How long after contact with an infected person do lice appear? - briefly
Lice infestations generally become visible 7–14 days after exposure to an infested person, as the eggs hatch within about a week. Early signs include itching and the appearance of nits attached to hair shafts.
How long after contact with an infected person do lice appear? - in detail
The interval between acquiring lice from an infected individual and observing the first nits or adult insects depends on the species and the stage at which the parasite was transferred.
Head lice (Pediculus humanus capitis) lay eggs that hatch in 7–10 days. If a nymph is transferred directly, it will mature to the adult stage in about 9 days, after which it begins reproducing. Consequently, visible signs—itching, live insects, or attached nits—typically appear within 10–14 days after contact.
Body lice (Pediculus humanus corporis) follow a similar timeline. Eggs hatch in 8–9 days, and nymphs reach adulthood in another 8–9 days. Because body lice live in clothing rather than on the scalp, detection may be delayed until the adult emerges and feeds, usually 10–12 days post‑exposure.
Pubic lice (Pthirus pubis) require 6–10 days for eggs to hatch and an additional 8–10 days for nymphs to become reproductive adults. Symptoms generally surface 2–3 weeks after the initial encounter.
Key factors influencing the onset of symptoms:
- Stage of transfer – eggs produce the longest delay; nymphs or adults shorten the period.
- Host hygiene – frequent washing can remove nits, delaying detection.
- Environmental conditions – temperature and humidity affect egg development speed.
- Individual susceptibility – variations in skin sensitivity alter the time when itching is perceived.
In practice, most infestations become noticeable between 7 and 21 days after the first exposure, with the shortest latency occurring when live insects are directly transferred and the longest when only eggs are acquired. Early inspection of hair, clothing seams, and the pubic area can confirm the presence before the full reproductive cycle is established.