Scabies & Pediculosis

The patient finished scabies treatment a week ago and is still scratching — the family insists it didn't work and wants to retreat. Retreating too soon is the second mistake; the first was not treating the whole household.

Scabies is caused by the mite Sarcoptes scabiei, which burrows into the epidermis to lay eggs. Transmission requires prolonged direct skin-to-skin contact (15–20 minutes). Presentation: intense pruritus that is significantly worse at night (a hallmark feature), caused by a delayed type IV hypersensitivity reaction to the mite, its eggs, and fecal matter. Characteristic linear burrows are found in web spaces of the fingers, flexor surfaces of the wrists, axillae, waistline, inner thighs, and genital area. In infants, the palms, soles, and scalp may also be involved. Treatment: permethrin 5% cream applied from the neck down to the toes (include scalp in infants), left on for 8–14 hours, then washed off. Treat ALL household members and close contacts simultaneously, even if asymptomatic — subclinical infestation is common. Repeat treatment in 1–2 weeks to kill newly hatched mites. Critical teaching: pruritus commonly persists for 2–4 weeks after successful treatment due to ongoing hypersensitivity to dead mite proteins — this does NOT indicate treatment failure. New burrows indicate treatment failure; continued itching alone does not. Crusted (Norwegian) scabies occurs in immunocompromised, elderly, or institutionalized patients — it involves thick hyperkeratotic crusts containing thousands to millions of mites and is extremely contagious. Treatment requires oral ivermectin plus topical permethrin and strict isolation. Pediculosis (lice): Pediculus humanus capitis (head lice) is the most common in school-age children. Nits (eggs) are cemented to the hair shaft near the scalp and cannot be shaken off (unlike dandruff). Treatment: permethrin 1% (OTC) or malathion 0.5%. Manual nit removal with a fine-toothed comb after treatment. Wash bedding and clothing in hot water (≥130°F/54°C), bag non-washable items for 2 weeks. Head lice are NOT a sign of poor hygiene — they prefer clean hair. The AAP and NASN recommend against no-nit school exclusion policies. Body lice live in clothing seams (not on the body) and are a vector for typhus, trench fever, and relapsing fever. Pubic lice: screen for other STIs when diagnosed.

Key Distinctions

Post-treatment pruritus persisting 2–4 weeks is normal and does NOT mean treatment failure — look for new burrows, not just ongoing itching. This is the #1 tested distinction. Crusted (Norwegian) scabies requires both oral ivermectin AND topical treatment (standard topical alone is insufficient) and much stricter isolation. Head lice do not transmit disease and are not a hygiene indicator — the most current guidelines oppose no-nit exclusion policies. Do not confuse scabies burrows (linear, in web spaces and skin folds) with other pruritic rashes — the distribution pattern and nocturnal worsening are diagnostic clues.

Clinical Pearl

Still itching after treatment? Look for new burrows, not old scratching. Dead mites itch for weeks — that's your immune system cleaning house.