Description

• A contagious parasitic infection caused by lice (blood-sucking insects, obligate parasites)
• 2 species of lice infest humans:
  • Pediculus humanus has 2 subspecies: The head louse (var. capitis) and the body louse (var. corporis). Both species are 1–3 mm long, flat, and wingless and have 3 pairs of legs that attach closely behind the head.
  • Phthirus 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

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

Prevalence
Head lice: 1–3% in industrialized countries

Risk Factors

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

General Prevention

• Environmental measures: Wash, dry-clean, or vacuum all items that may have come in contact with 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 (Note: Condoms do not prevent transmission nor does shaving pubic hair).
• Body lice: Proper hygiene

Pathophysiology

Itching is a hypersensitivity reaction to the saliva of the feeding louse.

Etiology

• Infestation by lice: P. humanus (var. capitis), P. humanus (var. corporis), or P. pubis
• Characteristics of lice:
  • Adult louse is dark grayish and moves quickly, but does not jump or fly.
  • Eggs (sometimes referred to as 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, and then sucking blood.
• Transmission: Direct human-to-human contact:
  • Head lice: Direct head-to-head contact or contact with infested fomite (less common)
  • Body lice: Contact with contaminated clothing or bedding
  • Pubic lice: Typically transmitted sexually (fomite transmission is unlikely)

Commonly Associated Conditions

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