Description

• 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

Epidemiology

Incidence

• 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

Prevalence
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 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 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.