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Head lice are ectoparasites. Basically, they suck up blood, glue their eggs (nits) onto a human hair out of which the nymph emerges. The nymph stage grows into an adult and the cycle repeats.

After a web search I find that head lice complete their entire life cycle on the human scalp. See for example the CDC page on lice for a credible and informative website. In fact, all websites I have found so far mention that lice live on the scalp. The wikipedia page explicitly states

Head lice [...] spend[...] their entire lives on the human scalp...

However, none of the sources I have dug up mentions explicitly that lice live only on the scalp and cannot survive anywhere else.

  • Notably, I am wondering if head lice can survive in a beard, or in other hairy parts of the human body. Is it enough to eradicate them from the scalp with lice shampoo and a lice comb, or can a few covert outcast lice survive somewhere else on the body just to repopulate the scalp after the war is over?
4

Human head lice generally do not survive very long when they are out of their preferred habitat. But they can adapt: "Head and body lice used to be designated Pediculus capitis and P. corporis but they are now known to belong to the same species, P. humanus x[16,17]. Fifty years ago Levene and Dobzhansky [18] showed that head lice could be trained or adapted to become the rather larger body lice by attaching them to the body in small pill boxes."

That should read as: an infestation should be looked at as a locally serious problem. Although any spread from there is not that likely, looking for them elsewhere is prudent advice. Treatment should be guided by actual symptoms. They are quite hungry. If a certain amount of time has passed after "the war is over" and no symptoms appeared elsewhere in the meantime, then a scalp repopulated from other sources on that body is very unlikely. Eggs surviving in the hair itself are much more likely.

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