• Common infestation, particularly in children; endemic worldwide
• Parasite is a blood-sucking insect 2-3 mm long with powerful claws found exclusively in humans (no animal host).
• Infestation involves hair shafts and scalp.
• Not a serious health hazard but associated with time lost from school and work

ICD-9 codes

• 132.0 pediculus capitis (head louse)
• 132.3 mixed pediculosis infestation
• 132.9 pediculosis, unspecified

Who is most affected

• Healthy children, especially girls
• Adults with poor hygiene
• Less common in African Americans (possibly due to differences in hair texture)
• Estimated 6-12 million U.S. cases per year

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Risk factors

• Head-to-head (i.e., shared combs) especially in schools, child-care centers, or “sleepover” camps
• Infected household members and overcrowded living conditions
• Poor hygiene (in adults)
• Factors not associated with increased risk
    — Hygiene (in children)
    — Length of hair
    — Socioeconomic class (all affected)

Complications and associated conditions

• Secondary bacterial skin infection (e.g., impetigo) is rare.
• Other louse infestation or scabies (especially in indigent, homeless, or refugee populations) 

Symptoms and diagnosis

• Pruritus/sensation of  “something crawling on head”
• Occasionally fever, lymphadenopathy, headache, rash, malaise, or irritability
• Often asymptomatic
• Procedure for scalp inspection 
    — Wet and comb hair.
    — Search with strong light (Wood’s lamp may facilitate detection) and magnifying glass using fine-toothed comb (teeth 0.2-0.3 mm apart) to trap lice. 
    — Insert comb near crown at scalp; comb entire head systematically at least twice, examining comb after each stroke.
    — Nits visible at proximal ends of hair shafts (may require magnifying lens)
    — Most easily seen at nape of neck or behind ears at hairline
    — Height of nits above scalp indicates duration of infestation (1 cm = one month).
• Rule out
    — Other causes of scalp pruritus (hairstyling products, contact or seborrheic dermatitis, etc.)
    — “Pseudonits” such as dandruff, dried hairspray, dirt, Trichosporon beigelii fungi (“white piedra”), or “hair casts” (desquamated epithelial hair plug)

Topical pediculicides (FDA approved)

• Resistance to common pediculicides reported to be emerging in United States
• Permethrin
    — Approved for adults and children >2 months old 
    — American Academy of Pediatrics (AAP) recommends as treatment of choice.
    — Reported adverse effects include asthma exacerbation, contact dermatitis, mild/transient burning, and pruritus.
• Malathion
    — Approved for adults and children >6 years old
    — Recommended when permethrin resistance suspected 
    — Flammable (do not use near hair dryer, cigarettes, etc.) 
    — May cause irritation of skin and scalp
• Lindane
    — Second-line therapy for patients who cannot tolerate or have failed first-line treatment with safer medications  (avoid in infants, children, and elderly)
    — Associated with persistent pruritus after treatment (not an indication for re-treatment) 
    — Pharmaceutical use banned in California since 2002