How do lice develop in humans? - briefly
Lice hatch from eggs (nits) attached to hair shafts, emerge as nymphs that undergo three molts over 7‑10 days, and become reproductive adults within two weeks of infestation. The entire cycle repeats as mature females lay additional eggs, perpetuating the infestation.
How do lice develop in humans? - in detail
Lice that inhabit humans belong to three species: Pediculus humanus capitis (head lice), Pediculus humanus humanus (body lice), and Pthirus pubis (pubic lice). Each follows an identical life cycle, differing only in preferred body region and environmental tolerance.
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Egg (nit) stage – Female lice embed oval eggs beneath hair shafts or clothing fibers. The shell hardens within 30 minutes, protecting the embryo. Incubation lasts 7–10 days at typical skin temperature (≈ 33 °C) and relative humidity of 70‑80 %. Temperature below 20 °C or humidity under 50 % prolongs development, often preventing hatching.
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Nymphal stages – Upon emergence, the immature insect is called a nymph. It undergoes three successive molts (instars), each lasting 2–3 days under optimal conditions. During each molt, the exoskeleton is shed, and the body enlarges. Feeding on blood is required for each molt; a nymph must ingest several microliters of blood to progress.
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Adult stage – After the final molt, the louse reaches sexual maturity. Adults measure 2–4 mm, survive 30 days on a host, and die within 1–2 days if removed from the human body. Females lay 5–10 eggs per day, depositing them near the scalp or in clothing seams, thereby sustaining the population.
Reproductive output determines infestation intensity. A single fertilized female can produce up to 150 eggs during her lifespan, leading to exponential population growth if unchecked. Transmission occurs through direct head-to-head contact for head lice, sharing of clothing or bedding for body lice, and sexual contact for pubic lice. Environmental reservoirs (e.g., hats, combs, linens) support egg survival but do not sustain adult lice, which die rapidly off the host.
Understanding each phase—egg incubation, nymphal development, and adult reproduction—provides the basis for effective control measures, such as targeted removal of nits, maintaining low humidity, and limiting close contact.