Head lice (pediculosis capitis): detection, treatment and myths

Head lice are tiny wingless insects that live in hair close to the scalp and feed on blood. NICE Clinical Knowledge Summaries notes that an infestation usually involves fewer than 10 live lice, and that left untreated it can persist for long periods. The eggs are glued to the hair shaft, and the empty egg cases left behind after hatching are the nits that many people see.

They are a nuisance rather than a danger. NHS inform describes head lice and nits as mostly harmless, and both NHS and NICE sources are clear that they are not caused by dirty hair. NICE says there is no evidence that lice prefer either clean or dirty hair, and NHS inform notes that they do not favour any particular hair type or length.

Head lice are spread by direct head-to-head contact, climbing from one person's hair to another. NHS inform says they cannot fly, jump or swim, are very unlikely to spread via hats, combs or pillows, and affect only people, so cannot be caught from animals. Once detached from the hair they usually die within 12 to 24 hours, according to the same source.

They are common in school-age children, particularly those aged 4 to 11, and they spread readily among families because of close contact. That is why guidance consistently advises checking everyone in the household or in close contact, and treating those with live lice on the same day.

Itching is a common clue but not proof. NICE says a diagnosis of active infestation should only be made if a live head louse is found, and that the presence of eggs alone, whether hatched or unhatched, is not proof. The reliable way to look is detection combing: systematically combing wet or dry hair with a fine-toothed detection comb that can be bought from pharmacies, supermarkets or online.

NHS inform describes wet detection combing as washing with ordinary shampoo, applying plenty of conditioner, untangling with a wide-toothed comb, then drawing the detection comb from the roots to the ends in every section, checking the comb after each stroke and repeating at least twice. Conditioner slows the lice, which is why wet combing is more effective than dry combing, although dry combing is quicker.

Treatment should be started only if a live louse is found, and everyone affected should be treated on the same day. NICE lists three main options depending on preference, treatment history and cost: a physical insecticide such as dimeticone 4% lotion, a traditional insecticide such as malathion aqueous liquid, or wet combing with a fine-toothed comb. It adds that no treatment guarantees success, but a treatment has the best chance if performed correctly.

The NHS wet combing schedule is to comb on days 1, 5, 9 and 13 to catch newly hatched lice, then check again on day 17. Medicated lotions and sprays kill lice in all types of hair, with the lice usually dying within a day, and some need repeating after a week. If a product fails, a pharmacist can suggest an alternative.

• Check that the treatment was used correctly and for the full time stated.

• Check household members and close contacts for a source of reinfestation.

• Repeat the same treatment or try a different one; consider resistance if malathion was used.

• Detection comb afterwards to confirm success.

Several popular approaches are not recommended. The NHS lists products containing permethrin, head lice repellents, electric combs, and plant oil treatments such as tea tree, eucalyptus or lavender oil, and herbal remedies, because they are unlikely to work. NICE similarly says essential oil and herbal treatments are not recommended because of the lack of good-quality evidence on safety and efficacy.

Lotions and sprays should not be used to prevent lice, as they can irritate the scalp and there is nothing that reliably prevents infestation. Hot washing of clothing and bedding is also not needed: NICE notes that a detached head louse survives only a short time, so there is no need to wash at high temperature or fumigate clothing and bedding.

Children being treated for head lice can still attend school, and the NHS says there is no need to keep them off. Regular home checks with a detection comb, particularly for primary school children, can identify infestation early. The aim is to stop transmission by treating promptly rather than excluding anyone.

Most infestations clear with correctly used treatment. NHS inform says that if a treatment does not work the first time you can try it again or use a different treatment, and a pharmacist can advise on whether to speak to the GP practice. The NHS advises checking the pack to see whether a product is suitable for you or your child, and how to use it.