VOL: 102, ISSUE: 26, PAGE NO: 45
Ian F. Burgess, MSc, MPhil, FRES, is director, Medical Entomology Centre, Insect Research & Development Limited, Shepreth, Royston, CambridgeshireFollowing recent developments in the diagnosis and control of head lice in the community, researchers are now looki...
Following recent developments in the diagnosis and control of head lice in the community, researchers are now looking at how people respond to head lice and how well they cope with them.
A ground-breaking study conducted in Belgium in 2005 looked at a group of over 6,000 children and measured all the factors that influenced whether they caught head lice and how well the families coped with them (Willems et al, 2005). The most important factors they identified for catching head lice were family size (number of siblings), socioeconomic status of the family unit, long hair, and hair colour. In terms of eradicating the head lice, the same basic underlying factors also had an effect but the most important was socioeconomic group, which means that for a vulnerable section of society least able to afford treatments, head lice simply add to their financial burdens. Having brown hair is also associated with difficulty finding and dealing with head lice.
The louse's life cycle
The life cycle of the head louse is the main factor that needs to be considered in the practical management of an infestation. Only one unpublished study has looked in detail at head lice living in an environment essentially the same as their natural habitat. By rearing head lice in containers strapped to his arm, Jim Lang, a PhD student in Arizona, was able to measure the length of time required for head louse eggs to hatch and for each of the nymphal development stages to complete its development (Lang, 1975). At skin temperature, louse eggs hatch between five and seven days after being laid. If the temperature drops below 33 degsC this period is extended. After hatching, the first-stage nymph feeds on blood until it is fully engorged and repeats this process several times each day for the rest of its life.
As the nymph feeds it uses the protein from the blood to grow new tissues until it reaches the point where it has outgrown its skin and the only way to grow further is to produce a new, larger skin and shed the old one. That process occurs three days after the nymph hatches. The second- and third- stage nymphs feed and grow in exactly the same way as first-stage nymphs, and in each case the development takes approximately three days.
After the third moult, the lice become adults, which means that development from hatching to maturity takes around nine days, although male lice become mature as much as one day earlier than females if conditions are favourable. Nymphs in the young juvenile stages (first- and second-stage nymphs) are relatively sedentary and do not normally move to a new host. Only third-stage nymphs and adults migrate.
Once the head lice are adults they can mate and produce offspring for themselves. Male head lice are extremely mobile and are constantly on the move in search of a mate. Females take about one day after their final moult to become fully mature. After mating, the females start to lay eggs within about 14 hours and for the first few days they lay between five and 10 eggs each day. However, this high rate of egg-laying does not last more than a few days and the females lay only four eggs a day for much of their lives.
Having a realistic idea of how fast head lice reproduce and knowing how fast they develop play a fundamental role in the timing of actions used to control them. For example, if all the mobile stages of head lice are removed or killed, the juveniles that hatch from eggs unaffected by the treatment will not pose an infestation risk for other people for about another six days. Also, after treatment, any viable eggs will have completed development and hatched seven days later, which is important for the timing of repeat treatments.
Diagnosis - the best method
There is controversy about how best to diagnose head louse infestation. Irrespective of the details, it is now largely agreed in Europe at least, that combing is more effective than visual inspection (Mumcuoglu et al, 2001; De Maeseneer et al, 2000). It does not matter whether this is carried out on dry hair or whether the hair is wet and lubricated with oil or conditioner before combing it. In most cases a plastic detection comb is more effective for finding head lice than a metal one. Plastic combs are more comfortable to use and less likely to damage hair or pull it out. However, the comb must have certain characteristics (see Box 1). The best detection comb is made from polystyrene, which may be more likely to break than a soft polypropylene one if it is handled roughly, but it is stiff enough to hook out even the smallest lice.
Developments in treating head lice
During the past year, new products for treating head lice have been released and new clinical studies evaluating treatments have been published. Two new products are Hedrin 4% dimeticone lotion (a 'P' classified medicine, available only from a pharmacist), and Full Marks solution (a class I medical device for use with a comb). Both these products act by coating the louse and preventing it from carrying out its normal physiological functions.
There is no clinical evidence yet to support the Full Marks product, but Hedrin was found to cure at least 70% of cases in two clinical trials (Burgess et al, 2005). The trials also found that most children needed more product applied than is often used. Many prescriptions provide doses that are too small: for example, a family involved in a study that comprised three teenage girls with long, thick hair had been prescribed one 200ml bottle of aqueous product for treatment before the study. However, because each of the girls required over 150ml for a single application for two treatments a week apart, they actually needed between 900ml and 1200ml (Burgess et al, 2005).
Combing as a treatment
Until recently, there has been little evidence to guide anyone wishing to consider combing as a treatment option. In a study published last year that compared combing with single applications of aqueous insecticide products (Hill et al, 2005), the combing option was found to be more successful, curing 32 out of 62 enrolled participants (52%), whereas malathion or permethrin cured only nine out of 71 people (13%). Wet combing showed a better performance in this study than previous ones. Burgess et al (2005) found that 168 of 260 people (65%) enrolled in their study had not used insecticides for over three months but still had lice. All had been wet combing during that time, and 69 (27%) had only ever used wet combing.
What does this mean in practice? More than likely it means that many people who wet comb do not fully understand the principles of what they are doing and, for example, use the wrong type of comb.
Combing is arduous, and many people do not comb enough. The most difficult problem to address is the fact that there is no clear way to determine whether all the adult lice have been removed. Any lice that remain will lay more eggs before the next combing session and so perpetuate the problem.
Advocates of wet combing usually recommend conditioner as a lubricant when combing the hair and to help straighten out tangles. What they don't realise is that, possibly, it is the conditioner that is the secret weapon in the fight against lice. Two recent publications indicate that a conditioner could be the active component in wet combing. A report from Belfast described applying Dove conditioner thickly from root to hair tip and leaving it in place for at least two hours. Two applications a week apart on eight people cured them all (Eames, 2004). In another study, 133 people used a conditioner-type lotion that was left on and dried using a hair dryer. Overall, 96% were cured, as assessed by wet combing the hair (Pearlman, 2004). The common factor in these two reports was that the conditioner was left in place longer than in normal use. Under these circumstances it is quite likely that the conditioner could work to kill the lice.
Pearlman (2004) suggests that the conditioner may block the spiracles (breathing holes) of the lice, but it is likely to be as a result of the pharmacological action of conditioner components on the integrity of the body surface of the louse. If conditioner does have a real effect on lice, it could be that they will develop a resistance to it.
Healthcare professionals need to ensure their patients know the treatment options for managing head lice. They also need to be aware that a treatment course must be completed to eradicate the problem.
NHS Direct - www.nhsdirect.nhs.uk.
Healthcare A-Z - www.healthcarea2z.org.