Treatment of head lice

The treatment of head lice is now complicated by the emergence of resistance to pediculicides. Most clinical trials were done before resistance emerged and reviews of these trials do not give clear guidelines to the clinician. In these circumstances, the choice of treatment will depend on local patterns of resistance, and where treatment has failed, recourse to testing for resistance is perhaps the best way forward.

Human head lice (Pediculus capitis) are ectoparasites with an obligatory blood feeding habit, which requires them to feed on their host's blood several times each day. Juvenile and adult forms both occur on the scalp, and eggs are attached to the hair shafts near the scalp. Infestation with head lice is a widespread condition that is seen most commonly, but not exclusively, in children of school age, although there is no proof of a link with school attendance. Prevalence does not vary significantly with season, and variations are probably due to changes in social behaviour rather than climatic conditions.¹ Considerable social stigma is associated with infection, which may cause psychological distress in the sufferer. This is often related to the common misconception that infestation with lice is indicative of uncleanliness, although this is unfounded as there is no evidence to support such a view.

Infestation with head lice is essentially harmless and most cases are symptomless. This means that most people may not know that they have lice up until pruritus develops as a result of sensitisation to louse saliva. This reaction may take up to three months to develop after initial infestation.² Secondary infection may occur as a result of bites being scratched, although bites may also become infected by the bacteria carried on the bodies or limbs of the lice or in their faeces. Head lice are thought to be one of the commonest causes of pyoderma of the scalp in developed countries.³,⁴

It is possible to contract head lice only by relatively prolonged head to head contact with an infected person, which typically means that the infection is passed between people who know each other well. Only the presence of live lice can confirm diagnosis of active infection. The presence of eggs alone is not sufficient for diagnosis as eggs may retain a viable appearance for weeks after death.

As most infections have existed for weeks rather than days before they are discovered, contacts over the previous month should be traced. Time spent curing an individual is wasted unless infectious contacts are also traced and treated. This reduces the risk of reinfection to the patient as well as the degree of transmission of lice on a wider scale.

The effectiveness of different treatments was assessed in two systematic reviews of poor quality trials. The first review⁵ found that permethrin was more effective than lindane.⁶,⁷ The subsequent Cochrane review set stricter criteria for quality of trials and excluded both of the trials on which the results of the earlier review were based.⁸ The Cochrane review found no evidence that any one pediculicide had greater effect than another. The two studies comparing malathion and permethrin with their respective vehicles showed a higher cure rate for the active ingredient than the vehicle.⁹,¹⁰ Another study comparing synergised pyrethrins with permethrin showed their effects to be equivalent.¹¹ A comparative trial of malathion lotion versus bug busting—wet combing with a hair conditioner—showed combing to be ineffective.¹² However, the emergence of drug resistance since most of these trials were conducted means there is no current evidence of the comparative effectiveness of these products. No evidence exists regarding the effectiveness of other chemical control methods, such as herbal treatments in the curative treatment of lice.

Resistance to synthetic pyrethroids has been reported from France, Czech Republic, Israel, and England, and more recently resistance to organophosphates has been reported. In some areas of the United Kingdom, in vitro levels of resistance to permethrin and malathion may be as high as 87% and 64% respectively.¹³ There is no evidence in the literature that widespread resistance has developed to carbamate pediculicides. However, patterns of resistance will vary geographically, so it is not possible to make an overall estimate of the level of resistance to the various pediculicides in different areas. The best choice of treatment will depend on local resistance patterns, and where treatment has failed this should be assessed on a case by case basis.