Key Facts

Scabies is a common, highly contagious ectoparasitic infestation. It is a significant public health concern, particularly in care homes, prisons, and overcrowded settings. Outbreaks require coordinated treatment of all contacts.

Epidemiology

• ~300 million cases/year globally
• Common in UK: increased incidence during/post COVID-19
• All ages affected; outbreaks in care homes, schools, prisons
• Not related to hygiene — anyone can be affected
• Crusted scabies: rare, almost exclusively in immunocompromised/elderly

Aetiology

• Sarcoptes scabiei var. hominis: obligate human parasite
• Transmission: prolonged skin-to-skin contact (>15–20 minutes); sexual transmission common
• Fomite transmission: rare in classical scabies; significant in crusted scabies
• Typical mite burden: 10–15 mites in classical scabies; thousands–millions in crusted scabies

Pathophysiology

• Female mite burrows into stratum corneum → lays eggs → larvae hatch in 3–4 days
• Type IV (delayed) hypersensitivity to mite proteins, faeces, and eggs → intense pruritus
• First infestation: sensitisation takes 4–6 weeks → asymptomatic but contagious during this period
• Re-infestation: symptoms develop within 24–48 hours (pre-sensitised)

Classical Scabies

• Intense generalised pruritus — characteristically worse at night
• Burrows: short (5–15 mm), wavy, grey-white lines — pathognomonic
  • Finger web spaces (most common)
  • Flexor aspect of wrists
  • Male genitalia (penile papules/nodules — almost pathognomonic)
  • Female nipples
  • Axillary folds, belt line, buttocks
• Excoriations, secondary eczematisation
• Papules, vesicles on hands and feet
• Face and scalp typically spared in adults (but NOT in infants)

Crusted (Norwegian) Scabies

• Hyperkeratotic, crusted, psoriasiform plaques
• May involve face, scalp, nails
• Pruritus may be minimal (impaired immune response)
• EXTREMELY contagious — thousands to millions of mites
• Risk groups: immunosuppressed, elderly, Down syndrome, neurological conditions

Scabies in Infants

• Lesions on palms, soles, face, scalp (unlike adults)
• Vesiculopustular lesions

Red Flags

• Crusted scabies — infection control emergency; isolate immediately
• Widespread secondary bacterial infection (impetigo, cellulitis)
• Treatment failure — consider crusted scabies, poor compliance, untreated contacts
• Institutional outbreak

Bedside

• Dermoscopy: 'delta-wing jet' sign (mite at end of burrow) — quick and sensitive; 'wake sign' of burrow
• Skin scraping: scrape burrow with scalpel blade → oil mount or KOH → microscopy for mites, eggs, or faecal pellets (scybala)
• Ink burrow test: apply ink over suspected burrow → wipe off → burrow retains ink

Bloods

• Not routinely required
• Eosinophil count: may be elevated
• HIV test: consider if crusted scabies in young patient

Special Tests

• Not usually needed — diagnosis is predominantly clinical
• Biopsy: rarely indicated; shows spongiotic dermatitis with eosinophils; mite may be seen in stratum corneum

Imaging

• Not applicable

Non-Pharmacological

• Treat ALL household contacts and sexual partners simultaneously — even if asymptomatic
• Launder bedding, towels, clothing at ≥60°C on day of treatment; items that cannot be washed — bag and seal for 72 hours
• Infection control: isolation for crusted scabies; barrier nursing
• Patient education: post-scabies itch may persist 2–4 weeks — not treatment failure

Pharmacological

Classical scabies:

• Permethrin 5% cream: first-line
  • Apply to whole body from chin down (include head/face in infants, elderly, immunosuppressed)
  • Leave on for 8–12 hours (overnight), wash off
  • Reapply if hands washed during treatment period
  • Repeat at day 7 (kills newly hatched mites)
• Oral ivermectin 200 mcg/kg: single dose, repeat at day 7 — alternative; particularly useful for outbreaks, treatment failure, or inability to apply topical treatment

Crusted scabies:

• Combined approach: oral ivermectin 200 mcg/kg on days 1, 2, 8, 9, 15 PLUS permethrin 5% daily for 7 days then twice weekly until cured
• Keratolytics to remove crusts
• Isolation and infection control precautions

Symptomatic relief:

• Crotamiton 10% cream: antipruritic, applied BD
• Emollients: for dry, eczematised skin
• Sedating antihistamine (chlorphenamine 4 mg TDS): for nocturnal itch
• Topical corticosteroid: short course for post-scabies eczema (after successful treatment)

Referral Criteria

• Dermatology: diagnostic uncertainty, treatment failure, suspected crusted scabies
• Public health/infection control: institutional outbreaks
• HIV/immunology: crusted scabies in young patient

• Classical scabies: excellent with appropriate treatment of patient AND contacts
• Treatment failure: usually due to untreated contacts, inadequate application, or missed diagnosis of crusted scabies
• Post-scabies itch: common, lasting 2–4 weeks after successful treatment; responds to emollients and mild topical steroids
• Crusted scabies: high morbidity; risk of secondary sepsis; requires aggressive treatment and infection control
• Scabies outbreaks in care homes: can be prolonged and difficult to eradicate without coordinated treatment

Scabies Treatment Summary

Key Distinguishing Features