Scabies Clinical Features
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Direct answer
Scabies is a contagious infestation by the mite Sarcoptes scabiei var. hominis that causes intense itch, classically worse at night. The diagnostic hallmark is the burrow, a thin, wavy, grey-white tunnel up to about 1 cm long, most often found in finger webs, wrists and flexor surfaces. Several household members itching together should prompt a search for scabies before any other diagnosis is made.
What you must remember
- Causative agent: Sarcoptes scabiei var. hominis, a female mite that burrows into the stratum corneum and lays eggs along the tunnel.
- Incubation: a first infestation usually takes about 2-6 weeks to cause itch, whereas reinfestation can itch within days, which explains rapid recurrence in households.
- Classic distribution: finger webs, wrists, elbows, axillae, periumbilical skin, genitals, areolae and buttocks; the face is usually spared in adults.
- Infants and young children: face, scalp, palms and soles are involved, and vesicles or pustules are common.
- Nodular scabies: firm, intensely itchy nodules, mostly on the genitals, groin and axillae, that can persist for weeks after the mite has been killed.
- Confirmation: dermoscopy shows the delta-wing jet sign, and skin scraping in mineral oil shows mites, eggs or scybala (faecal pellets).
- Common mimics: eczema, prurigo, papular urticaria and insect bites; the household pattern of itch is the clue that separates them.
Following the burrow to a diagnosis
A family of four presents with a week of night-time itch, and the youngest child's wrists show thin grey lines. The approach moves from pattern recognition to a confirmatory test, with each step narrowing the differential.
- Recognise the pattern: Generalised itch that is worse at night and shared by several household members makes scabies likely before any rash is examined. Eczema alone rarely explains a whole family itching together.
- Search the typical sites: Examine finger webs, wrists, elbows, axillae and genitals under good light and magnification for burrows. A burrow looks like a thin, slightly raised, wavy grey-white line that is easy to miss without a close look.
- Use dermoscopy: Place a drop of oil on the suspected burrow and look for the delta-wing jet sign, a dark triangular mite head with forelegs at one end of the tunnel. This confirms the diagnosis without any harm to the patient.
- Take a skin scraping: Scrape the end of the burrow with a blade, place the material on a slide with mineral oil and examine it under the microscope for mites, eggs or scybala. A negative scraping does not exclude scabies when burrows are few.
- Exclude the mimics: Consider prurigo nodularis, papular urticaria and eczema when the picture is atypical. Treat contacts empirically if the household pattern fits, even when the index patient's diagnosis remains uncertain.
Where the exam tries to catch you
Scabies items usually test the burrow, the distribution by age and the handling of contacts.
- Trap: expecting the face to be the classic site in every patient. Correct: the face is typically spared in adults but involved in infants, where the scalp, palms and soles are also affected.
- Trap: calling persistent itch after treatment a failure and repeating the scabicide. Correct: post-scabietic itch can last several weeks because of residual hypersensitivity, so the diagnosis should be reconsidered before any reflex retreatment.
- Trap: prescribing a steroid cream for the rash and never finding the mite. Correct: steroids suppress inflammation without killing the mite, so scabies must be confirmed and treated with a scabicide.
- Trap: treating only the index patient. Correct: household and close sexual contacts are treated at the same time to prevent reinfestation from an untreated person.
Frequently asked questions
Which clinical finding is most specific for scabies?
The burrow is the most specific clinical clue in scabies, most reliably found in finger webs, wrists and flexor surfaces. Pruritic papules alone are non-specific, so a burrow or a dermoscopic finding should be sought before the diagnosis is accepted.
What is the delta-wing jet sign in scabies?
The delta-wing jet sign is the dermoscopic image of the scabies mite seen at the end of a burrow, appearing as a dark triangular head with forelegs. It allows non-invasive confirmation of scabies without taking a skin scraping.
Why does itch persist after successful scabies treatment?
Itch can persist for several weeks after effective scabies treatment because of a delayed hypersensitivity reaction to mite antigens. Persistent itch alone does not mean the scabicide has failed, so new burrows should be looked for before the treatment is repeated.
Why must household contacts be treated in scabies?
Scabies spreads through prolonged skin-to-skin contact, so household and close sexual contacts can carry mites before symptoms appear. Treating them at the same time as the index patient prevents reinfestation from an untreated contact.
Which features suggest nodular scabies?
Nodular scabies presents as firm, intensely itchy nodules mainly on the genitals, groin and axillae that remain after the mite has been eliminated. The nodules reflect a hypersensitivity response and can take weeks to months to settle.