Pediculosis (Lice)

Pediculosis (Lice) is a topic covered in the 5-Minute Clinical Consult.

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  • A contagious parasitic infection caused by ectoparasitic blood-feeding insects (lice)
  • Two species of lice infest humans:
    • Pediculus humanus has two subspecies: the head louse (var. capitis) and the body louse (var. corporis). Both species are 1 to 3 mm long, flat, and wingless and have three pairs of legs that attach closely behind the head.
    • Pthirus pubis (pubic or crab louse): resembles a sea crab and has widespread claws on the 2nd and 3rd legs
  • System(s) affected: skin/exocrine
  • Synonym(s): lice; crabs


  • In the United States: 6 to 12 million new cases per year
  • Predominant age
    • Head lice: most common in children 3 to 12 years of age; more common in girls than boys
    • Pubic lice: most common in adults

Head lice: 1–3% in industrialized countries

Etiology and Pathophysiology

  • Characteristics of lice:
    • Adult louse is dark grayish and moves quickly but does not jump or fly.
    • Eggs (nits) camouflage with the individuals’ hair color and are cemented to the base of the hair shaft (within 4 mm of the scalp).
    • Nits (empty egg casings) appear white (opalescent) and remain cemented to the hair shaft.
    • Lice feed solely on human blood by piercing the skin, injecting saliva (anticoagulant properties to allow for blood meal), and then ingesting blood.
    • Itching is a hypersensitivity reaction to the saliva of the feeding louse.
  • Transmission: direct human-to-human contact
    • Head lice: direct head-to-head contact or contact with infested fomite (less likely)
    • Body lice: contact with contaminated clothing or bedding
    • Pubic lice: typically transmitted sexually (fomite transmission much less likely)

Risk Factors

  • General: overcrowding and close personal contact
  • Head lice
    • School-aged children, gender (girls; longer hair)
    • Sharing combs, hats (including helmets), clothing, and bed linens
    • African Americans rarely have head lice; theories include twisted hair shaft and increased use of pomades.
  • Body lice: poor hygiene, homelessness
  • Pubic lice: promiscuity (very high transmission rate)

General Prevention

  • Environmental measures: Wash, dry-clean, or vacuum items that may have contacted infected individuals.
  • Screen and treat affected household contacts.
  • Head lice: Follow-up by school nurses may help to prevent recurrence and spread.
  • Pubic lice: Limit the number of sexual partners (condoms do not prevent transmission nor does shaving pubic hair).
  • Body lice: proper body hygiene

Commonly Associated Conditions

Up to 1/3 of patients with pubic lice have at least one concomitant STI.

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