Head lice (pediculosis capitis) is a very common problem worldwide. The head louse, Pediculus capitis, has been dependent on humans for thousands of years. Currently, millions are affected in North America. It affects all ages and races. It’s less common in blacks as their hair is curly, and elliptical in cross section, making it more difficult for lice to lay their eggs. Head lice are most frequently seen in children with girls affected more than boys. Girls tend to have longer hair and are more likely to spread the lice via shared brushes and hair accessories.
The head louse is a six-legged wingless insect about 2-3 mm in length. It is an obligate human parasite that feeds on blood and then lays eggs on scalp hair. It undergoes a lifecycle from egg (nit) to 3 nymph forms, ultimately becoming an adult male or female louse. The whole process from copulation to adult takes roughly 25 days. An adult female lives for nearly 30 days, laying up to 10 eggs per day.
The head louse will die without a human blood meal after 36 hours. In hot, humid conditions, nits can survive for up to 10 days away from the human host.
Head lice are transferred from scalp to scalp by a variety of means:
They’re not known to transmit specific diseases, unlike the body louse, but may transmit bacteria such as S. aureus and Group A Streptococci from one host to another.
The most common complaint is an itchy scalp. Pruritus can be severe, but varies. As with scabies, the itch isn’t felt immediately. After initial infestation, it may take up to one month before the scalp itches. With repeat exposure, itch appears much quicker, within days.
On examination of the scalp, there may be non-specific findings such as excoriations, and erythematous crusted papules. Pay close attention to the postauricular areas — and occiput — as they’re favoured locations of the insect.
Clinching the diagnosis requires seeing the head louse and/or its nits. Nits are light brown concretions, less than 1 mm, on the proximal hair shaft (usually within 1 cm of the scalp). When hatched, they are whiter, and can be mistaken for dandruff.
The main complications to watch for is a secondary bacterial infection of the scalp. This could be the presenting concern.
All patients should have at least two treatments — on days 1 and 8, and preferably a third one week later if the treatment is not strongly ovicidal (such as pyrethroids).
Repeating treatments helps to ensure the destruction of nymphs that may have hatched between treatments, reduce the chance of re-infestation from fomites (objects that can transmit the infection), reduce the likelihood of resistance to the topical agent.
After treatment, nits may still be seen in the hair, but these are likely dead.
In Canada, several topical pediculicides are available:
Pyrethrin and piperonyl butoxide
In Canada, a pyrethrin 0.33% and piperonyl butoxide 3% shampoo is available. This shampoo should be applied generously to dry hair, rubbed in, and left in place for at least 10 minutes before washing out.
Given that naturally-derived pyrethrins are instable and less effective, a synthetic pyrethroid was developed. Permethrin has the same mechanism of action as its natural counterpart but doesn’t require piperonyl butoxide to enhance effectiveness.
Permethrin is available as 1% crème-rinse preparations and as a 5% lotion and cream. For the 1% crème-rinse preparations, the hair should be shampooed and towel-dried. The crème-rinse should be shaken, then applied generously to hair and left in place for at least 10 minutes before washing out.
For more challenging cases, the higher concentration 5% cream, normally reserved to treat scabies, can be used. The lotion or cream is applied thoroughly to the scalp and hair and left on under a shower cap for 8-12 hours or overnight before being washed out.
Pyrethroids are safe topical agents with no serious reported adverse effects. It isn’t known whether or not it’s found in breast milk, so breastfeeding should be stopped around the time of treatment.
John Kraft, MD, is in his third year of the Dermatology Residency Program at the University of Toronto.
Charles Lynde, MD, FRCP(C) is an assistant professor of dermatology at the University of Toronto.