Head lice

As the name implies, head lice are specialized to live among the hair present on the human head and are exquisitely adapted to living mainly on the scalp and neck hairs of their human host. Lice present on other body parts covered by hair are not head lice but are either Pubic lice (Pthirus pubis) or Body lice[?] (Pediculus humanus).

Description The adult head louse resembles a miniature crab when viewed through a strong magnifying glass[?]. Head lice have six legs, but their two front legs are very large in order to grab onto the hair shafts. Head lice are tan to greyish-white in color.

Life cycle The female louse glues her eggs, called "nits", which look like tiny white beads, to hair shafts.

There are three main stages in the life of a head louse: the nit, the nymph, and the adult.

• Nit: Nits are head lice eggs. They are hard to see and are found firmly attached to the hair shaft. They are oval and usually yellow to white. Nits take about 1 week to hatch.
• Nymph: The nit hatches into a baby louse called a nymph. It looks like an adult head louse, but is smaller. Nymphs mature into adults about 7 days after hatching. To live, the nymph must feed on blood. It metamorphoses 3 times before it reaches the adult stage.
• Adult. Females lay nits (a few hundreds of eggs); they are usually larger than males. To live, adult lice need to feed on blood. If the louse falls off a person, it dies within 1-2 days.

Symptoms The louse feeds on human blood, and the bite causes itching. Bites can become secondarily infected; scratching may break the skin and help cause this secondary infection. The most common symptom is itching of the scalp.

Head lice are normally spread by close contact but can also be spread by sharing clothes or bedding.

Treatment Head lice can be killed by a 1% permethrin or pyrethrin lice shampoo, but the hair must be combed with a fine-toothed comb after treatment to remove the nits. In the Western world resistance to commercially available anti-lice shampoos is increasing strongly. In some countries, such as the UK, resistance is so strong that many families do not know what to do anymore. Lice can survive in bedding and clothing, so these items must be treated, sterilized, or contact with them must be avoided for two weeks, after which time any lice will have died.

Lindane (1%), another pediculocide, is not recommended for pregnant or nursing women or for children less than 2 years old.

• See also Pediculosis