> Table of Contents > Pediculosis (Lice)
Pediculosis (Lice)
Kaelen C. Dunican, PharmD
Cynthia Jeremiah, MD
image BASICS
  • 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.
    • Phthirus pubis (pubic or crab louse): resembles a sea crab and has widespread claws on the second and third 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
  • 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 (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 sucking 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 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 (condoms do not prevent transmission nor does shaving pubic hair).
  • Body lice: proper hygiene
Up to 1/3 of patients with pubic lice have at least one concomitant STI.
  • Diagnosis is confirmed by visualization of live lice.
  • P. capitis (head lice)
    • Found most often on the back of the head and neck and behind the ears (warmer areas)
    • Eyelashes may be involved.
    • Eggs, found cemented on the base of a hair shaft, are difficult to remove.
    • Pruritus may be accompanied by local erythema and small papules.
    • May see excoriations around hairline
    • Scratching can cause inflammation and secondary bacterial infection.
    • Pyoderma and lymphadenopathy may occur in severe infestation.
  • P. corporis (body lice)
    • Poor general hygiene
    • Adult lice and nits in the seams of clothing
    • Intense pruritus involving area covered by clothing (trunk, axillae, and groin)
    • Uninfected bites present as erythematous macules, papules, and wheals.
    • Pyoderma and excoriation may be seen.
  • P. pubis (pubic lice)
    • Pubic hair is the most common site, but lice may spread to hair around anus, abdomen, axillae, chest, beard, eyebrows, and eyelashes.
    • Eggs are present at the base of hair shafts.
    • Anogenital pruritus
    • Blue macules may be seen in surrounding skin.
    • Delay in treatment may lead to development of groin infection and regional adenopathy.
  • Scabies and other mite species that can cause cutaneous reactions in humans
  • Dandruff and other hair debris sometimes look like head lice eggs and nits but is less adherent.
  • Diagnosis is based on visualization of live louse.
  • Head lice: Comb hair thoroughly with a fine-toothed louse comb (0.2 to 0.3 mm between teeth) to identify live lice (1)[C]. Wet hair may limits static electricity. Simple visual inspection has same sensitivity as wet combing but is only ˜25% as effective as dry combing with a metal comb (2).
  • Body lice: Examine the seams of clothing to locate lice and eggs (3).
  • Lice and eggs are more easily visualized under a microscope.
  • In contrast to dandruff, eggs and nits cannot be removed easily from a hair shaft.
Follow-Up Tests & Special Considerations
  • Empty nits remain on hair shafts for months after eradication of the live infestation. On Wood lamp exam, live nits fluoresce white and empty nits fluoresce gray.
  • Pubic lice: Evaluate for concurrent STIs.
Permethrin (over the counter [OTC]), synergized pyrethrin (OTC), spinosad (Rx), benzyl alcohol (Rx), malathion (Rx), and topical ivermectin (Rx) are all effective for head lice (1)[A],(2)[C]. Permethrin, synergized pyrethrin, and malathion are effective for pubic lice (2)[C]:
  • Permethrin generally preferred because it has residual activity for up to 3 weeks. However, newer shampoos and conditioners may reduce the residual effect (1)[C].
  • Malathion and spinosad are considered second line for head lice but may not require a second application due to ovicidal activity (1)[B],(2)[C].
  • Ivermectin 0.5% lotion is also effective for head lice (2)[C].
First Line
  • Head and pubic lice
  • Pyrethrum insecticides: permethrin 1% cream rinse (Nix) or pyrethrins 0.33% with piperonyl butoxide 4% (synergized pyrethrin, Rid, Pronto): are first line unless there is proven resistance in the community.
    • Apply for 10 minutes, then wash.
    • Reapply synergized pyrethrin in 7 to 10 days (day 9 is optimal). Also be necessary with permethrin, if live lice are observed.
    • Side effects: application-site erythema, ocular erythema, and application-site irritation
  • Body lice: best treated with synergized pyrethrin lotion applied once and left on for several hours
  • Eyelash infestation: Apply petroleum jelly BID for 10 days.
  • Precautions
    • Pyrethrin: Avoid in patients with ragweed allergy (may cause respiratory symptoms).
    • Pediculicides should never be used to treat eyelash infections.
Second Line
Head lice and pubic lice
  • Malathion 0.5% lotion (Ovide)
    • Apply for 8 to 12 hours, then wash off
    • Excipients isopropyl alcohol (78%) and terpineol (12%) may contribute to its efficacy:
      • Flammable and has a bad odor
      • Despite ovicidal activity, a second application may be necessary after 7 to 10 days (day 9 is optimal) if live lice are observed.
    • Lindane 1% shampoo, no longer recommended by the American Academy of Pediatrics
      • Apply for 4 minutes, then wash (should not be repeated).
      • Side effects: neurotoxicity (seizures, muscle spasms), aplastic anemia
      • Contraindications: uncontrolled seizure disorder, premature infants
      • P.773

      • Precautions: Do not use on excoriated skin, in immunocompromised patients, conditions that increase seizure risk, or with medications that decrease seizure threshold.
      • Possible interactions: concomitant use with medications that lower the seizure threshold
  • Head lice
    • Spinosad 0.9% lotion (Natroba)
      • Apply to dry hair and scalp for 10 minutes, then rinse with warm water. Repeat in 7 days if live lice are observed.
      • Side effects: application-site erythema, ocular erythema, and application-site irritation
    • Benzyl alcohol 5% lotion (Ulesfia)
      • Apply to dry hair using enough to saturate scalp and hair (amount depends on hair length), rinse after 10 minutes, and repeat in 7 days.
      • Side effects: pruritus, erythema, pyoderma, ocular irritation, application-site irritation
    • Ivermectin 0.5% lotion (Sklice)
      • Apply to dry hair by using enough to saturate the scalp and hair (max. 4 oz) then rinse after 10 minutes.
      • Side effects: burning sensation at application site, dandruff, dry skin, eye irritation
    • Mechanical removal of lice and nits by wetting hair and then systematically combing with a fine-toothed comb every 3 to 4 days for 2 weeks to remove all lice as they hatch
  • Head lice: Clean items that have been in contact with the head of the infected individual within 48 hours.
  • Wash all bedding, towels, clothes, headgear, combs, brushes, and hair accessories in hot water (60°C).
  • Vacuum furniture and carpets
  • Seal any personal articles that cannot be washed in hot water, dry cleaned, or vacuumed in a plastic bag and store for at least 2 weeks.
  • Examine and treat household members and close contacts concurrently.
  • Insecticide sprays are not necessary.
  • Pubic lice: Avoid sexual activity until both partners are successfully treated.
  • Nit and egg removal
    • Remove eggs that are within 1 cm of the scalp to prevent reinfestation.
    • After treatment with shampoo or lotion, eggs and nits remain in the scalp or pubic hair until mechanically removed. Hair conditioner facilitates nit removal.
    • Eggs and nits are best removed with a very fine nit comb.
Pediatric Considerations
  • Avoid synergized pyrethrin and permethrin in infants <2 months of age. Avoid benzyl alcohol, topical ivermectin, and spinosad in children <6 months of age; and avoid malathion in children <2 years of age.
  • Lindane: not recommended in patients <50 kg
Pregnancy Considerations
Permethrin, synergized pyrethrin, malathion, spinosad, and benzyl alcohol are pregnancy Category B. Lindane and topical ivermectin are Category C.
  • For “difficult to treat” cases of head lice, oral ivermectin 400 &mgr;g/kg (not approved by the FDA for lice), given twice at a 7-day interval, is superior to topical 0.5% malathion lotion (4,5)[B].
  • Ivermectin: 200 &mgr;g/kg PO repeated in 10 days or 300 &mgr;g/kg PO repeated in 7 days
    • Should not be used in children <15 kg; pregnancy Category C
    • Not approved by the FDA for lice
  • Dual therapy with 1% permethrin and oral trimethoprim/sulfamethoxazole (TMP/SMX) only for cases of multiple treatment failures or suspected cases of lice-related resistance to therapy (TMP/SMX is not approved by the FDA for lice).
  • Permethrin 5% cream (Rx) is not FDA approved for lice and is unlikely to be effective for lice that are resistant to 1% cream rinse (1)[B].
Head lice
  • Dry-on, suffocation-based pediculicide: Cetaphil lotion
    • Apply thoroughly to hair, comb, dry with hair dryer, shampoo after 8 hours.
    • Repeat once a week until cured, up to a maximum of three applications.
    • Not approved by the FDA for lice
  • Dimethicone 4% lotion: Apply to hair for 8 hours; repeat in 1 week (not approved by the FDA for lice).
  • No home remedies (e.g., vinegar, isopropyl alcohol, olive oil, ylang ylang oil, mayonnaise, melted butter, and petroleum jelly) have been proven effective to treat head lice infestations.
  • Herbal shampoos and pomades have not been evaluated in clinical trials and are not approved by the FDA for lice.
  • Lavender oil and tea tree oil have been implicated in triggering prepubertal gynecomastia in boys and should not be used to treat lice.
  • Electronic louse combs have not proven effective and are not approved by the FDA.
Children may return to school after completing topical treatment, even if nits remain in place. No-nit policies are not necessary.
Patient Monitoring
Drug resistance should be suspected if no dead lice are observed in 8 to 12 hours after treatment.
  • National Pediculosis Association: http://www.headlice.org/
  • CDC: http://www.cdc.gov/parasites/lice/
  • http://www.guideline.gov/content.aspx?id=46429&search=lice
  • With appropriate treatment, >90% cure rate
  • Recurrence is common, mainly from reinfection or treatment nonadherence. Resistance to synthetic pyrethroids is increasing.
1. Devore CD, Schutze GE. Head lice. Pediatrics. 2015;135(5): e1355-e1365.
2. Burgess IF. Current treatments for pediculosis capitis. Curr Opin Infect Dis. 2009;22(2):131-136.
3. Gunning K, Pippitt K, Kiraly B, et al. Pediculosis and scabies: treatment update. Am Fam Physician. 2012;86(6):535-541.
4. Chosidow O, Giraudeau B, Cottrell J, et al. Oral ivermectin versus malathion lotion for difficult-to-treat head lice. N Engl J Med. 2010;362(10):896-905.
5. Feldmeier H. Treatment of pediculosis capitis: a critical appraisal of the current literature. Am J Clin Dermatol. 2014;15(5):401-412.
Additional Reading
  • Cole SW, Lundquist LM. Spinosad for treatment of head lice infestation. Ann Pharmacother. 2011;45(7-8):954-959.
  • Durand R, Bouvresse S, Berdjane Z, et al. Insecticide resistance in head lice: clinical, parasitological and genetic aspects. Clin Microbiol Infect. 2012;18(4): 338-344.
  • Pariser DM, Meinking TL, Bell M, et al. Topical 0.5% ivermectin lotion for treatment of head lice. N Engl J Med. 2012;367(18):1687-1693.
See Also
Arthropod Bites and Stings; Scabies
  • B85.0 Pediculosis due to Pediculus humanus capitis
  • B85.1 Pediculosis due to Pediculus humanus corporis
  • B85.3 Phthiriasis
Clinical Pearls
  • School-based no-nit policies are not necessary because empty nits may remain on hair shafts for months after successful eradication.
  • Proper product application is essential; consider improper product application when assessing treatment failure.
  • Prevalence of resistant infestations is increasing, so if no dead lice are observed in 8 to 12 hours after treatment, suspect resistance and use an alternative agent.
  • Routine retreatment on day 9 is recommended for nonovicidal products (permethrin and synergized pyrethrin).
  • With all treatment options, reinspect hair after 7 to 9 days and, if live lice are detected, repeat treatment on day 9.