head lice climbing around the offspring of lawyers, bankers and…. ahem… professors. One reason for this re-emergence is evolved resistance to the compounds used to treat lice infestation, but likely our ever increasing busy lifestyle is to be blamed too, as effective treatment really only is doable with the painstakingly gruesome louse combing (which most little ones think is torture).

Contrary to most people, I think lice are fascinating critters – at least as long as they stay out of my hair. They are extremely adapted to their niche: the flowing hair of the human skull. Inside this jungle, the lice dart around fast as lightning from one end to the other, usually without us knowing it. However, if you catch one and put it on a flat surface you’ll see that it is unable to move, as the louse’s legs stretches horizontally, adapted to efficiently grab hairs, rather than strolling around. You can also see whether they have fed or not, and from that gather whether a new generation of lice are being produced.

Furthermore, consider the boom and bust dynamics that goes on day after day at the louse population level. Each human head is an isolated patch of habitat, and migration to other patches dependent on the behavior of the host – when we hug each other, swap caps, or in other ways put our hair together – behaviors that differ dependent on age class and gender of the host. For example, in my thinning hair, the poor lice have limited chance of long-term survival (a windswept barren heath land, from a louse perspective), while in my youngest daughters the hair is long and interactions with other suitable hosts frequent. Even so, most colonizations of a patch are likely to involve only a single or a few lice, from which a new generation need to stem. Sexual reproduction is the norm, so either the adult female needs to fertilized before moving to a new host, or she has to have company of a male to start a new colony.

But when the population of lice increase, so does our likelihood to notice them. And once noticed, full war is waged in order to exterminate them. This involves various expensive shampoos, often containing substances that immobilize and suffocate the lice, and, of course, good ol’ fashioned combing. The effect is recurrent population bottlenecks, where numbers of lice go down to very low level before bouncing back. One wonders how they manage to avoid deleterious inbreeding effects.

It isn’t an easy life being louse, loathed and hated by their hosts – but it is the only life they know. Simply, lice are good at being lice, and are unlikely to ever (as in evolutionary times) leave their fitness landscape peak and abandon parasitic life. As for now, they are the unwanted gift that keeps on giving.