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The emerging nymphs stay on the original host for about seven days until they are adults. Only adult lice are contagious as nymphs cannot spread to another host. Adult lice are spread when sustained, direct head to head contact allows them to walk from one head to another. Head lice cannot jump or fly. Although they can survive for a couple of days without a host, head lice are probably not viable as soon as they leave the head.¹

How is it diagnosed?

Although the presence of nits (empty egg shells), eggs or dead lice may indicate a previous infection, a living, moving louse must be found to confirm the diagnosis. Louse specimens can be attached onto sticky tape for checking by a healthcare professional. `Imaginary lice', i.e. other objects found in the hair such as `hair muff' or foreign bodies, are often confused with head lice.

In a typical infection, fewer than ten lice will inhabit a head of hair. The eggs are pinhead size, while adult lice are about the size of a sesame seed. Other clues may also be found, e.g. lice droppings, which may be seen as black specks on pillows or collars.

The best method of detecting live lice is thought to be by `wet combing' (see patient information leaflet). Families should be encouraged to use a fine tooth detector comb to perform detection combing routinely (about once a week). Such combs are available to buy from pharmacies.

Asmanycasesareasymptomatic, lice may be difficult to find without regular detection combing. Itching may not develop for up to three months in some cases.⁴ Nowadays, head louse infection is rarely associated with serious clinical consequences, although in rare cases inflammation of the scalp and secondary infection can occur.²

What treatment methods are available?

Treatment should only be started if infection is confirmed by a healthcare professional. Ideally, live lice must be seen on the patient. Alternatively, patients may present louse specimens to a healthcare professional for assessment.

Treatment with insecticides

Three insecticides are listed in the Nurse Prescribers' Formulary (NPF): malathion and the synthetic pyrethroids, phenothrin and permethrin. To ensure the effective elimination of lice and eggs, it is vital that insecticides are used correctly and in sufficient amounts (see table 1 and the enclosed patient information leaflet). Carbaryl can currently only be prescribed by a doctor.

What is the evidence?

The evidence for the use of insecticides to treat head lice is of poor quality. Most studies were done in developing countries where the participants had no prior exposure to insecticide. The results may not be applicable to other countries; for example, lice in the UK have often been previously exposed to insecticides and may not be fully susceptible, or even resistant to some products.

A published systematic review assessed the clinical efficacy of various treatments.⁵ Only seven of the 28 studies identified were considered to be of sufficient quality for inclusion. Clinical cure rate was assessed 14 days after applying the treatment.

This review included one study of malathion 0.5% lotion and one of carbaryl 0.5% lotion. Both agents were found to cure over 90% of patients after a single application. The six studies involving permethrin 1% cream rinse also found cure rates of around 90%. However, a study showing a similar cure rate for phenothrin 0.2% lotion was excluded, as it measured cure rate at day 21 rather than day 14.

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