Head Louse

Pediculus humanus capitis — the head louse — is one of three lice that parasitize humans (alongside the body louse Pediculus humanus humanus and the pubic/crab louse Pthirus pubis) and the only louse adapted exclusively to the human scalp. The species is an obligate ectoparasite: she cannot survive more than 24-48 hours off a human head, cannot complete her life cycle on any other host, and depends entirely on host body heat, blood, and the dense hair shaft architecture of the human scalp for survival. She glues her eggs (nits) to individual hair shafts close to the scalp; nits hatch in 7-10 days and progress through three nymphal instars before adulthood. The head louse is considered globally cosmopolitan and infests an estimated 6-12 million humans per year (mostly children aged 3-11) — making her one of the most common human ectoparasites worldwide. Despite the high infestation rate, head lice do NOT transmit any human disease in clinical practice — this is in marked contrast to the closely related body louse, which transmits typhus (Rickettsia prowazekii), trench fever (Bartonella quintana), and relapsing fever (Borrelia recurrentis). The head louse / body louse divergence has been used by molecular anthropologists to date the origin of human clothing: head and body lice diverged genetically approximately 170,000 years ago (Kittler, Kayser, Stoneking 2003), with body lice evolving as a specialized form to inhabit clothing — implying that humans began wearing clothes regularly enough to provide a stable body-louse niche around that time.