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- 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
- 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
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 hypersensitivity reaction to the saliva of the feeding louse.
- 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)
- 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)
- 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.