(Redirected from Lice)


Drawing of a louse clinging to a human hair.
  • Caused by lice/louse: small wingless insects, dorsoventrally flattened, with reduced or no eyes and enlarged tarsal claws for clinging[1]

Taxonomy of Human Lice

  • Pediculus humanus
    • Head louse (capitus)^
    • Body louse (coporis)
  • Pthirus pubis
    • Pubic louse


  • Pediculus humanus capitis^
    • Affects millions of school-age children all over the world
    • Transmission is by close head-to-head contact and rarely through fomites such as hats, clothes, or pillowcases
  • Pediculus humanus corporis
    • Propagated when clothes are not washed or changed regularly
    • Usually affect homeless people, displaced persons, or prisoners in poor conditions
    • Vector for:
  • Pthirus pubis
    • Infests pubic hair and occasionally other hairy areas, such as eye lashes
    • Usually transmitted during intimate body contact, such as during sexual intercourse. Treatment must therefore include the patient's partner.

^Most common[2]

Clinical Features

Lice vs. Common Hair Debris.
Heavily infested hair with pediculus humanus capitis (arrow).
Massive infestation of pediculus humanus corporis of: (A) head (B) beard and (C, D) pubis.

Pediculus humanus capitis

  • May see live mites or nits (egg sacks) on hair shaft close to scalp
  • May be asymptomatic or pruritic

Pediculus humanus corporis

Pthirus pubis

Lice vs. Common Hair Debris

  • Desquamate epithelial cells (DEC) may occur when oil glands compensate for drying effects of chemicals (e.g. post initial treatment).
    • They are bright, white, irregularly shaped clumps stuck to hair
  • Hair casts are thin, elongated cylinder-shaped segments of dandruff
    • Encircling hair shaft and easily dislodged
  • Nit (louse egg) is smooth, oval shaped, attached to side of hair shaft
    • Ranging in color from off-white to brown
    • Slightly smaller than a sesame seed
    • Always the same shape (never irregular, fuzzy, or encircling hair (although glue produced by louse may encircle tightly around the shaft)
  • Actual lice
    • Have legs

Differential Diagnosis

Domestic U.S. Ectoparasites

See also travel-related skin conditions


  • Generally a clinical diagnosis


  • Usually transmitted during intimate body contact, such as during sexual intercourse. Treatment must therefore include the patient's partner.

Over the Counter (OTC)

  • Permethrin 1% lotion shampoo (if >2 months old)[3]
    • Wash hair with non-conditioned shampoo
    • Apply Permethrin for 10 min and rinse
    • Repeat on day 9
  • Pyrethrin lotion
    • Apply to affected areas and wash off after 10 min
    • Repeat in 7 days
  • Dimethicone therapy
    • First, apply the product to dry hair, then wait 10 min. Next, with product still in the hair, separate hair into small sections and comb hair to remove lice and eggs. Use a lice comb to remove lice and their eggs from hair. Finally, shampoo hair thoroughly with regular shampoo and warm water. Repeat as needed.[4]


Reserved for failed OTC treatment

  • Spinosad 0.9% topical suspension (if >6 months old)
    • Apply to scalp and air and wash off after 10 min.
    • Repeat in 10 days
  • Malathion 0.5% lotion (if >6 years old)
    • Applied to affected areas and wash after 8 hrs
    • Repeat in 7 days
  • Benzyl Alcohol 5% lotion (> 6 months old)
    • Apply to dry hair and wash off after 10 min
    • Repeat in 7 days
  • Ivermectin 400mcg/kg PO
    • Once on day 1 THEN once in 7 days
    • Reserved for patients failing topical treatment
  • Lindane therapy
    • Only consider if patient has failed two prior prescription treatments
    • Avoid in children <50 kg due to seizure association

Eyelash Infestation

  • Apply ophthalmic petroleum jelly q12hrs x 10 days

Pediatrics <2yo

  • Wet combing is an alternative to medical therapy


  • Discharge
    • Children can return to school after first topical treatment has been applied
    • Instruct parents and all close contacts to wash clothing and bedding in hot soapy water to avoid disease spread

See Also


  3. Devore CD and Schutze G. Head Lice. Pediatrics. 2015; 135(5) e1355-e1365.
  4. Ihde ES, Boscamp JR, Loh JM, Rosen L. Safety and efficacy of a 100% dimethicone pediculocide in school-age children. BMC Pediatr. 2015;15:70.