How do head lice infest? - briefly
Head lice are transferred primarily by direct head‑to‑head contact, during which mobile nymphs and attached eggs move to a new host; secondary transmission occurs via shared items such as combs, hats, or hair accessories.
How do head lice infest? - in detail
Head lice (Pediculus humanus capitis) colonize a new host through direct contact with hair that already contains live insects or their eggs. When an infested person’s head touches another’s, adult females or nymphs can crawl onto the clean scalp within seconds. The insects are attracted to the warmth and carbon‑dioxide exhaled by the host, which guide them toward the hair shaft.
Females lay 5–10 eggs (nits) per day, attaching each to a hair strand close to the scalp with a cement‑like secretion. The cement hardens within minutes, securing the egg in place. Eggs hatch after 7–10 days, releasing nymphs that resemble miniature adults. Nymphs undergo three molts over 9–12 days before reaching reproductive maturity. The entire life cycle, from egg to egg‑laying adult, lasts about three weeks under optimal conditions.
Transmission is most efficient in environments where heads are in close proximity for extended periods, such as schools, daycare centers, sports teams, or during shared use of hats, hairbrushes, and pillows. Lice cannot survive more than 24–48 hours off a human host because they require blood meals every 3–4 hours. Consequently, infestations spread primarily through person‑to‑person contact rather than through inanimate objects, although contaminated items can serve as secondary sources.
Factors that facilitate colonization include dense hair, which provides a stable substrate for egg attachment, and the presence of a scalp temperature around 34 °C, which supports rapid development. Poor hygiene does not cause infestations but may increase detection difficulty, as heavy shedding of hair or frequent washing can dislodge some insects, reducing visible signs.
Effective control requires removing live lice and nits simultaneously. Mechanical removal with a fine‑toothed comb, applied to wet, conditioned hair, extracts insects and loosens cemented eggs. Chemical treatments, such as topical pediculicides containing permethrin or dimethicone, incapacitate lice but do not dissolve the cement; therefore, a second combing session after 7–10 days is essential to eliminate newly hatched nymphs. Environmental decontamination—washing bedding, clothing, and personal items in hot water (≥ 50 °C) or sealing them in plastic bags for two weeks—reduces the risk of re‑infestation.
In summary, head lice infest a new host by crawling onto the scalp during close head‑to‑head contact, laying cemented eggs near the scalp, and completing a rapid life cycle that enables swift population growth. Prevention focuses on minimizing direct head contact, regular inspection of hair, and prompt, thorough treatment of any detected infestation.