Pediculosis and scabies. A revision.

Pediculosis

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)

Diagnosis:
– 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.

Treatment
– 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.

1

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)

Scabies

– 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)

Capture

– 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.

Summary

Capture

Reference:

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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