Pediculosis: Head Lice and Body Lice

Pediculosis is infestation with the human louse, Pediculus humanus. It has two subspecies: the head louse (P. h. capitis) and the body louse (P. h. humanus). Both are ectoparasites, and humans are their only known hosts. Recent molecular data suggest the two are ecotypes of a single species, with continuing evolution between the two populations.

Life cycle

The life cycle of the head louse. CDC DPDx.

A head louse goes through three stages:

• Egg (nit). The adult female cements each egg to a hair shaft close to the scalp. Nits are oval, usually yellow to white, 0.8 mm by 0.3 mm, and hard to see — they are often mistaken for dandruff or drops of hair spray. Viable eggs usually sit within 6 mm of the scalp. They hatch in about a week (6 to 9 days).
• Nymph. Once the nymph hatches, the empty shell turns a more visible dull yellow and stays stuck to the hair. The nymph looks like a small adult, about the size of a pinhead, and molts three times before becoming an adult about 7 days after hatching.
• Adult. An adult is about the size of a sesame seed, with six clawed legs, and is tan to grayish-white — darker on people with dark hair. Adult females are 3–4 mm long, males slightly smaller. A female can lay up to 8 nits a day. Adults can live up to 30 days on a person's head but must feed on blood several times a day; off the host, without blood, a louse dies within 1 to 2 days.

Body lice look much the same but live differently: they live and lay their eggs on the clothing and belongings of the infested person and move onto the body to feed. Although body lice usually stay on the torso and head lice on the head and face, either can move. Both can be told apart from pubic lice (Pthirus pubis) by their longer, narrower bodies, and their legs are built for gripping hair.

Adult head louse. CDC DPDx.

Adult female head louse, with eggs visible inside. CDC DPDx.

Adult Pediculus humanus. CDC DPDx.

A head louse egg with a first-stage nymph hatching. CDC DPDx.

The same egg's empty shell after the nymph left. CDC DPDx.

Who gets lice

Head lice are very common worldwide. Children of preschool and elementary-school age, 3 to 11, are infested most often, and girls more often than boys, probably because of more frequent head-to-head contact. Body lice are also found worldwide but are less common and are usually seen amid poverty, war and homelessness.

How they spread

Head lice spread mainly through direct head-to-head contact with an infested person, common during play at school and at home — sports, playgrounds, camp, sleepovers. Less often they spread through shared items, which is more common for body lice: wearing an infested person's hats, scarves, coats, sports uniforms or hair ribbons; using their combs, brushes or towels; or lying on a bed, couch, pillow, carpet or stuffed animal they recently touched.

Symptoms

Most head lice infestations cause no symptoms. When they do, symptoms include a tickling feeling of something moving in the hair, itching from an allergic reaction to louse saliva, and irritability. Scratching can lead to secondary bacterial infection. Body lice can carry Rickettsia prowazekii (epidemic typhus), Bartonella quintana (trench fever) and Borrelia recurrentis (louse-borne relapsing fever).

Diagnosis and treatment

The best way to diagnose pediculosis is to find a live nymph or adult louse on the scalp or in the hair. Many nits within 6 mm of the scalp strongly suggest an active infestation; nits found only farther than 6 mm from the scalp point to a past one. Treatment information is on CDC's lice pages.

Sources

• Centers for Disease Control and Prevention, DPDx — Laboratory Identification of Parasites of Public Health Concern: "Pediculosis," citing Veracx and Raoult, "Biology and genetics of human head and body lice," Trends in Parasitology 28 (2012).