Scabies – Rapid Review Notes

Epidemiology

  • Global distribution; affects all ages, races, socioeconomic groups
  • Key spread enhancers: overcrowding, delayed treatment, poor public awareness

Transmission

  • Prolonged skin-to-skin contact (sexual or non-sexual)
  • Household clustering; family members often share infestation
  • Fomites (clothing, bedding, towels) may transmit, especially in crusted cases

Pathogenesis

  • Causative mite: Sarcoptes scabiei var. hominis (human-specific)
  • Life cycle entirely in epidermis; duration 3030 days
    • Female lays 60−9060{-}90 eggs → mature in 1010 days
  • Mite can survive off host ≤3\le 3 days (up to ≤7\le 7 days in crusted scabies)
  • First infestation: sensitization & symptoms appear after 2−62{-}6 weeks; subsequent infestations cause rapid pruritus

Clinical Features (Typical)

  • Severe, generalized nocturnal pruritus
  • Symmetric papules, vesicles, excoriations at classic sites:
    • Interdigital webs, flexor wrists, axillae
    • Waistline, buttocks, belt area, ankles, feet
    • Male genitalia; areolae/nipples in women
  • Pathognomonic: burrow (serpiginous intra-epidermal tunnel)

Special Populations

  • Infants, elderly, immunocompromised: lesions may involve scalp, face, palms, soles; nodules, vesicles, eczematous dermatitis; high risk secondary infection
  • Crusted (Norwegian) scabies: millions of mites, thick hyperkeratotic crusts, highly contagious; seen in immunosuppressed (e.g., PLWHA, frail elderly)

Diagnosis

  • Microscopy of mineral-oil skin scrapings: mites, eggs, or scybala (fecal pellets)
  • Skin biopsy rarely required

Treatment Principles

  • Topical scabicide applied overnight to entire skin surface (include scalp/face in infants & crusted cases)
  • Treat all household/close contacts simultaneously; consider environmental decontamination:
    • Wash/heat-dry clothes, linen, towels used within previous week
    • Seal unwashable items in plastic for ≥3\ge 3 days
  • Pets do not require treatment (different mite species)
First-Line Agents
  • Permethrin 5%5\% cream (single overnight application; repeat day 77)
  • Benzyl benzoate (variable concentrations)
  • Oral ivermectin 250−400 μg/kg250{-}400\,\mu g/kg; repeat after 11 week (especially crusted/scabies outbreaks)
Alternatives / Cautions
  • Lindane 1%1\% lotion (neurotoxicity risk; avoid in infants, pregnant or lactating patients)

Post-Treatment Issues

  • Post-scabietic pruritus may persist 2−42{-}4 weeks; reassure patient, use antipruritics/emollients
  • Second treatment on day 77 prevents resistance & kills newly hatched nymphs

Key Takeaways

  • High index of suspicion in any patient with nocturnal pruritus and grouped papules
  • Early, simultaneous treatment plus fomite control stops community spread
  • Crusted scabies = dermatologic emergency due to heavy mite load and contagion