Pediculosis (Lice)



  • 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



  • In the United States: 6 to 12 million new cases per year
  • Head lice are most common in children 3 to 11 years of age; more common in girls than boys. Pubic lice are more common in adults.

Head lice: 1–3% in industrialized countries

Etiology and Pathophysiology

  • Characteristics of lice:
    • Adult louse is dark grayish and moves quickly by crawling (does not jump or fly). It has claws that allow it to cling to individual strands of hair. 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 delayed hypersensitivity reaction to the saliva of the feeding louse, which may take 4 to 6 weeks to develop after the first exposure. Subsequent exposures may take 1 to 2 days for symptoms to develop (1).
  • Transmission: direct human-to-human contact
    • Head lice: direct head-to-head contact or contact with infected 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
  • 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 30% of patients with pubic lice have at least one concomitant STI.

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