Pediculosis and scabies. A revision.


Caused by human lice (Pediculus humanus capitis—head louse, Phthirus pubis—crab or pubic louse, and Pediculus humanus corpus—body louse)

Transmitted by person-person contact

Pediculosis capitis:

Life cycle
– An egg glued firmly to head shaft
– In 1 week, it hatches as nymph.
– In 10 days, it hatches as sexually mature male or female
– Within 24 hours of mating, the female starts to lay eggs.
– Outside the host, they only survive around a day.

Main symptom is itching, due to allergic reaction to the louse saliva (takes 2-3 weeks to develop)

– Finding lice or nits (the eggs)
– Combing the hair with a louse comb and examining the teeth may detect more cases than visualization alone>
– Finding one viable louse confirms the diagnosis.
– If nits are found, they need to be examind microscopically for the presence of embryo.

– First line: permethrin
– Second line: piperonyl butoxide–pyrethrins, malathion lotion
– Lindane shampoo may also work, but concern for neurotoxicity.
– Oral ivermectin kills the adult lice and nymph but not the eggs, so reapplication 1 week later is needed. NOT FDA approved.
– Alternative therapy (not proven to work): suffocation by applying olive oil or petrolatum oil and cover the head for 4-6 hours per day.
– Environmental measure: need to screen for household contacts and treat those who are infested.

  • Removal of nits has not been found to improve efficacy, and is difficult.
  • Use of a 50 percentvinegar and water rinse after shampooing may help slightly with nit removal.
  • Wet combing may help
  • Hot water washing or soaking may help to remove nits.

Treatment failure
– Failure to follow instructions properly (most common)
– Resistance: suspected if live lice can still be found after 12-24 hours after treatment and other causes have been excluded. Use a different class of agent.


Pediculosis pubis

– Spread through sexual contact, so screen for STDs eg gonorrhea and chlamydia.

Pediculosis corporis

– Treatment similar to pediculosis capitis.

– Human body lice can also transmit epidermic typhus (R.prowazeckii) and trench fever (B.quintana)


– Caused by mite Sarcoptes scabiei

– Transmitted by human-human contact.

Life cycle:

  • Scabies mites can survive up to four days off the host. During that time, reinfestation is possible.
  • The female mite burrows under the skin and, before dying, lays 10 to 25 eggs.
  • Three days later, the eggs hatch.
  • The larvae move to the skin surface and mature into adults after 14 to 17 day

– Symptoms: pruritic papular rash (due to delayed hypersens to mites, eggs and fecal pellets)

  • Normally concentrated in skin folds
  • In children can be vesicular/pustules. In elderly can be bullous
  • Consider scabies when the rash is intensely pruritic (especially at night)


– Diagnosis: normally a clinical one

  • Skin scraping and examination under microscopy may confirm the diagnosis, but has low sens.
  • If symptoms persist after 2-3 weeks, skin scrappings are essential for diagnosis.

– Treatment

  • Permethrin cream

    – Apply to all parts of body including scalp, leave it for 12 hours, then reapply 1 week later
    – Persistent pruritus can occur up to 4 weeks. Therefore, do not evaluate for treatment efficacy until after 4 weeks.
    – Antihistamine can be used to manage the pruritus.
  • 2nd line: malathion
  • Other options: crotamiton cream, lindane, benzyl benzoate
    – Lindane can be used but neurotox risk
    – 10% crotamiton cream can be used in children (is very safe). need to leave for 24 hours, wash off then reapply for another 24 hours.
    – benzoyl benzoate is irritative to skin, also need to leave for 24 hours, wash off then reapply for another 24 hours.
  • Environmental measures
    – Wash linen under hot water or heated drying. If cannot be washed under hot water, it should be dry cleaned and sealed in a plastic bag for 4-5 days (the period of days a mite can survive outside a host).

For crusted scabies

picture here

– Skin crusting limits the penetration of topical drugs, also crusting may alter the systemic absorption of the topical drugs

  • Topical agents eg permethrin may not be effective in crusted scabies.

– Ivermectin (PO) is normally the choice in crusted scabies, although ivermectin is not FDA approved for treatment of scabies.




DAVID C. FLINDERS, M.D., and PETER DE SCHWEINITZ, M.D. Pediculosis and Scabies. AMERICAN FAMILY PHYSICIAN, 2004: 69(2);341-48


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