When are lice contagious? - briefly
Lice can spread as soon as live insects or viable eggs are present on a person’s hair, usually through direct head‑to‑head contact or sharing items such as combs, hats, or bedding. Transmission ceases only after all active lice and unhatched nits have been removed.
When are lice contagious? - in detail
Lice become transmissible as soon as a viable nymph or adult emerges from an egg. The hatch occurs approximately 7‑10 days after the egg is laid; at that moment the newly‑born insect is capable of moving across the scalp and attaching to a new host. Adult lice, which develop after an additional 5‑7 days, continue to be contagious for the duration of their life span, typically 30 days without a blood meal.
Key periods of contagion:
- Egg hatching (day 7‑10): Nymphs can crawl to another head within minutes of emergence.
- Early nymph stage (first 2‑3 days): Nymphs feed frequently and are highly mobile, increasing the risk of transfer.
- Mature adult stage (day 12‑30): Adults lay new eggs and maintain the ability to move between hosts.
- Post‑treatment (first 24‑48 hours): Surviving lice may still be present after chemical or mechanical removal; they remain contagious until all live insects are eliminated.
Transmission requires direct head‑to‑head contact, which is the most common route in schools, camps, and households. Indirect transfer via hats, combs, or upholstery is possible but less efficient; lice survive off a host for only 24‑48 hours under typical indoor conditions.
Effective control hinges on interrupting these contagious phases. Immediate removal of live lice and nits, repeated inspection every 2‑3 days for at least two weeks, and thorough cleaning of personal items reduce the likelihood of further spread. Failure to eradicate nymphs before they mature re‑establishes the contagious cycle.