Scabies is diagnosed clinically far more often than it is confirmed, which is fine when the presentation is classic and a problem when it is not. A patient with generalised itch, an unimpressive rash and a partner who also itches is scabies until proven otherwise, whatever the referral letter says about eczema.
Look where the mite lives: finger webs, wrists, axillae, periumbilical skin, genitals in men, areolae in women, and in infants the palms, soles and scalp. Dermoscopy showing the delta-wing sign takes seconds. Where the diagnosis is doubtful, a scrape from an unexcoriated burrow is still worth doing before committing a household to treatment.
Treatment failures are almost never resistance. They are untreated contacts, a single application instead of two a week apart, missed sites — scalp, under the nails, between the toes — and permethrin applied to skin still hot from a bath. Treat every household member on the same day, cut and scrub the nails, and warn that itch persists for two to four weeks after successful treatment, or the patient returns convinced the drug failed.
- Examine finger webs, wrists, axillae, genitals and areolae; in infants add palms, soles and scalp
- Use dermoscopy for the delta-wing sign before resorting to a scrape
- Treat all household and sexual contacts on the same day, symptomatic or not
- Two applications a week apart, to cool dry skin, including under the nails
- Warn about post-scabetic itch lasting two to four weeks so it is not mistaken for failure
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