How long before lice appear? - briefly
Lice eggs hatch in about 7‑10 days, after which nymphs appear and become noticeable. Adult lice can be seen within two weeks of the initial infestation.
How long before lice appear? - in detail
Lice become detectable after a specific interval that follows egg deposition. Female head‑lice lay 6‑10 eggs (nits) each day, attaching them firmly to hair shafts within 1 mm of the scalp. The eggs require 7‑10 days at typical indoor temperatures (20‑30 °C) to complete embryogenesis and hatch. Newly emerged nymphs, called crawlers, are mobile for 1‑2 days before they settle and begin feeding. During this period they are visible only as tiny, moving specks.
After settlement, nymphs undergo three molts. Each molt lasts about 5‑7 days, so the entire immature phase spans roughly 9‑12 days. By the end of the third molt, the insects reach adult size (≈2–3 mm) and are readily seen moving along hair shafts or on the scalp. Adults live 30‑40 days and continue laying eggs, maintaining the cycle.
Summarized timeline:
- Egg stage: 7‑10 days
- Crawler stage: 1‑2 days
- First molt: 5‑7 days
- Second molt: 5‑7 days
- Third molt (adult emergence): 5‑7 days
- Total time from egg to visible adult: approximately 18‑30 days, depending on temperature and host hygiene.
Factors that can shorten or extend this interval include:
- Ambient temperature: higher temperatures accelerate development; lower temperatures delay hatching.
- Host hair density: dense hair provides a more stable microenvironment, supporting faster growth.
- Personal hygiene: frequent washing does not kill nits but may reduce visibility, potentially delaying detection.
Consequently, the earliest visible signs—live lice or moving nits—typically appear about three weeks after the initial egg‑laying event, with most infestations becoming apparent within one month. Early detection relies on regular scalp inspection, especially in environments where transmission is common.