head lice infestation

Head Lice Infestation

     Infestation with head lice is a common problem in all age groups, especially in children. It occurs even in those who maintain good hygiene. It is caused by the bug pediculosis humanus capitis.

      Pediculosis humanus capitis or head lice have been a source of irritation and disgust for thousands of years. Infestation with this parasite is common throughout the world, crossing all economic and social boundaries (Sladden, 2005). These parasites usually infest the scalps of school age children, between 4 to 11 years.  Scalp is the most preferred site of dwelling because of warmth and moisture. They are most often found behind the ears and at the back of the neck.

     Infestation with head lice is a persistent and recurrent problem. It causes itching and irritation. In some societies, parents are often embarrassed if children have head lice, because of the misconception that lice are associated with poor hygiene (Sladden, 2005).  Individuals with head lice infestation may scratch the scalp to alleviate itching and this may lead to secondary infections (Sladden, 2005). Many insecticides have been tried in the past for treatment of these parasites. The main problem with medical treatment is development of resistance. And an even more problem is that of increased risk of toxicity in children. Hence, medications have to be given weighing risks of toxicity, possibility of resistance and effectiveness of treatment. Although current insecticides registered for use against head lice are generally considered safe for occasional use, they may pose a greater risk of direct or cumulative toxicity if used frequently.

Description of the parasite:

     The adult louse is 2 to 3 mm long, almost the size of a sesame seed.  Its color is usually pale gray, although it may vary from brown to jet black. It feeds by injecting small amounts of saliva and taking tiny amounts of blood from the scalp every few hours. The female lives up to 3 to 4 weeks and lays 5-10 eggs a day which are incubated by body heat. They hatch in 10-14 days to release nymphs. Nymphs grow in about 9 to 12 days. The eggs are called nits which are firmly attached to the hair shaft close to the scalp. The louse produces a glue-like substance to facilitate the attachment of the nits to the shaft. Viable nits are most easily seen at the posterior hairline of the infested person because, elsewhere, they are camouflaged with pigment to match the hair color of the infested person. Empty nit casings appear white in color and may be mistaken for dandruff.

      Nymphs, after they grow, mate. The females lay eggs. This cycle repeats every 3 weeks (Frankowski, 2002).

Clinical manifestations:

     Head lice do not transmit any disease agents. Their saliva may cause itching, which may be severe in a sensitized individual. Itching usually starts 4-6 weeks after infestation. Scratching due to itching may lead to secondary infections like impetigo and local adenopathy (Sladden, 2005).


     Head lice are most common in children 4 to 11 years of age. Younger children, adolescents and adults may also be affected. All socioeconomic groups, all over the world are affected. In the United States, the infestation is less common in blacks than in individuals of other races. This could be because blacks have oval-shaped hair shafts that are harder for lice to grasp (Frankowski, 2002). Head lice infestation is not significantly influenced by hair length or by frequent brushing or shampooing. Transmission occurs by direct contact with the head of another infested individual.

     The gold standard for diagnosing head lice is finding a live louse on the head.

A wide range of treatments for head louse infestation have been proposed over the past 2000 years. As early as 1945, Molner reported the use of 20% phenyl cellulosive as lousicide. Another compound reported was ascobial which is said to be similar to 50% benzoyl benzoate used in the treatment for scabies (Molner, 1945).There are two treatment methods for head lice: mechanical removal by wet combing; and topical pediculicide lotion/rinses. However, there are not many reports in the literature comparing the effectiveness of these two treatment methods. Well controlled clinical trials of insecticide treatments are indeed sparse.

Topical pediculicides:

Malathion- Malathion is available in alcohol dissolved form in 0.5% solution. It has to be applied to dry hair until the scalp and hair are wet and thoroughly coated. It must be left on for at least eight hours for it to work. The alcohol in malathion lotion and the terpenoids may cause stinging (Nash, 2003). Also, it is because of this alcohol that it is inflammable. If lice are found 7-10 days after treatment, it is necessary to treat again with the same or different medication. The drug is considered safe in pregnancy or breastfeeding. However it must be kept out of reach of children because ingestion can cause OP poisoning and respiratory depression. The drug is used only in resistant cases.

Lindane- Lindane is as effective as malathion and other insecticide based agents but its use is limited by its potential neurotoxicity (Nash, 2003). It is applied all over the scalp and hair for only four minutes. 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. Resistance has been reported worldwide for many years (Frankowski, Leonard & Weiner, 2002).

Permethrin- Pyrethroids are synthetically developed compounds which are structurally and chemically related to pyrethrins. The characteristic of the pediculicide activity is that it is shown to provide residual activity lasting for 10 or more days. Permethrin, the most commonly used pyrethroid is available as a 1% cream which has to be applied for 10 minutes and then rinsed. It is actually a potential pediculoside but the parasites might have developed resistance to it (Polack et al, 1999). A second treatment may be necessary usually seven days after the first treatment. Proper thorough treatment must be given with this drug; otherwise resistance to the drug may develop. 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 naturally occurring insecticides derived from chrysanthemums and are highly photo-unstable, losing their potency within 12-24 hours after exposure to light. They are available as shampoos or mousse in 0.33% concentration. They are neurotoxic to lice but have extremely low mammalian toxicity. Possible allergic reaction in patients who are sensitive to ragweed and chrysanthemum can occur. They work as well as permethrin. They are usually available in combination with Piperonyl Butoxide. The medicine has to be left for 10 minutes after application and then washed off. A second application after 7-10 days is a must to ensure treatment of lice emerging from eggs that have not been killed by the first application.

Dimethicone- Dimethicone is dispensed in 4% lotion form. It has no conventional insecticide activity. It contains 4% long chain linear silicone (dimeticone) in a volatile silicone base (cyclomethicone). After thorough application to the hair and scalp, it has to be left over night or for about 8 hours. It dries naturally by evaporation of the cyclomethicone solvent. It has to be repeated after 7 days. This silicone compound is the first medical product with a formulation specifically designed for use against head lice. There lower incidence of irritant adverse events with this drug (Burgess, Brown and Lee, 2005).

Crotamiton- It is available as 10% lotion and is mainly used as a treatment for scabies. One study has reported it to be effective against head lice when applied to the scalp and left on for 24 hours before rinsing out. However, safety and absorption in children, adults, and pregnant women have not been evaluated.

     All topical pediculicides should be rinsed from the hair over a sink rather than in the shower or bath to limit exposure to other parts of the body. Also, rinsing should be done with cool water and not hot water to minimize absorption attributable to vasodilatation (Frankowski, Leonard & Weiner, 2002).

Efficiency of treatment:

      Stichele and others reviewed various clinical efficacy trials of topical treatments for head lice. They reported that only permethrin has sufficient evidence published to show efficacy. Less expensive treatments such as malathion did not have much evidence of efficacy. Lindane and the natural pyrethrines are not sufficiently effective to justify their use (Stichele, Dezeure & Bogaert, 1995). The timing of head lice maturation most favorable to their survival in the presence of anti-lice agents is the maximum time as an ovum (12 days) and the shortest possible time of maturing from newly hatched nymph to egg-laying adult (8.5 days). Pyrethroids and lindane are not ovicidal and hence require 2 to 3 treatments per cycle to eradicate lice. Malathion is ovicidal and hence requires only 1 to 2 treatments. Once the lice develop genetic resistance to any pesticide, they do not get killed with that pesticide.

Failure of treatments:
Inadequate application of medication, poor compliance, development of resistance by the parasite to the medication, failure to retreat nymphs that have recently emerged from the eggs and reinfection- all these can lead to failure of treatments.

Resistance to topical pediculicides:

     The treatment of head lice is now complicated by the emergence of resistance to pediculicides. Since most clinical trials pertaining to the efficacy of the drug were done before the emergence of resistance, the choice of treatment will depend on local patterns of resistance. Recourse to testing for resistance is necessary where treatment has failed. .

Mechanical removal by wet combing:

     ‘Bug busting’ or wet combing with conditioner was proposed by the UK Charity Community Hygiene Concern as a method of detecting head lice and as a means of treatment. The method involves combing thoroughly wet hair with fine toothed comb. A Bug Buster kit was developed after trials comprising instructions and materials to undertake four sequential combings on wet, conditioned hair, leaving three days between each. The efficacy of this method of treatment is still unproven. The modified Bug buster kit which is in current use is the 1996 version.

      Some have recommended shaving of heads in children for treatment of lice because it can be safer than insecticides (Lwegaba, 2005

Treatments that need further study:

1. Mechanical removal of lice and eggs by combing: Many combs have been developed for lice removal. There is little evidence to support the use of any combing method.

2. Herbal treatments and aroma therapy: Tea oil, olive oil, etc.: No studies have evaluated their efficacy or possible toxicity.

3. Oral anti-parasites:

Thiabendazole- The primary action of this drug is inhibition of parasite microtubule polymerization by binding to β-tubulin. It also interferes with the synaptic transmission of lice through its probable cholinergic effect. This drug is recommended as mass and simultaneous treatment rather than individual and isolated treatment because, new infestations can occur during the drug-free interval.  Trials are still warranted. The drug may prove to be useful in resistant cases (Namazi, 2003).

Cotrimoxazole- This is an antibiotic containing Sulfamethoxazole and Trimethoprim. It acts by killing the symbiotic bacteria in the gut of the louse. It may also have some direct toxic effect on the louse.

Ivermectin- This drug is basically an antihelmenthic. However, it is neurotoxic in children and must not be given in those less than 15 kg.

4. Occlusive agents- Standard petroleum jelly, a viscous substance, acts by obstructing the respiratory spiracles of the adult louse and the holes in the operculum of the eggs, thus blocking efficient air exchange. The jelly has to be massaged on the entire surface of the hair and scalp and left overnight with a shower cap on. Diligent shampooing is usually necessary for at least the next 7 to 10 days to remove the residue.

5. Glue-loosening agents- Many such products are available in the markets. Vinegar or vinegar-based products, formic acid (8%), acetone and bleach to vodka are intended to be applied to the hair for 3-10 minutes before combing out the nits. There is no evidence of any studies reporting the efficacy of these organic solvents (Frankowski, Leonard & Weiner, 2002).

     Treatment with pesticides should not be started unless live lice are found. Nits are not a sign of active infestation in case of head lice. There is no evidence to support the cleaning of bed sheets, pillow covers and clothing, or treating of any head and neck gadgets, baseball helmets, and furniture with insecticide sprays is effective in preventing infestation with the parasite. Head lice are harmless. If detached from their host they are vulnerable and effectively dead. Lice seen on chairs, pillows, and hats are dead, sick, or elderly or are cast skins of lice and these cannot infect a person (Nash, 2003). Head lice usually survive for less than 1 day away from the scalp at normal room temperature, and their eggs cannot hatch at an ambient temperature lower than that near the scalp. Hence unnecessary insecticide sprays only pose a health hazard. Banning children with nits in schools is no good a policy. Many schools treat all children with nits as though they are contagious and such children are unnecessarily removed from school until they are treated.  Head lice are probably more common in girls. This is because girls are more likely to have close contacts during play and not because they have longer hair. Cutting hair or tying it back is not helpful and it may actually increase the incidence of infestation by making it easier for lice to move off of and on to the scalp.


     It is actually impossible to totally prevent head lice infestations. Children come into close head-to-head contact with each other frequently while playing. Older children can be taught not to share personal items such as combs, brushes and hats. Appropriate treatment in those with infestation prevents spread to others.


Burgess, I. F, Brown, C. M, Lee, P. N (2005). Treatment of head louse infestation with   4% dimeticone lotion: randomised controlled equivalence trial. British Medical Journal (330),1423.

Frankowski, B. L., Weiner, L. B., Committee on School Health, Committee on Infectious Diseases, (2002). Head Lice. Pediatrics 110: 638-643.

Lwegaba, A. (2005). Shaving can be safer headlice treatment than insecticides. British Medical Journal (330), 1510.

Molner, J. G. (1945). Treatment of Pediculosis Capitis. American Journal of Public health (35), 1302-1303.

Namazi, M. R. (2003). Treatment of pediculosis capitis with thiabendazole: a pilot study. International Journal of Dermatology (42)12, 973–976.

Nash, B. (2003). Treating head lice. British Medical Journal (326), 1256-1257.

Polack, R. J., Kiszewski, A., Armstrong, P., Haln, C., Wolfe, N., Rahman, H. A., Laserson, K., Telford, s. r., & Spielman,a. (1999).Differential permethrin susceptibility of head lice sampled in the United States and Borneo. Arch Pediatr Adolesc Med. (153), 969-973.

Sladden, M. J. (2005). More common skin infections in children. British Medical Journal (330), 1194-1198.

Stichele, R.V.H, Dezeure, E. M. & Bogaert, M. G. (1995). Systematic review of clinical efficacy of topical treatments for head lice. British Journal of Medicine (311), 604-608.


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