Head Lice

Body lice (including crab lice) are discussed in a separate article.

Pediculus humanus capitis, or head lice, are an ectoparasite whose only known host is humans. The life cycle of the head louse has three stages; egg, nymph and adult. The head louse egg, more commonly referred to as a nit, is laid by the adult female on the base of the hair close to the scalp. The eggs are small and yellowish white, and may be mistaken for flakes of dandruff. They take approximately one week to hatch, releasing a nymph, or immature louse, which takes a further week to develop into a mature adult louse.

Adult lice are grey/white and have six legs. Females are slightly larger than males and can lay up to 8 eggs per day. Adult lice survive by taking a feed of blood from their host several times a day. They can live for about 30 days. Head lice may live for up to 2 days without a blood feed, and may survive for that length of time away from a host. Lice may be spread by person to person transmission, or by using infested articles, such as clothing, combs or pillows.

• Head lice infestation is extremely common worldwide. It affects millions of children in industrialised countries.¹ In developing countries, it is thought that infection rates are even higher.
• Head lice are endemic in the U.K., with over 50% of school children aged 7-8 (the peak age for infection) being infected.²
• Although head lice are transmissible, the potential for epidemic spread is minimal, as spread requires direct person to person contact, or transmission via infected articles such as clothing, combs or pillows.
• Girls are more commonly affected than boys. It is thought that this may be due to closer contact between girls.

• The majority of infestations are totally asymptomatic. Presentation is usually when adult lice or nits have been seen.
• Itching may be caused by the movement of the louse, or occasionally by an allergic reaction to louse saliva.

• Investigation is rarely required, as both the eggs and lice can be seen if the hair is combed with a fine tooth comb, particularly on the back of the head, and behind the ears.
• If confirmation is required, the presumptive material can be saved, fast moving lice caught on transparent adhesive tape, and examined under a microscope.
• An estimate of the duration of infestation can be derived from non-viable egg-casings furthest up the hair from the scalp, given that hair grows a centimeter a month.
• Nits and lice fluoresce with a Woods light.

• As lice can survive without a host for up to two days, all linen, clothing etc. should be washed at the same time as treatment.
• Any article that cannot be washed, e.g. soft toys etc., should be sealed in a plastic bag for 2 weeks.
• All combs, brushes etc. should be cleaned thoroughly.
• Wet combing: head lice may be removed by combing wet hair meticulously with a plastic detection comb (probably for at least 30 minutes each time) over the whole scalp at 4-day intervals for a minimum of 2 weeks; hair conditioner or vegetable oil help to facilitate the process.³

Drugs

• Malathion and the pyrethroids (permethrin and phenothrin) are effective against head lice, although resistance is becoming increasingly common and in some areas resistance to more than one of these agents has been reported.³
• Careful application of dimeticone, which acts on the surface of head lice, is also effective. Benzyl benzoate is licensed for the treatment of head lice but it is less effective than other drugs.³
• Advice should be sought from local laboratories if resistance appears to be a problem. The policy of rotating treatments over a complete district is no longer used.³
• Lotion or liquid formulations should be used. Shampoos are diluted too much to be effective. Alcoholic formulations are effective but aqueous formulations are preferred in severe eczema, for patients with asthma, and small children.
• A contact time of 12 hours or overnight treatment is recommended for lotions and liquids; a 2-hour treatment is not sufficient to kill eggs. A course of treatment for head lice should be 2 applications of product 7 days apart to prevent lice emerging from any eggs that survive the first application.
• Rotating insecticides on a district-wide basis is now considered outmoded. To overcome the development of resistance, a mosaic strategy is required whereby, if a course of treatment fails to cure the infestation, a different insecticide is used for the next course. If a course of treatment with either permethrin or phenothrin fails, then a non-pyrethroid parasiticidal product should be used for the next course.³
• Several "alternative" therapies exist which can be bought over the counter, although no trials have been performed to look at their efficacy.

• Secondary infection may be a problem in children who scratch their heads.
• Head lice have not yet been proven to transmit disease.
• However, experimentally the 3 disease organisms transmitted by the body louse (Epidemic or Louse-borne Typhus, Trench fever and Louse-borne relapsing fever), have been shown to proliferate and reproduce in the head louse.
• It may only be the head louse's behaviour, lower fecundity, less hardiness, and lower numbers, that prevent it acting as a vector role.

• It is very difficult to control the spread of head lice in children due to the close contact that children normally have with each other.
• If a head lice infestation is noted in a school, vigilance amongst the parents and treatment of affected children will help to prevent a cycle of reinfestation.
• Tying back long hair may reduce transmission of infection.
• Regular (every 3-4 days) methodical combing with a fine-toothed comb will help to prevent infestation.
• Piperonal has been used as head lice repellent, but has not yet been proven to be effective.