Scabies

Overview

•Nocturnal pruritus - intense itch worse at night, often disrupting sleep; single most characteristic symptom

•Burrows - fine, grey-white, linear or curved tracks 2-10 mm long in superficial skin (the mite's tunnel)

•Erythematous papules - small, excoriated papules from scratching and immune reaction

•Genital nodules - firm, reddish-brown nodules on genitals and axillae (persistent post-inflammatory response)

•Classic sites: finger webs (look here first), wrists, elbows, axillae, nipples, umbilicus, genitals - face/scalp/back spared in adults

•Close contacts are commonly simultaneously affected - key historical clue

🎯
Nocturnal itch + linear burrows in finger webs = scabies until proven otherwise. The itch is a type IV hypersensitivity to mite proteins - NOT direct burrowing injury - hence itch lags 2-6 weeks after first infestation.

Investigations

•Clinical diagnosis - based on nocturnal itch, burrows in web spaces, affected contacts; no investigations routinely required

🏆 Gold standard

•dermoscopy - visualises mite at end of burrow ('delta-wing jet' sign); or skin scraping with microscopy confirming mite/eggs/scybala

Differential diagnosis

Scabies vs bedbug infestation
FeatureScabiesBedbug infestation
Causative agentSarcoptes scabiei miteCimex lectularius
Itch timingNocturnal, intensePruritic but not exclusively nocturnal
Lesion typeBurrows, papulesPapules/wheals in a line or curve
DistributionFinger webs, wrists, genitalsExposed areas; hands/feet/groin often spared
Contacts affectedYes - household spread commonMultiple residents in same accommodation
Setting clueCare homes, schools, householdsHotels, hostels, temporary accommodation

Feature	Scabies	Bedbug infestation
Causative agent	Sarcoptes scabiei mite	Cimex lectularius
Itch timing	Nocturnal, intense	Pruritic but not exclusively nocturnal
Lesion type	Burrows, papules	Papules/wheals in a line or curve
Distribution	Finger webs, wrists, genitals	Exposed areas; hands/feet/groin often spared
Contacts affected	Yes - household spread common	Multiple residents in same accommodation
Setting clue	Care homes, schools, households	Hotels, hostels, temporary accommodation

Management

•Treat patient AND all close contacts (household members, sexual partners within 6-8 weeks) simultaneously - regardless of symptoms

🥇 First-line

•permethrin 5% cream - apply to entire body neck to soles (including under nails, skin folds, between toes), leave 8-12 hours overnight, wash off; repeat after 7 days

🥈 Second-line

•malathion 0.5% aqueous liquid - if permethrin contraindicated or fails; same whole-body technique; two applications 7 days apart

🥉 Third-line

•ivermectin (oral) - unlicensed in UK; used off-label for crusted scabies or topical treatment failure; specialist guidance required

•Apply permethrin to cool, dry skin (not after hot bath - vasodilation reduces efficacy); also apply to face/scalp in infants, elderly, and immunocompromised

•Environmental decontamination: wash clothing, bedding, towels used in 72 h before treatment at 60°C; items that cannot be washed sealed in plastic bag for 72 hours

•Symptom relief: non-sedating antihistamine (e.g. cetirizine) by day; sedating antihistamine (e.g. chlorphenamine) at night

⚠️
Post-scabetic itch persists for 2-6 weeks after successful treatment due to ongoing hypersensitivity to dead mite debris - this does NOT mean treatment failure. Review at 4 weeks; only re-treat if new burrows appear.