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Marjorie Bekaert Thomas
Advances in health and medicine.
Children's Health Channel
Reported November 2, 2012

Head Games: Wiping out Lice Infestation


(Ivanhoe Newswire) -- Although anyone can get head lice, it most commonly occurs in children three to 12 years old. In the United States, approximately six to 12 million children have head lice infestations each year.  It’s commonly perceived as a sign of poor hygiene and resistance of head lice to insecticides led researchers to look for alternatives.
Head lice are parasitic insects that can be found on the head and neck and survive by feeding on human blood. Lice are spread most commonly by close person-to-person contact. Only a live louse on the head should define infestation, with detection enhanced by fine-toothed combing of hair. The head louse, Pediculus humanus var. capitis, is an obligate, ectoparasite that feeds three to six times daily.
Medical consequences of infestation can include itching and scratching, which may lead to cutaneous bacterial superinfection and possible transmission of Bartonella quintana, the trench-fever pathogen that is usually transmitted by body lice and that has been isolated from head lice.
The relevant primary end point of studies is the louse-free rate of the infested population, ideally at the end of the louse's life cycle, because not all drugs are active against all developmental stages (e.g., eggs). In the mid-1980s, pyrethrin and pyrethroid insecticides, with claimed efficacy (of up to 95%), safety, convenience, cost-effectiveness, and a supposedly favorable environmental-safety profile, were marketed for head lice and progressively replaced other insecticides but in the early 1990s, a randomized, controlled trial involving French schoolchildren showed that 24 hours after application, 0.5% malathion lotion, an infrequently used insecticide at that time, achieved a better louse-free rate (92%) than 0.3% d-phenothrin (40%). Ex vivo parasitologic testing of freshly sampled lice gave the same results, raising the possibility that lice might have acquired a pyrethroid-resistant phenotype.
Pyrethroids are neurotoxins that modify louse voltage-gated sodium channels (VGSC), causing spastic paralysis and death. 
Ivermectin is an antiparasitic drug used for onchocerciasis and lymphatic filariasis. It induces arthropod and nematode paralysis and death by interrupting neurotransmission, acting on glutamate-gated or γ-aminobutyric acid–gated chloride channels. A recent cluster-randomized, controlled trial showed that a single oral dose (400 μg per kilogram of body weight) repeated within 7 days achieved higher louse-free rates on day 15 than 0.5% malathion among patients with head lice refractory to insecticides (95.2% vs. 85.0%) and their household members (92.4% vs. 79.1%).
Although oral ivermectin (at a dose of 400 μg per kilogram) is used off-label, its safety for patients with head-louse infestation remains unknown. However, more than 45 million people have taken oral ivermectin at a dose of 200 μg per kilogram for other infections. The lotion is convenient (i.e., applied to dry hair, left for 10 minutes, then rinsed with water), which should increase compliance. Notably, in a randomized, controlled trial in which malathion lotion was applied for 8 to 10 hours on parent and child volunteers, compliance reached only 50 percent. In an in vitro experiment, topical ivermectin acted as a posteclosion nymphicide by reducing head-louse blood-feeding, a finding that explains the single-application protocol of the current studies. However, the louse-free rate on day 15 was lower than on day 2, suggesting less in vivo activity.
Strong ivermectin selection pressure in a population requires prudence, because it will eventually select resistant lice. Phenotypic and genotypic studies have shown emerging ivermectin resistance in Onchocerca volvulus in countries (e.g., Ghana and Cameroon) where ivermectin is widely used to control onchocerciasis. 
So, how should head-louse infestation be managed? Good comparative-effectiveness research is still lacking. As of right now, options include wet combing or treatment with dimethicone or other topical agents, depending on the availability of the agents in the country. Nit removal is useful. Ivermectin should be the last choice, whether topical (for still-infested persons) or oral (especially for mass treatment). Management should also include more frequent checking for head-louse infestation in families and schools.
Source: New England Journal of Medicine


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