Head lice have become extremely common. There can be few households with school age children that have not suffered infestation at some time, or received repeated warnings from the school authorities that there is a head lice problem. Head lice successfully colonise the heads of children from both poor and affluent backgrounds and it is clear that they are happy living in clean or dirty hair.

The widespread problem means that people are increasingly well informed about head lice, and although it can still be distressing to find head lice on family or friends, the stigma associated with this has lessened. Head lice can live on adults, but are less common in men than women. Head lice are not known to cause serious harm.

Head lice do not fly or jump; they can only walk from head to head when two heads are close together. Full grown lice are about 3mm in length, but are not easily seen on the hair. Lice feed on blood which they suck from the scalp. Female lice glue their eggs to the hair shaft where it is warm enough to incubate the eggs (generally close to the scalp). They may lay about two to five eggs in 24 hours and the eggs take 7 to 10 days to hatch. Development from hatching to maturity takes 9 to 14 days. In most cases, the head is not "crawling with lice"; often there are fewer than 10 lice present.

Although some people who have head lice will experience itching of the scalp, others have no obvious symptoms. Careful inspection of the hair may reveal empty egg cases (seen as white dots glued to the hair) called "nits". Live eggs are more difficult to see because they can appear the same colour as their surroundings. The most likely sites for eggs (but not lice) are behind the ears and at the nape of the neck. Live lice may be seen anywhere close to the scalp.

Combing wet hair with a detector comb is the best way to determine whether live lice are present. Detector combs have fine teeth so even newly hatched lice cannot pass between them. Combing with a detector comb can be difficult, particularly with long hair. Hair conditioner and prior untangling with a wide-tooth comb makes using the detector comb more comfortable. Lice can be seen on the comb or trapped between the teeth.

Odd flakes of skin may also be seen on the comb, but lice can be identified by shape and sometimes movement. In dry hair, lice move rapidly away from any disturbance while in wet hair they remain motionless.

Regular use of head lice combs has come to the fore in both the detection and treatment of head lice infestations. Three main factors have contributed to this - public concern about the safety of parasiticidal treatments (highlighted in recent television and press reports), problems of resistance and ineffective treatment, and reinfestation in children at schools and nurseries.

There is no need to wash bedding and so forth when treating head lice. Any lice found off the head, unless actively removed, will be dead or dying. Lice removed from the host by combing may be able to survive for one or two days, but will probably suffer dehydration and be unable to feed after a much shorter time.

If live head lice are found, regular systematic wet combing, although time consuming, can be used to clear them. It is recommended that four combing sessions at halfweekly intervals over a two-week period are required. At the first session, all hatched lice are removed. Lice which hatch from eggs laid before the first combing session are removed at subsequent sessions. Newly hatched lice do not move from the host during the first few days of life, so between sessions the patient will not infect others. If full grown lice are found at the second, third or fourth session then the patient has caught lice again or some have been missed at the first combing session. This means the combing sessions must continue at half-weekly intervals for another two weeks.

At each session, the hair is systematically combed through from the roots (starting with the teeth of the comb touching the scalp) to the tips with the hair covered in plenty of conditioner. Conditioner is wiped off the comb with a white cloth after each stroke so lice are visible on the cloth. Then the hair is combed again when the conditioner has been rinsed off to check that all adult lice have been removed. The hair is left very wet rather than towel dried for combing, because lice are less able to move when wet. Once clear, combing every week should prevent reinfestation, with the more frequent combing regimen adopted if live lice are again detected.

It is estimated that this combing regime can be effective in about 80 per cent of cases. However, this leaves a minority who do not find the method effective and may need extra support.

Another development in combing is a battery operated electric comb (Robi Comb). The comb passes an electric current through anything caught in its teeth. The electric shock kills lice of all ages but does not affect eggs. Any debris caught between the teeth has to be removed before the current can be reactivated.

The British National Formulary lists malathion, carbaryl, permethrin and phenothrin for treatment of head lice. Carbaryl, although still available, was made prescription only in 1996, because animal studies suggested that it is a potential human carcinogen. However, the Department of Health (UK) has emphasised that the risk from intermittent use of head lice preparations is likely to be extremely small.

Malathion, an organophosphate, remains a pharmacy medicine. Both malathion and carbaryl are available as alcoholic lotions or aqueous liquids. Aqueous formulations are recommended for people who have asthma or eczema, and for young children. Shampoos are inadvisable because they have to be reapplied and are less effective than other formulations. The BNF recommends that lotions and liquids are left on the head overnight to ensure all the eggs are killed, rather than the two hour contact times suggested by some manufacturers. It is important to use the full quantity of liquid or lotion, otherwise the product may not be effective. An adult requires about 50ml; children may need proportionately less but this depends on the thickness of the hair.

The BNF advises that a second application of head lice treatment is used seven days after the first, to kill any lice emerging from eggs that survived the first treatment. Combing with a detector comb should determine whether this is necessary.

Most areas still operate some sort of rotation policy for head lice treatments. The rotation policy was originally adopted as a means of delaying the development of resistance. Such policies operate on a local basis and further information can be obtained from the local pharmaceutical adviser or community services pharmacist. However, since resistance has been demonstrated with all agents, flexibility in treatment is needed - if one treatment does not work, another should be tried.

When head lice are found, all family members should be carefully examined for signs of infestation. Current opinion is that treatments should now only be used on those where infestation is demonstrated by finding live lice, rather than treating the whole family as a matter of routine. Likewise, when schools report that some pupils have head lice, individuals only need treatment if lice are found. Most of the head lice treatments still include instructions to treat the whole family, so patients need to be advised that this is not necessary. All the parasiticides carry warnings concerning use on permed or colour treated hair. While the manufacturers report that problems appear to be uncommon, products can be tested on a strand of hair before use on the whole head. This may be particularly important if colour or perm lotions have been applied in the previous four weeks.

Chlorine inactivates some pediculocides, and this is why there are sometimes cautions concerning swimming when head lice preparations are used. If a patient has been swimming on the day of treatment, the hair should be shampooed and dried before the treatment is applied. There is no reason to avoid swimming the day after application of any product.

There are on-going concerns about the possible adverse effects of exposure to parasiticides and they should not be used more than once a week or for more than three consecutive weeks. Malathion is reported to be one of the "safer" organophosphates, although a small percentage is still absorbed through the skin.

It is important that patients have sufficient information to choose the strategy, whether combing or insecticidal treatment, that is most suited to their individual needs and circumstances.

Preventing infestation and reinfestation with head lice is a difficult issue. Head lice treatments should not be used on a prophylactic basis. Piperonal (Rappell) is a head lice repellent which is sprayed on to the hair and provokes a negative response from receptors on the antennae of the lice so they do not move in that direction. The spray is applied daily. It does not kill lice or eggs. However, clinical studies have not been reported and at present the value of this approach is uncertain.

There has been some interest in the use of tea tree oil products for head lice. Theoretically, there is some justification for this since tea tree oil contains terpenes and these substances have been reported to have insecticidal and ovicidal activity. However, at present, there is no evidence to support the use of tea tree oil products in treatment of head lice and there have been a number of reports of adverse skin reactions caused by the oil.

Head lice seem to have reached epidemic proportions in many schools. However, the tide can be turned by wet combing once a week with a detector comb, so problems are quickly recognised. Weekly wet combing is a sensible precaution for all nursery and school aged children. When head lice are found, other contacts should be notified.