Pediculosis (Lice)

Key Points

  • Pediculosis, or louse infestation, is a common clinical issue that is of limited morbidity but is of significant public health concern, especially in school-age children.
  • Humans may be parasitized by three louse types: body lice, head lice, and pubic lice.
  • Growing resistance of lice to permethrin, and pyrethrins has led to the development of alternative treatment options that include ivermectin, benzyl alcohol, and spinosad.


Head Lice

Head lice are found worldwide, and are most prevalent amongst elementary school children. Girls may be more susceptible to infection, possibly because they have longer hairstyles. Incidence of pediculosis is seasonal and peaks during the warmer months of the year and during the time corresponding to the start of the school year. Transmission occurs via head-to-head contact. Head lice are obligate parasites that feed on human blood every 6 hours and die in several days without a host. Visible to the naked eye, they appear grayish in color, have six legs, and can reach up to 6 mm in length. The life cycle of the head louse is approximately 21 days in length and has three stages: egg, nymph (or instar), and adult louse. During its reproductive stage, a female louse can produce an average of five eggs (also known as nits) daily. Eggs are attached to the hair shaft close to the scalp with concretions secreted by the louse. Incubation time is 5-10 days. Viable eggs are yellow/brown in color and found close to the scalp, whereas older hatched eggs appear white and are found farther (>1 cm) from the scalp surface.

Body Lice

Body lice infestations are associated with poor living conditions and poverty. Body lice tend to live in clothing and not directly on the body. They are often found along seam lines and in bedding. Body lice may reach 2.5-4 mm in length. They may be associated with the spread of relapsing fever (Borrelia recurrentis), epidemic typhus (Rickettsia prowazekii), and trench fever (Bartonella quintana).

Pubic Lice

Pubic lice are smaller, about 1 mm in length, and on magnification resemble a crab. In addition to infesting hairs of the pubic area, they may infest other body hairs including eyelashes, axillae, the perianal region, chest, scalp, and beard hairs. Transmission of pubic lice most commonly occurs during sexual contact, and rarely may transfer through the sharing of towels or underwear. Patients with pubic lice should be counseled on prevention of other sexually transmitted infections (STI) and on safe sex practices. Testing for STIs, including HIV, should be offered. Pubic lice diagnosed in a child should raise suspicion of sexual abuse.

Though not considered a serious threat to human health, louse infestation is contagious and can become a public health concern, especially when infestations affect school-age children, their caretakers, and household members. Widespread and localized resistance to treatments has been observed in recent years, and as a result therapeutic strategies may differ based on geographic location.

There is limited morbidity associated with lice. The primary complaint is intense pruritus occurring 4-6 weeks following the onset of infestation, which is primarily due to a hypersensitivity reaction to components of the louse saliva. Severe pruritus may occasionally result in excoriations and can be complicated by secondary skin infections such as impetigo.

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