Experimental and Clinical Pharmacology
Treating head lice
- Orli Wargon
- Aust Prescr 2000;23:62-3
- 1 March 2000
- DOI: 10.18773/austprescr.2000.066
Head lice (Pediculosis capitis) is a common condition in children. It is usually detected in schools, when children are seen scratching their heads. The diagnosis can be confirmed by finding live adult lice or viable eggs. Lice are usually adequately treated with an over-the-counter topical permethrin preparation. Treatment failure results from inadequate therapy, re-infection, resistance to the insecticide or an underlying immunosuppression. Prescription drugs such as ivermectin may be needed in more difficult cases.
Blood sucking lice of the order Anoplura are successful obligate ectoparasites of humans.1 Pediculus humanus capitis, the head louse, is a distinct clinical form. It clings to hair with its claws and feeds by sucking blood from the scalp. Its life cycle is about 17-20 days with eggs hatching 7-10 days after they are laid, and adults are fully developed about 10 days later.2 Head lice spread by head to head contact.
A child found to be scratching their head at school is often checked by the teacher, school nurse or public health personnel. If they are well trained or experienced they will look for live adult lice on the crown of the scalp, immature nymphs or viable eggs. Empty shells or nits adhere to the hair, 1 cm from the scalp surface, with a glue-like substance. They are unlike seborrhoeic scales, hair casts and hair spray, which are easily brushed off.
Three types of insecticide are marketed worldwide. In my local pharmacies all insecticide products for head lice are either permethrins 10 mg/mL as shampoos, lotions or creme rinses or maldison products with 0.5% alcohol bases, 1% foam bases, shampoos or soaking solutions.
In theory, one application should kill all lice and eggs. However, in vitro studies3,4 suggest that some eggs can survive and require a second application after seven days. Surviving eggs can cause reinfestation if not removed.
Everyone who has been in contact with the patient should be examined and treated if affected. It is also advisable to wash all clothes, head gear, towels, bed linen, combs and brushes as head lice can survive away from the human host for about three days and eggs can survive for up to 10 days.
Permethrin (3 phenoxybenzyl cis trans3(2,2-dichlorvinyl)-2,2 dimethyl cyclopropane carboxylate) is a synthetic pyrethroid. It acts on parasitic nerve cell membranes and has low mammalian toxicity, but incomplete ovicidal properties.6 Patients apply permethrin to the hair for 10 minutes then wash it off. As permethrin is not ovicidal, treatment may need to be repeated after a week.
Maldison is a moderately toxic organophosphate insecticide and a fast-acting ovicide which acts by non-reversibly blocking acetylcholine.6 Patients apply maldison to their hair then wash it off after 12 hours.
Evidence of therapeutic efficacy
The most often quoted systematic review of the topical treatments for head lice7 concludes that there is only sufficient evidence to support the efficacy of permethrin. However, a recent review by the Cochrane Collaboration could not make a recommendation about which treatment is best.8
A visit to the local pharmacy revealed naturally occurring substances including echinacea and melaleuca oil being marketed to treat head lice as well as an electronic lice comb. Reports supporting mechanical methods are anecdotal.9
Other anecdotal reports include the use of petrolatum used for its occlusion properties.10
Ivermectin is an antiparasitic agent used extensively in veterinary practice and more recently has been used in human medicine. The mode of action is via chloride ion channels in cell membranes, leading to an influx of negatively charged ions that block cellular action potentials and cause muscle paralysis. Much higher concentrations are required to affect neurological function in mammals than in parasites. Open studies on oral ivermectin, using a single 200 microgram/kg oral dose with or without a second dose at 10 days11,12 suggest that further trials are warranted.
In the UK resistance to permethrin is widespread. In Australia resistance to maldison has been reported.13
Safety and adverse effects
If used correctly the treatments have no major adverse effects. Patients may develop stinging or tingling of the skin, erythema of the scalp or red eyes. Maldison does not have the potential to cause a specific polyneuropathy as, unlike other organophosphates, it does not bind to the relevant target protein.14
Parents are generally shocked when they discover that their children have head lice. It often becomes very difficult to trace contacts as parents do not wish to admit to their friends and family that their children have lice, because of the associated embarrassment and social stigma. On a practical level, whenever a school discovers even a few eggs, the whole class is treated and the recommendation from schools is often that the entire family also be treated.
Systematic review shows that only topical permethrin is efficacious for head lice. Clinical experience and information on the ovicidal properties of maldison suggests that further randomised controlled clinical trials are necessary to assess the efficacy of this alternative and cheaper insecticide. Immuno suppression may require the consideration of other medications and again appropriate clinical trials are needed.