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Head lice

Head lice (Pediculus capitis) are one of the many varieties of sucking lice (singular "louse") specialized to live on different areas of various animals.

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).

Table of contents
1 Description
2 Life cycle
3 Symptoms
4 Treatment


The adult head louse resembles a miniature ant that appears flat when viewed from the side through a strong magnifying glass. Head lice have a head, thorax and abdomen with 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

Lice eggs on the hair very close to the scalp are the primary sign of an active infestation. The female louse glues her eggs, sometimes called "nits", which look like tiny white beads, to hair shafts very close to the scalp. Eggs are very small, about the size of a period in normal printing. Eggs may appear yellowish, brownish or greyish, but almost always lighter colored. Eggs normally undergo a 7-9 day incubation before hatching as a baby nymph. Classically, a louse egg does not become a "nit" until after it has completed its incubation stage, thus leaving a "nit". A "nit" is either the empty shell remaining after the nynmph has departed or the dead egg that remains if incubation was not successful. Dead eggs will appear dark, or raisonlike, as they dry out. "Nits" are usually found one-half inch or more away from the scalp and are not considered a sign of an active infestation.

There are three nymph instar stages as the baby louse matures, with the louse shedding its exoskeleton at the end of each stage as it gets larger. The nymph stage typically takes 10 to 12 days.

Whether a louse is male or female is not apparent until they are nearly mature. Fertilization of eggs takes place once as the female reaches the mature stage. The female can then lay 3-7 eggs each day for the next 28 to 30 days, her normal life span.

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


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.


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.

One alternative to insecticidal treatments, particularly where resistance is common, is to use an electric comb. Alternating teeth carry a high voltage (though the developable current is small, and so the device is safe). Fine combing causes any contact with the lice to result in their electrocution. This allows diligent combing to iradicate an infestation.

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