the head louse, is an insect of the order Anoplura and is an
ectoparasite whose only host are humans. The louse feeds on blood
several times daily and resides close to the scalp to maintain its body
The life cycle of
the head louse has three stages: egg, nymph, and adult.
Eggs: Nits are head lice eggs. They are hard to see and are
often confused for dandruff or hair spray droplets. Nits are laid by
the adult female and are cemented at the base of the hair shaft nearest
They are 0.8 mm by 0.3 mm, oval and usually yellow to white. Nits take
about 1 week to hatch (range 6 to 9 days). Viable eggs are usually
located within 6 mm of the scalp.
Nymphs: The egg hatches to release a nymph
The nit shell then becomes a more visible dull yellow and remains
attached to the hair shaft. The nymph looks like an adult head louse,
but is about the size of a pinhead. Nymphs mature after three molts (
and become adults about 7 days after hatching.
Adults: The adult louse is about the size of a sesame seed, has 6
legs (each with claws), and is tan to grayish-white
In persons with dark hair, the adult louse will appear darker. Females
are usually larger than males and can lay up to 8 nits per day. Adult
lice can live up to 30 days on a personís head. To live, adult lice
need to feed on blood several times daily. Without blood meals, the
louse will die within 1 to 2 days off the host.
infestation is very common and is distributed worldwide. Preschool and
elementary-age children, 3 to 10 years of age are infested most often.
Females are infested more often than males, probably due to more
frequent head to head contact. In the United States, African-Americans
are rarely infested with head lice. This is believed to be due to the
American louseís preference for the shape and width of the hair shaft of
The majority of
infestations are asymptomatic. When symptoms are noted they may include
a tickling feeling of something moving in the hair, itching, caused by
the an allergic reaction to louse saliva, and irritability. Secondary
bacterial infection may be a complication.
The main mode of transmission is contact with a person who is already
infested (i.e., head-to-head contact). Contact is common during play
(sports activities, playgrounds, at camp, and slumber parties) at school
and at home.
transmission via fomites may occur. Wearing clothing, such as hats,
scarves, coats, sports uniforms, or hair ribbons worn by an infested
person; using infested combs, brushes or towels; or lying on a bed,
couch, pillow, carpet, or stuffed animal that has recently been in
contact with an infested person may result in transmission. Of note,
both nymph and adult lice forms need to feed on blood to live. If an
adult louse does not have a blood meal, it can die in 2 days.
The diagnosis of
pediculosis is best made by finding a live nymph or adult louse on the
scalp or in the hair of a person. Finding numerous nits within 6 mm of
the scalp is highly suggestive of active infestation. Finding nits only
more than 6 mm from the scalp is only indicative of previous
either a prescription or an over-the-counter (OTC) medication.