How quickly do lice spread across the head? - briefly
Lice can colonize an entire scalp within 48–72 hours, as mobile nymphs crawl outward from the initial site and lay eggs. Egg hatching adds new insects, accelerating the infestation to full coverage in just a few days.
How quickly do lice spread across the head? - in detail
Lice move across a scalp by crawling, not by flying, and their spread rate depends on several biological and environmental factors. An adult female can lay up to 10 eggs (nits) per day, each attached to a hair shaft near the scalp. Eggs hatch in 7–10 days, releasing nymphs that reach maturity after another 7–10 days. During this 14–20‑day window, newly emerged nymphs crawl a few centimeters per hour, allowing them to reach adjacent hair strands quickly. Within 24 hours of hatching, a nymph can travel from its birth site to neighboring hairs, establishing a dense infestation in as little as 2–3 days if the host’s hair density is high.
Key factors influencing the pace of infestation:
- Hair density and length – dense, long hair provides more pathways for crawling and a larger surface for egg attachment.
- Host behavior – frequent head-to-head contact (e.g., sharing hats, combs, or close physical interaction) accelerates transfer of adult lice and nits between individuals.
- Environmental conditions – warm, humid environments favor faster development; temperatures above 30 °C can shorten the egg‑to‑nymph period by 1–2 days.
- Population pressure – a higher initial number of adult females increases egg output, leading to a rapid rise in nymph numbers.
Typical progression on a single head:
- Day 0–2 – Introduction of adult lice through direct contact; initial crawling across the scalp.
- Day 3–7 – First eggs laid; early nymphs begin to appear.
- Day 8–14 – Eggs hatch; nymphs spread to adjacent hairs, creating visible clusters.
- Day 15–21 – Nymphs mature into reproductive adults; exponential increase in egg production.
- Day 22+ – Full‑scale infestation, with multiple generations coexisting and extensive coverage of the scalp.
Because each generation can develop within three weeks, an untreated infestation can expand from a few individuals to a dense population covering the entire head in roughly one month. Prompt detection and treatment interrupt this cycle, preventing the rapid escalation described above.