Trying to Keep Ahead of Lice: A Therapeutic Challenge

C. E. Malcolm, MD, CCFP; J. N. Bergman MD, FRCPC


Skin Therapy Letter. 2006;11(10):1-6. 

In This Article


Pediculosis capitis, or head lice, is a world-wide public health concern affecting persons of all ages and socioeconomic backgrounds. It is caused by Pediculus humanus capitis, an obligate ectoparasite that lives on human hair and feeds on the blood from the skin. Upon diagnosis, treatment should be initiated, since established infestations with head lice generally do not spontaneously resolve. Chemical pediculicides are currently the standard treatment, however, issues of resistance have made it necessary to explore new alternatives. If an infestation is resistant to these drugs, then the physician should consider treating with an agent from a different class of pediculicides or, potentially, with newer nonpediculicides.


In the US, the number of head lice infestations annually is estimated between 6 — 12 million among children 3 — 12 years of age.[1] The social, economic and educational impact of head lice infestations is considerable. In the US, the total direct costs for treatment and indirect costs for lost wages, educational programs, and school and nursing home monitoring programs have been estimated at more than $1 billion annually.[2]

Infestation is most common in school-aged children with girls being more commonly affected than boys. African-American children are less often affected; this variation is thought to be the result of differences in the hair shaft structure, which may be oval shaped and thus more difficult for a louse to grasp.[1] Transmission of head lice most commonly occurs through close physical contact, especially head-to-head contact, but fomites, such as hats also play a role. Louse transfer has been found to be optimal when hairs are relatively stationary and parallel, suggesting that louse transmission is more likely to occur while children are at rest, than during periods of vigorous play.[3]

Head lice infestation is caused by the obligate ectoparasite Pediculus humanus capitis, a wingless, elongated, dorsoventrally flattened insect. The adult louse feeds 4 — 5 times/day and can normally only survive for 1 — 2 days away from the scalp. Eggs are glued to the hair in egg castings, or nits, close to the scalp and can survive up to 10 days away from the human host. Lice typically lay nits within 1 — 2mm of the scalp and for practical purposes, nits within 1cm of the scalp should be counted as a sign of active infestation.[4]


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