Why do we have head lice?

I was recently asked: “why do we have head lice?”

My first, rather facetious, answer was that we have head lice because we have heads, and obviously they’d be called something different it they lived elsewhere on the human body….

But then I realized a slightly better answer was needed. So let’s start at the beginning.

Humans are host to three species of louse: head lice (on heads), body lice (not on heads) and pubic lice (self-explanatory). Except things are never quite this easy. So, in reverse order:

The pubic louse (also called the crab louse),  Phthirus pubis, is actually the generalist human body louse. It is adapted to coarser sparser hair, so is most at home around the genitals, but also extends up men’s hairy chests and backs, into armpits, and can be found in beards. Its spread by casual sex is sometimes over-emphasized, but it needs close physical bodily contact to crawl from one victim to another. This is usually in double beds, but there are reports of innocent children with crab lice in their eyelashes, perhaps caught from some not quite so innocent parents.

The body louse, Pediculus corporis, is perhaps better called the clothing louse, and an old name, Pediculus vestimenti, echoes this. It actually lives in the weave of the clothes, and lays its eggs in the threads, but visits the body to take its blood meals several times a day.

The head louse? Well, we all know about that.

Pubic lice are not very common these days, mainly because, on the whole, the close physical contact necessary for its spread is only between sexually active adults. Once monogamy of some form kicks in, infestations are likely to be dealt with quickly.

The body or clothing louse is also not very common these days, mainly because we have the luxury of taking our clothes off. This, effectively removes the lice. Ever since the discovery of regular hygienic clothing changes, the body louse has been under attack. Changing clothes, say, once a week gets rid of this louse quickly. Adult lice are dead within 24 hours, and any nits that survive the week off and hatch when next against the skin will be despatched the next week off, before they have a chance to lay any more eggs. Today the clothing louse only exists where people are unable to take their clothes off, for weeks, or months, or years — down-and-outs, refugees during famine or war, the victims of natural or man-made disasters. These are people who own only the clothes they stand up in, or lay down to sleep in. They do not, cannot, change their clothes.

The head louse, though, lives in a microhabitat we all possess — head hair. There is no taboo against hugging or cuddling our children, friends and family members. Head louse transmission is quick and easy.

Is there anything that might reduce head louse abundance? Perhaps if the world is swept with an almost universal fashion (in adults and children) for baldness. Otherwise, not. Head lice are here to stay.

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