Dr Tony Downs looks at the latest evidence on epidemiology and treatment options
Accurate evidence-based advice is becoming less available to patients as the management of head lice moves from GPs to health visitors, school nurses, and pharmacists. This shift is taking place when widespread drug resistance is being largely ignored by the Department of Health and pharmaceutical industry.
During a head lice consultation you should endeavour to identify a living louse with a louse detector comb, and examine other family members present. Only members within a family unit who have head lice require treatment. Treatment should always start with combing before moving on to insecticide treatments.
Head louse biology
The head louse has a lifespan of about 30 days and a variable reproduction rate (one to 10 eggs per day). The scalp provides a relatively warm and humid environment. Neither louse nor egg is able to survive outside this environment for more than a few hours.
Hair length of less than 2mm will not maintain the required controlled environment. The lice stay close to the scalp and are regularly seen wandering through and over the hairs. They suck blood every four hours.
Head lice can survive immersion under water and exposure to detergent solutions for more than 30 minutes without ill-effects.
The school health returns to the Medical Officer were abandoned nationally in 1987. As a result there has been no ongoing information on the prevalence of head lice in school children the major group affected.
Fifty-three GP practices across England and Wales collect information on head lice as part of the RCGP Birmingham Research Unit Weekly Returns Service.
The mean weekly consultation rate for head lice infestation per 100,000 population in the age groups up to 14 years suggests the incidence peaked in 1996 and has been steadily declining.
However, lower rates of GP episodes may not necessarily mean a fall in head lice incidence. Children may be being treated without GP consultation (75 per cent of all insecticide preparations for head lice are sold over the counter; parents may use a single prescription item to treat all affected family members; and de-lousing combs are non-prescription items). In addition, half of infestations are asymptomatic, particularly those with a small louse load.
My own experience from our research group in Bristol would suggest a relatively fixed infestation percentage of 20 per cent of school children during the winter, declining during the summer.
Our assessment of point prevalence in other parts of England, however, shows that there is a wide variation (5-24 per cent infestation rates) of head lice within a community which would be dependent on a variety of factors. At times, head lice in a school may be common.
Factors that affect transmission
The RCGP data shows head lice are not confined to school children and are present at a lower incidence in adults. A seasonal variation is seen in all age groups, but is more obvious in children, with a low incidence in the summer.
A significantly higher incidence is seen in females. When schoolchildren (aged four to 11) are assessed more closely, it can be seen that four- to six-year-olds mainly have lower infestation rates than seven- to eight-year-olds, and usually much lighter louse loads. After the age of eight, infestation rates tend to decline.
The behaviour patterns in girls and boys at different ages may affect transmission and therefore infestation rates. Primary school children are organised into small learning groups around desks where co-operation and sharing is encouraged and hair contact is more likely.
With other children, independence is fostered so there may be more separation between individuals, reducing the potential for transmission.
Head lice are present throughout the world. A low incidence has been noted in African-Americans. No such low incidence is seen in Africans or UK Afro-Caribbeans. African head lice have been reported to show a slight anatomical variation allowing a better grip of African-type hair.
This might explain the low incidence seen in African-Americans. The regular use of pomades and gels to straighten curly hair in this community may, however, be an alternative explanation for this low incidence. These products, as well as head shaving, can provide an inhospitable climate for head lice.
The regular act of combing to aid hair straightening will also remove head lice. Curly hair can be difficult to comb with a louse detector comb. It is important not to exclude African-type hair when assessing and treating for head lice. Extra time may be required to avoid missing infested cases.
Which insecticide is best?
Carbaryl is highly likely to cure a head lice infestation, but there are concerns that it is carcinogenic and it is important that this product is not overused.
To date, clinical evidence suggests that pyrethroids (permethrin and phenothrin) are no better than placebo. The effectiveness of malathion will vary from region to region in England and Wales. When taking into account the research carried out by different institutes, malathion has a 50 per cent chance of curing an infestation.
Resistance to pyrethroids (phenothrin and permethrin) is an international problem. Our exposure tests on head lice to phenothrin and permethrin from samples taken across England in 2002 showed a universal inability of these products to kill head lice. Genetic sampling of English head lice shows a high prevalence of pyrethroid super-resistant genes.
A randomised controlled trial in Bristol schoolchildren presented at the American Academy of Dermatologists meeting in 2002 and now awaiting publication showed a kill rate of 46 per cent to placebo (normal shampoo) and a 0 per cent kill rate to 1 per cent permethrin (Lyclear Creme Rinse). The detergent effect of normal shampoo will kill and remove some head lice but a much higher kill rate would be needed to eradicate a community-wide infestation.
Increasing the concentration of permethrin or phenothrin, or switching to an alternative pyrethroid, will not overcome this drug resistance, because the mutation conferring resistance is at the drug target-binding site.
Malathion resistance is common in England and Wales. Rates vary reflecting different resistant mechanisms in head lice. Where resistance to malathion is as a result of altering the drug target-binding site (the acetylcholinesterase enzyme), then resistance is absolute. Where resistance to malathion is generated by accelerated detoxification of malathion, then resistance is less complete and likely to vary from generation to generation of head lice. This is because these types of resistance mechanisms tend to depend on more than one genetic mutation.
For such a common and relied upon treatment, there is very little evidence to support its use. There is still controversy as to the most appropriate combing method.
Combing takes time, and can be impractical for large families. The minimum combing strategy appears to be twice a week for two weeks. It is not known if more frequent combing, and combing for more than two weeks, is more effective. How many times the hair should be combed during a combing session is also unclear.
Lice detector combs have teeth that are 0.2 to 0.3mm apart to trap the lice. Plastic combs are less traumatic to the scalp than metal ones, but they do not last very long with regular use. Advocates of the 'bug busting' technique insist that maximum benefit is obtained by combing on days one, five, nine and 13. Hair must be shampooed, rinsed, and conditioner applied to facilitate combing.
In one UK randomised clinical trial observers carried out either phenothrin lotion treatment one week apart, or the 'bug busting' protocol. By day 14, 13 per cent of participants were louse-clear in the phenothrin group, and 53 per cent of participants were louse-clear in the combing group.
Tea tree oil
Essential oils, including tea tree oil, are promoted as treatments for head lice by alternative medicine therapists. Tea tree (melanenca alternifolia) oil is a complex mixture of 100 hydrocarbons and terpenes. Of the 15 compounds found in highest concentration, 12 are monoterpenoids. One of these monoterpenoids, alpha-terpeniol, is also an incipient ingredient in at least one standard insecticide-containing head louse treatment lotion (Suleo M).
It is felt to be more effective than its non terpene-containing counterparts. Exposure tests of head lice to tea tree oil and alpha-terpeniol show that these products are both effective insecticides. High concentrations, however, are needed to obtain a 90 per cent kill rate. Tea tree oil is a volatile substance, and there may be insufficient chemical remaining on the scalp following application to kill lice.
By and large, shampoos are less effective than lotions when it comes to killing head lice, and this may be true for tea tree oil as well. Proper dose mortality curves have not been established.
Long-term safety data is also lacking, which may be particularly pertinent, given that head lice treatments are applied frequently throughout the year on the scalps of children. The promotion of commercially available essential oils as treatments should therefore be discouraged until more data is available.
Battery-powered devices such as the Robbi comb have electrodes substituted for the teeth of the comb. When a louse-sized object falls between the electrodes, it is cauterised.
This device may be little more than an expensive novelty item, and has undergone no published clinical testing.
Key messages to get across to patients
·Head lice are transmitted by close contact.
·Short hair and regular grooming decreases transmission.
·Drug resistance is prevalent, but patients should avoid gimmicks such as electric combs and alternative medicines.
·The patient and family should not be concerned about the presence of nits (empty eggshells).
·Head lice infestations are, by and large, asymptomatic. The commonest symptom is itching. Occasionally, secondary infection (fungal or bacterial) is seen.
·Head lice only survive a few hours off the host.
·They may or may not wish to discuss the infestation with the school nurse. If prevalence is high within the classroom, then a co-ordinated treatment may be helpful.
Tony Downs is consultant dermatologist, Royal Devon & Exeter Hospital