Stat consult: Head lice

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Head lice cases in the United States are estimated at 6 to 12 million cases per year primarily affecting girls aged 3 to 12 years.
Head lice cases in the United States are estimated at 6 to 12 million cases per year primarily affecting girls aged 3 to 12 years.


  • common pediatric ectoparasite infestation

Also called 

  • pediculosis capitis

Who is most affected

  • healthy children aged 3 to 12 years regardless of hygiene
  • girls > boys
  • household contacts of infested children
  • adults with poor personal hygiene 
  • less common in African Americans


  • common, estimated 6 to 12 million cases in the United States per year
  • reported prevalence 3.4% to 35% 

Likely risk factors 

  • head-to-head contact, especially in schools, child care centers, or "sleepover" camps
  • household contacts
  • overcrowded living conditions 

Possible risk factors

  • hair traits

Factors not associated with increased risk

  • hygiene (in children)
  • length of hair 
  • frequency of brushing or shampooing
  • socioeconomic class 

Associated conditions 

  • other louse infestation or scabies (especially in indigent, homeless, or refugee populations)


  • head louse Pediculus humanus var capitis 


  • life cycle repeats every 3 weeks
    • female louse lives 3 to 4 weeks, lays up to 10 eggs (nits) daily
    • eggs initially translucent, attached to base of hair shaft by glue-like substance from louse
    • empty egg casings are white, viable nits pigmented
    • eggs incubate (need heat of scalp) and hatch in 7 to 12 days
    • female nymphs grow for about 9 to 12 days, then mate and lay eggs
  • no transmission of other disease agents to humans 


Chief concern (CC)

  • scalp pruritus (due to injected louse saliva)
  • "something crawling on my head"
  • often asymptomatic
Social History (SH) 
  • ask if any cases of head lice at child's school or day care 



  • primary findings are nits or adult lice (may require magnifying lens) 
    • nits at proximal ends of hair shafts 
    • viable nits most easily seen at nape of neck or behind ears at hairline 
    • height of nits above scalp due to hair growth indicates duration of infestation (1 cm/month) 
    • number of lice found may vary by cultural hair grooming practices 
  • secondary skin reactions to lice or scratching 
    • excoriation 
    • secondary bacterial infections 


  • posterior cervical local adenopathy if secondary skin infection

Making the diagnosis

  • presence of live adult louse, nymphs, or viable-appearing eggs (nits) on head 
  • nits appear viable if found <1 cm of scalp, firmly attached to hair, and have microscopic "eye spot" 

Differential diagnosis

  • other causes of scalp pruritus 
    • irritation from hairstyling products 
    • allergic contact dermatitis 
    • seborrheic dermatitis 
  • dead eggs or empty egg gases
  • "pseudo-nits" — nodules resembling nits (can be easily separated from hair) 
    • dandruff
    • dried hairspray 
    • dirt, lint, sand 
    • white piedra — fungi (Trichosporon beigelii)
    • hair casts — desquamated cylindrical epithelial cell plug that encircles hair, easily movable  

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