Scabies

A contagious parasitic skin infestation caused by the mite Sarcoptes scabiei var. hominis, which burrows into the stratum corneum of the skin.

Aetiology

  • Causative organism: Sarcoptes scabiei var. hominis
  • Risk factors:
    • Crowded living conditions (dormitories, nursing homes, prisons)
    • Poor hygiene or limited access to sanitation
    • Close contact with infected individuals
    • Immunocompromised patients → risk for crusted (Norwegian) scabies

Pathophysiology

  1. Female mites burrow into the stratum corneum to lay eggs.
  1. Eggs hatch into larvae → mature into adult mites.
  1. The host develops a hypersensitivity reaction to mite antigens, causing pruritus.
  1. Intense itching leads to scratching → possible secondary bacterial infection.
Life cycle duration: ~10–14 days.

Clinical presentation

  • Intense pruritus, worse at night
  • Burrows (canaliculus): thin, wavy, gray-white lines in the skin
  • Papules, vesicles, excoriations from scratching
notion image
  • Common sites:
    • Finger webs (interdigital spaces)
    • Flexor surfaces of the wrist
    • Elbows, axillae
    • Umbilical area
    • Waistline
    • Genital area (scrotum, penis)
    • In infants: palms, soles, scalp may also be involved
Special form:
Crusted (Norwegian) Scabies
  • Occurs in elderly, immunosuppressed, or neurologic impairment
  • Thick crusts, minimal itching, massive mite burden
  • Highly infectious

Investigations

  • Mostly clinical
  • Confirmation tests:
    • Skin scraping of burrows → microscopic visualization of mites, eggs, or fecal pellets
    • Dermoscopy: “hang glider” or “triangle with jet trail” sign (mite in burrow)

Management

  • First-line treatment
    • Permethrin 5% cream
      • Apply from neck down (entire body), leave overnight 8–14 hours, repeat in 1 week.
  • Alternative treatments
    • Oral Ivermectin (200 μg/kg) → single dose, repeat after 7–14 days
        • Preferred in crusted scabies or outbreaks
    • Benzyl benzoate 10-20% lotion leave 24 hours
    • Sulfur Percipitatum 5–10% ointment (safe in infants <2 months old & pregnancy)
      • Leave overnight 8 hours for 3 days consecutively
  • General measures
    • Treat all household contacts simultaneously
    • Wash clothing, bedding, towels in hot water & dry under heat
    • Items that can’t be washed → seal inplastic bags for 3–7 days
    • Antihistamines/topical steroids for pruritus relief