Why are there no lice but there are nits? - briefly
Adult head lice have a brief lifespan and are often missed during inspection, whereas their eggs—nits—remain glued to hair strands for several days after the insects have died. Therefore, a scalp may display nits while no live lice are present.
Why are there no lice but there are nits? - in detail
Adult head‑lice are mobile, feed frequently on blood, and die within a few weeks without a blood source. Their eggs, commonly called nits, are firmly attached to hair shafts and can survive several days after the adult dies. Because nits are immobile and protected by a cement‑like substance, they remain visible even when no living insects are present.
The life cycle proceeds as follows:
- Egg (nit): laid by the adult, incubates 7–10 days at typical indoor temperatures.
- Nymph: hatches from the egg, undergoes three molts over 4–6 days, each stage resembling a smaller adult.
- Adult: emerges, begins feeding, and reproduces for up to 30 days.
When treatment eliminates the feeding insects, the adults disappear quickly, but the eggs persist until they hatch or are removed manually. Consequently, a person may observe only nits after successful eradication of the parasites.
Factors that increase the likelihood of finding only eggs include:
- Effective insecticide that kills adults but does not dissolve the egg coating.
- Incomplete coverage of the scalp during treatment, allowing adults to die while some eggs remain untouched.
- Environmental conditions that slow hatching, extending the period during which only nits are present.
Detecting nits involves visual inspection of the hair close to the scalp, where the cemented eggs appear as oval, translucent or yellowish structures. Their attachment point can be distinguished from hair fragments by the presence of a small, white or brownish operculum at one end.
Management strategies focus on both stages:
- Apply pediculicidal agents that contain ovicidal ingredients, such as dimethicone or certain pyrethrins, to disrupt the protective coating.
- Follow with a fine‑tooth nit comb to physically remove remaining eggs after the recommended waiting period (usually 7–10 days).
- Re‑inspect the scalp after 24 hours and again after 7 days to ensure no new adults have emerged.
Understanding the distinction between the mobile parasite and its immobile developmental stage explains why adult lice may be absent while their eggs continue to be observable.