Lice (Pediculosis)

Description

Infestation of the head, body, or anogenital region by parasitic, wingless insects that feed exclusively on human blood

Epidemiology

• Head lice
  • Spread by head-to-head contact
  • Most common among children 3 to 12 years old
  • Associated with female gender, crowded living conditions
  • Less common among African Americans
  • Point prevalence estimates range from <1% in some places to >90% in others.
  • In the United States, seasonal peak of prescriptions for treatment are filled in July to September coinciding with the back-to-school period.
• Body lice
  • Spread by close physical contact with infested persons, clothing, or bedding
  • Associated with poor sanitation, cool climates, homelessness, war, disasters, refugee camps
  • No racial or gender differences
• Pubic lice
  • Usually sexually transmitted
  • Can also spread through contact with clothing or bedding recently used by infested person
  • Most common among young adults

General Prevention

Humans are the only host for all three types of lice. Recurrences are common and may be prevented by examining and treating close contacts, especially bedmates.

• Head lice
  • Avoid head-to-head contact with infested persons; don’t share brushes, hats, or hair ties.
  • Avoid lying on pillows, furniture, or stuffed toys used by infested persons within last 2 days.
  • Wash clothing and bedding used by infested persons with hot water (≥130°F) and set dryer to highest heat setting. Items may also be dry-cleaned or sealed in a plastic bag for 2 weeks.
  • Treatment of furniture, upholstery, and carpets is not necessary because lice only survive for a short while away from host, making transmission via textiles extremely unlikely.
  • Environmental insecticide is not helpful.
  • Treatment of pets is not necessary.
  • “No-nit” school policies do not control head lice transmission and are not recommended.
• Body lice
  • Regularly wash clothes.
  • Avoid using clothing or bedding used by infested persons.
• Pubic lice
  • Avoid close body contact or sharing clothes with infested persons.
  • Not prevented by condom use

Pathophysiology

• Lice bites are painless.
• To facilitate the blood meal, lice inject enzymes, anticoagulant, and vasodilators. These provoke host inflammatory response causing pruritus.
• Bites cause intradermal hemorrhage with infiltrates of eosinophils and lymphocytes.
• Excoriation can introduce secondary infections.
• Vector-borne pathogens (body lice only) can cause chronic bacteremia, angiomatosis, or endocarditis.

Etiology

• Head lice (Pediculus humanus capitis)
  • Adult lice are white to gray, 2- to 4-mm long, have six legs, and no wings. They crawl quickly away from threat or bright light and cannot jump or fly. If removed from host, lice will die within 2 days.
  • Females lay up to eight eggs (also called “nits”) per day over a 2- to 3-week lifespan, attaching nits to base of hair shafts with adhesive.
  • Nymphs hatch from nits in 7 to 12 days, leaving behind empty white nit casings on hair.
  • Emerging nymphs die without a blood meal within a few hours. Nymphs molt 3 times over 9 to 11 days to become nit-laying adults.
  • Typical infestation includes lice in all stages of development.
• Body lice (Pediculus humanus corporis)
  • Morphology and life cycle are similar to head lice, but adults are slightly larger.
  • Live and lay eggs on clothing and only come to the skin to feed 4 to 5 times per day
  • Able to live longer off host than head lice
  • Nits hatch in 6 to 10 days
• Pubic lice (Phthirus pubis)
  • Crab-like appearance with larger talus adapted to coarser hair
  • Predilection for pubic hair; may also infest axillary hair, perianal area, eyelashes, beard, and rarely scalp

Commonly Associated Conditions

• Body lice
  • May act as a vector for epidemic typhus (Rickettsia prowazekii), relapsing fever (Borrelia recurrentis), trench fever (Bartonella quintana), or plague (Yersinia pestis)
• Pubic lice
  • Commonly occurs with other sexually transmitted infections
  • Although pubic lice on children’s eyelashes usually result from close contact with infested parent, must also consider sexual abuse