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Head lice prevention, diagnosis and treatment



Head lice (Pediculus humanus capitis) are parasites that cause an infestation of the human head and feed on blood from the scalp of their host. They are not thought to be a vector of disease.

Key learning points

  • The highest prevalence of head lice infestation occurs in children between the ages of three and 11
  • Often the parents feel a sense of shame as they do not realise how commonly children are infested – personal hygiene is not connected to infestation
  • Misdiagnosis is common – patients may need to be examined with a magnifying glass

Head lice (Pediculus humanus capitis) are parasites that cause an infestation of the human head and feed on blood from the scalp of their host. They are not thought to be a vector of disease. The severity of such infestations varies, with a typical infestation being approximately 30 lice per head but with as few as 10 or up to a maximum of 1,000 in severe cases. 

Susceptibility and prevalence

There is little doubt that head lice infestations are extremely common worldwide, usually affecting millions of children. The highest prevalence of head lice infestation occurs in children between the ages of three and 11.1,2 Girls show a higher prevalence than boys.3,4 Several suggestions have been made for this difference: girls’ close contact with each other, their preference for longer hair and their interchange of fomites, like hair bands and clips.5,3 

Prevalence does not vary significantly with season and any variation seen is probably due to an alteration in social behaviour rather than climate conditions.6

It is a problem in families of four or more children and families living in the lower socioeconomic bracket. Schools will often send home infested children, insisting that they be treated before returning to class. Often there is a sense of shame on the part of parents whose children have infestations, as many do not realise how commonly children are infested.1 

There are three forms of head lice: nits, nymphs and adults.

Nits

Louse eggs (ova or nits) are small, oval and yellowish white and attach to the hair shafts, taking between seven to 10 days to hatch. At this stage they can be easily mistaken for dandruff. The eggs remain after hatching and many nits are empty egg cases. 

Nymphs

Nymphs hatch from the nits. The baby lice look like the adults, but are smaller. They take about seven days to mature to adults, and feed on blood to survive.

Adults

Adults are about the size of a sesame seed, approximately 3mm in length. They have six legs, and are tan to greyish-white. The legs have hook-like claws to hold onto the hair. Adults can live up to 30 days and feed on host blood.

How are they spread?

Lice are spread by close person-to-person contact. It is possible, but uncommon, to get lice by sharing personal belongings such as hats or hairbrushes. 

Personal hygiene has nothing to do with getting head lice. Many infestations are totally asymptomatic. Presentation to a healthcare professional is usually the result of adult lice or nits being seen, while other patients will present with frequent itching, often described as a tickling feeling in the hair sometimes associated with sores from scratching.7

Head lice are at their most active at night, causing irritability and difficulty sleeping for the human host. Itching of the scalp is not sufficient for diagnosis of active infestation. 

Evidence suggests that itching may not develop for several weeks or months after the initial infestation and may persist for days or weeks after successful eradication of the head lice. 

Diagnosis

Nits alone are also not sufficient to diagnose active head lice, because of the difficulty of distinguishing between dead and live eggs with the naked eye. 

Misdiagnosis of head lice infestation is common. The diagnosis is best made by finding a live nymph or adult louse on the scalp or hair.

Since adult and nymph lice are very small, move quickly, and avoid light, they may be difficult to find. Use of a fine-toothed louse comb may facilitate identification of live lice.

If crawling lice are not seen, finding nits attached firmly within ¼ inch of the base of hair shafts suggests, but does not confirm, the person is infested. Nits are frequently seen on hair behind the ears and near the back of the neck. Nits that are attached more than ¼ inch from the base of the hair shaft are almost always non-viable (hatched or dead). 

Head lice and nits can be visible with the naked eye, although use of a magnifying lens may be necessary to find crawling lice or to identify a developing nymph inside a viable nit. Nits are often confused with other particles found in hair such as dandruff, hair spray droplets, and dirt particles. If no nymphs or adults are seen, and the only nits found are more than ¼ inch from the scalp, then the infestation is probably old and no longer active — it does not need to be treated.

Treatment

Treatment is recommended for people who have an active infestation of head lice. All household members and other close contacts should be checked and treated if necessary. It is important to treat everyone at the same time.

Prophylactic treatment is recommended for persons who share the same bed with actively infested individuals. All infested persons (household members and close contacts) and their bedmates should be treated at the same time.

Some head lice medications have an ovicidal effect. For pediculicides that are only weakly ovicidal or not ovicidal, routine retreatment is recommended seven days after the first application. For those that are more strongly ovicidal, retreatment is recommended only if live (crawling) lice are still present several days after treatment (see recommendation for each medication). To be most effective, retreatment should occur after all eggs have hatched but before new eggs are produced.

When treating head lice, supplemental measures can be combined with recommended medicine. However such additional (non-pharmacologic) measures are generally not required to eliminate a head lice infestation. For example, hats, scarves, pillow cases, bedding, clothing and towels worn or used by the infested person in the two-day period before treatment is started can be machine washed and dried using the hot water and hot air cycles, because lice and eggs are killed by exposure for five minutes to temperatures greater than 53.5°C (128.3°F).  

Items that cannot be laundered may be dry-cleaned or sealed in a plastic bag for two weeks. Items such as hats, grooming aids and towels that meet the hair of an infested person should not be shared. Vacuuming furniture and floors can remove an infested person’s hairs that might have viable nits attached.

Treatment options

If a live head louse is found, treat with one of the following: 

  • A physical insecticide — these silicone or fatty acid ester-based products kill the lice by physically coating their surfaces and suffocating them, therefore resistance is unlikely to develop.
  • Use of mechanical methods such as wet combing combined with hair conditioner and a fine-tooth comb until all head lice are removed. This should be repeated at three-to-four-day intervals for a two week period. It is an alternative to insecticides, but requires motivation on the part of the parent in the case of young children or the patient themselves. There is no evidence to support the use of electric combs, aromatherapy such as tea tree oil or herbal treatments.
  • Dimeticone (lotion or spray)7 such as dimeticone 4% lotion, dimeticone 92% spray and isopropyl myristate and cyclomethicone solution. This coats the lice and interferes with their water balance by preventing the excretion of water. Patients should be advised to rub it into dry hair and scalp in the evening, allow it to dry naturally and then shampoo it out the following morning. This process should be repeated after seven days.

Insecticides

Malathion, phenothrin and permethrin are all used for both first- and second-line treatment options and although they are available over the counter, NHS prescriptions are often sought. Malathion 0.5% aqueous liquid7 is the only chemical insecticide. It works by poisoning the lice, but resistance has been reported. The Bug Buster kit is the only head lice removal (and detection) method that has been evaluated in randomised controlled trials and it is available on the NHS. 

The choice of treatment will depend on the preference of the person and their parents or carers after considering the advantages and disadvantages of each treatment, what has been previously tried, and the cost of the treatment. 

It is important to be aware that wet combing or dimeticone 4% lotion is recommended first line for pregnant or breastfeeding women, young children aged six months to two years, and patients with asthma or eczema.

Shampoos are generally not recommended because they are too dilute and have an insufficient contact time to kill eggs.

Patient advice 

In general, all affected family members should be treated on the same day to avoid reinfection. A small bottle of insecticide is sufficient for treating short or shoulder length hair, but a larger bottle would be needed for longer, thicker hair.

For insecticides: 

  • Treatment should be applied to all areas of the scalp and to all of the hairs, from root to tip. 
  • The product should be left on for the time recommended by the manufacturer, then washed off. This varies from 15 minutes (for example with Hedrin Once Spray Gel and Vamousse) to at least eight hours (for example with Hedrin Lotion and NYDA). A contact time of eight to 12 hours (or overnight) is recommended for lotions and liquids.
  • It is generally recommended that insecticides are applied twice, at least seven days apart, in order to treat any lice hatching from eggs before they lay more eggs themselves. Some experts suggest that two applications may be insufficient and that three applications may be required to achieve a complete cure.
  • Inappropriate use can lead to treatment failure and may increase the risk of resistant lice.
  • The hair should be kept away from naked flames, cigarettes and other sources of ignition during treatment with dimeticone-containing products (although not flammable, dimeticone is not water based and will not prevent hair from burning).

For wet combing: 

  • Four sessions should be spaced over two weeks (on days one, five, nine and 13).
  • It takes about 10 minutes to complete the process on short hair, and 20-30 minutes for long, frizzy, or curly hair. Two combing procedures are recommended at each treatment session.
  • Detailed information on wet combing is provided in the Bug Buster kit and is also available on the Community Hygiene Concern website (chc.org).

Confirming treatment success 

Advise that after treatment, detection combing should be done to check that the treatment has been successful. 

The UK Medicines Information (UKMi) Cymru Wales recommends that detection combing be done two or three days after the second application of treatment and again after an interval of seven days. Treatment has been successful if no lice are found.

Advice from Community Hygiene Concern is that regardless of the treatment used, detection combing should be done on days five, nine and 12 or 13 after the first application to avoid the risk of continuing infestation. 

Treatment has been successful if no lice are found on any of these occasions. 

If the treatment has been wet combing, a fifth detection combing session should be done on day 17. Treatment has been successful if no lice are found on day 17.

Prevention

Key advice to offer parents and patients:6 

  • Avoid head-to-head (hair-to-hair) contact during play and other activities at home, school, and elsewhere (eg sports activities and in the playground).
  • Do not share clothing such as hats, scarves, coats and sports uniforms.
  • Do not share combs, brushes or towels. Disinfect combs and brushes by immersing them in hot water (at least 130°F) for five to 10 minutes.
  • Do not lie on beds, couches, pillows, carpets, or stuffed animals that have recently been in contact with any affected person. 
  • Clothing and bedding should be washed at a hot temperature and tumble dried at a high heat if appropriate.
  • Clothing and items that are not washable can be dry-cleaned or sealed in a plastic bag and stored for two weeks.
  • The floor and furniture should be vacuumed, particularly where the infested person sat or lay. However, spending time and money on housecleaning activities is not necessary to prevent reinfestation by lice or nits that may have fallen off the head or crawled onto furniture or clothing, due to the organism’s short lifespan. Head lice survive fewer than two days if they fall off a person and cannot feed. Nits cannot hatch and usually die within a week if they are not kept at the same temperature as that found close to the scalp.

References

1 Van de r Wouden J, Klootwijk T, Le Cleach L et al. Interventions for treating head lice. Cochrane Database of Systematic Reviews 2011.

2 Jones K, English J. Review of common therapeutic options in the United States for the treatment of pediculosis capitis. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America 2003;36:1355-61.

3 Meinking T. Infestations. Current Problems in Dermatology 1999;11:73-120.

4 Willems S, Lapeere H, Haedens N et al. The importance of socio-economic status and individual characteristics on the prevalence of head lice in schoolchildren. European Journal of Dermatology 2005;15:387-92.

5 Williams L, Reichert A, MacKenzie W et al. Nits and school policy. Pediatrics 2001;107:1011-15.

6 Burkhart C, Burkhart C. Fomite transmission in head lice. Journal of the American Academy of Dermatology 2007;56:1044-7

7 Wickenden J. Head lice, schools, teachers and parents. Health at School 1985;1:18-19.

8 Burgess I. Head lice. Clinical Evidence 2006. BMJ Publishing Group.