Treating head lice

Description Head lice are parasites that usually infest the scalps of school age children, although adults also get them. Lice attach their eggs to hair shafts near the scalp and lay five to six eggs a day. Lice never willingly leave the head; they stay close to the scalp for food, shelter, warmth, and moisture. They are most often found behind the ears and at the back of the neck. Hatched eggshells (nits) may be confused with dandruff. The mature louse is the size of a sesame seed and has six legs and hook-like claws that grasp the strand of hair tightly, making it difficult to dislodge. It feeds on the host's blood every three to six hours, which can cause scalp itching, though most cases are asymptomatic. The diagnosis of lice infestation can be made definitively only if living lice are present.

Figure 1.

DR JEREMY BURGESS/SPL

Treatment

Treatments that are likely to work

Malathion

Malathion works as well as other agents used to kill lice. Malathion must be left on for at least eight hours for it to work. It is applied to dry hair until the scalp and hair are wet and thoroughly coated. If lice are found 7-10 days after treatment, treat again with the same or different medication. The alcohol in malathion lotion and the terpenoids may cause stinging. Malathion is considered safe in pregnancy or breastfeeding.

Lindane

Although lindane may work as well as other insecticide based agents, its use is limited by its potential neurotoxicity. It is applied, for only four minutes, to wet the hair and skin or scalp of the affected area and surrounding hairy areas. Treatment may be repeated after seven days if necessary. Lindane should be avoided in pregnancy and breastfeeding. If it must be used in pregnancy, it should only be used once.

Permethrin

Permethrin may work better than lindane, though most trials were done before permethrin was widely used and resistance to the drug developed. It is used as 1% creme rinse left on for 10 minutes. If the first application fails to kill the eggs (that is, if young hatching lice are found using a suitable detection comb) a second treatment is indicated, usually seven days after the first treatment. Undertreatment in this situation could contribute to the development of resistance to the drug. There is no role for a third application, as this will contribute to resistance and is not likely to be effective. Permethrin has not been studied in pregnancy.

Pyrethrins

Pyrethrins are used as a 0.33% shampoo or mousse, by applying enough to thoroughly wet the hair and leaving it on for 10 minutes. All pyrethrum products that conform to the US Food and Drug Administration criteria for use in humans require a second application after 7-10 days, to ensure treatment of lice emerging from eggs that have not been killed by the first application. Pyrethrins work as well as permethrin.

Treatments that need further study

Herbal treatments and aromatherapy

Herbal treatments (including tea tree oil) and aromatherapy are sometimes used to treat head lice. No studies have evaluated their efficacy or possible toxicity.

Mechanical removal of lice or viable louse eggs by combing

There is insufficient evidence to determine the efficacy of “bug busting” (wet combing with conditioner). Many combs have been developed for lice removal, but many that are used are inappropriate. There is little evidence to support the use of any combing method. In one trial, significantly more people treated with malathion than with bug busting had no lice at seven days (31/40 (78%) v 12/32 (38%)). A new study (which we have not yet added to the BestTreatments site) looked at permethrin creme rinse with and without combing; permethrin without combing was marginally more effective.¹

Misconceptions about head lice: what does the research really show?

• There is no evidence to support the cleaning of sheets and clothing, or the treating of earphones, baseball helmets, and furniture with insecticide sprays.

• Infection is spread between people only by relatively prolonged head to head contact; thus it is typically spread between people who know each other well. Lice seen on chairs, pillows, and hats are dead, sick, or elderly or are cast skins of lice—these cannot infect a person.

• School based “no-nits” policies (banning children with nits until all nits are removed) do not make sense—less than 20% of school children with nits will go on to develop infestation within 14 days. About half of children sent home for head lice don't have them. Many public health experts believe that “no-nits” policies should be abandoned.

• Treatment should not be started unless live lice are found. Nits are not a sign of active infestation with head lice.

• Cutting hair, or tying it back, is not helpful and may increase the incidence of infestation by making it easier for lice to move off of and on to the scalp.

• Head lice are probably more common in girls because girls are more likely to have close contacts during play—not because they have longer hair.

• Head lice are harmless. If detached from their host they are vulnerable and effectively dead.