Pubic lice and body lice are discussed in the separate article Pubic and Body Lice.

Head lice infestation (pediculosis capitis) is caused by the parasitic insect Pediculus humanus capitis, which lives on and among the hair of the scalp and neck of humans. The adult louse feeds on blood.¹

• Louse eggs (ova or nits) are small, oval and yellowish white, and are attached to the hair shafts. They usually take 7-10 days to hatch.
• Immature lice (nymphs) take 6-10 days to mature into adults, which are up to 3 mm long.
• Adult lice have six legs (which have hook-like claws to hold on to the hair) and are grey-white to black in colour. The female louse lays 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.
• Transmission of head lice requires head-to-head contact. Head lice can only live on humans and the lifespan is very short (several days) once detached from a human head..

Epidemiology

• Head lice infestation is extremely common worldwide. It affects millions of children in industrialised countries.² In developing countries, it is thought that infection rates are even higher.
• Head lice are endemic in the UK, with over 50% of schoolchildren aged 7-8 (the peak age for infection) being infected.³
• Risk factors include age less than 12 years, female gender, families with four or more children, lower socioeconomic status and long hair.¹
• There is no evidence that head lice have a preference for either clean or dirty hair.

Presentation¹

• The majority of infestations are totally asymptomatic. Presentation is usually when adult lice or nits have been seen.
• Itching of the scalp is not sufficient to diagnose active infestation. Itching may not develop for several weeks or months after becoming infested and may persist for days to weeks after successful eradication of head lice.
• Nits alone are not sufficient to diagnose active head lice infestation because it is difficult to distinguish between dead and live eggs with the naked eye.
• A diagnosis of active head lice infestation can only be made if a live head louse is found. Detection combing (systematic combing of wet or dry hair with a detection comb) should be used to confirm the presence of lice.
  • Wet combing takes 5-15 minutes per head; lice are immobilised by hair conditioner, so are easy to see on the comb.
  • Dry combing takes 3-5 minutes per head. Using a comb on dry hair may produce static electricity; when a louse is spotted on the comb, placing a thumb on it before drawing the comb out of the hair prevents the louse being flicked off the comb by the static electricity in the hair.

Differential diagnosis¹

• Itching may also be due to:
  • Psychogenic itch on hearing that there are head lice within the school.
  • Other itchy scalp conditions, such as eczema.
  • Successfully treated head lice infestation but with persisting itch, which may last for weeks.
• Nits can be confused with:
  • Seborrhoeic scales.
  • Hair muffs (secretions from the hair follicle that are wrapped round the hair shaft).
  • Hairspray.
• Nits can usually be differentiated as they stick firmly to the hair even after vigorous brushing.

Management¹

• Treat the person only if a live head louse is found. Treat all affected household members simultaneously.
• There is no need to wash clothing or bedding that has been in contact with lice.
• Children who are being treated for head lice can still attend school.

Treatment options

• Dimeticone 4% lotion (Hedrin® lotion):
  • Should be applied twice, with 7 days between applications. Dimeticone is left on the hair and scalp for 8 hours, or overnight, and then washed out using shampoo.
• Wet combing using the Bug Buster® comb and method:
  • Involves methodically combing wet hair with the fine-toothed Bug Buster® comb to remove lice. This is undertaken for four sessions over 2 weeks.
  • Wet combing should be continued until no full-grown lice have been seen for three consecutive sessions.
• Isopropyl myristate and cyclomethicone (Full Marks Solution®):
  • Not suitable for children younger than 2 years of age or people with skin conditions.
  • Should be applied twice, with 7 days between applications.
  • It is left in place for 10 minutes and the hair is then systematically combed with a fine-toothed comb to remove lice and then washed using shampoo to remove the solution.
• Coconut, anise, and ylang ylang spray (Lyclear SprayAway®):
  • Not suitable for children younger than 2 years of age, people with skin conditions, or those with asthma.
  • Should be applied twice with 7 days between applications. The spray is left in place for 15 minutes. The hair is then washed using shampoo to remove the spray, and then systematically combed with a fine-toothed comb to remove lice.
• Malathion 0.5% aqueous liquid:
  • Should be applied twice, with 7 days between applications.
  • It should be applied to the hair from the roots to the tips, left on the hair and scalp for 12 hours or overnight, and then washed out using shampoo.
• Benzyl benzoate is licensed for the treatment of head lice but it is less effective than other drugs.⁴
• Women who are pregnant or breast-feeding can be treated with wet combing or dimeticone lotion. Malathion is recommended if a traditional insecticide is thought to be necessary during pregnancy.

Phenothrin and carbaryl are no longer available in the UK. Permethrin is not recommended because there are concerns that the contact time is not long enough for the product to be effective, and there is evidence of resistance to permethrin in the UK.

Checking to ensure treatment has been successful

• Insecticide treatment:
  • Advise people to check whether treatment was successful by detection combing on day 2 or day 3 after completing a course of treatment, and again after an interval of 7 days. Treatment has been successful if no lice are found at both sessions.
  • If treatment has been repeated because the first treatment course was unsuccessful: advise people to use detection combing 5 days after the first application of treatment, 2 days after the second application of treatment, and again after a further 7 days.
• Wet combing using the Bug Buster® comb and method:
  • This method has been successful if no lice are found on day 17. If lice are found on day 17, continue use of the Bug Buster® comb until no full-grown lice have been found on three consecutive sessions.

Management of confirmed unsuccessful treatment

• Household members, close family, and close friends (both adults and children) should be assessed using detection combing to identify possible sources of reinfestation.
• Check whether the treatment was used correctly.
• Repeat the same treatment or switch to a different treatment: the choice will depend on the preference of the person or child's parent, and whether resistance to a traditional insecticide is suspected.
• Ensure that all affected household contacts are again treated simultaneously.
• 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.⁴

Complications¹

• Pruritic rash on the back of the neck and behind the ears, caused by a hypersensitivity reaction to louse faeces.
• Excoriation and skin infection are rare.
• Loss of sleep caused by continuous itching is not usually a problem.
• Anxiety and distress for the child and parents.

Prognosis¹

If left untreated, infestation with head lice may persist for long periods, often for more than 1 year.

Prevention

• 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.
• There is no evidence for any benefit of head lice repellents, or using head lice treatments prophylactically.
• There is no evidence that measures beyond normal personal hygiene, housekeeping, and laundry can prevent reinfestation.
• 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 and regular (every 3-4 days) methodical combing with a fine-toothed comb may help to prevent infestation.

Document references

1. Head lice, Clinical Knowledge Summaries (March 2010)
2. Heukelbach J, Feldmeier H; Ectoparasites--the underestimated realm. Lancet. 2004 Mar 13;363(9412):889-91. [abstract]
3. Downs AM; Managing head lice in an era of increasing resistance to insecticides. Am J Clin Dermatol. 2004;5(3):169-77. [abstract]
4. British National Formulary

Internet and further reading

• Head lice (Pediculosis); Health Protection Agency
• The National Pediculosis Association

Acknowledgements