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- 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
- 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
Head lice: 1–3% in industrialized countries
- 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)
- 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
Itching is a hypersensitivity reaction to the saliva of the feeding louse.
- 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.