Sarcoptes scabiei

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Sarcoptes scabiei /
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Background

  • A mite that causes scabies

Microbiology

  • Eight-legged mite (arachnid)
  • Lives on human skin, but can survive 24 hours on clothes and linens

Epidemiology

  • Transmitted by person-to-person direct contact
    • They don't jump
    • Transmission via unwashed bed sheets or heavily contaminated clothing
    • Animal scabies (mange) can transiently infect humans
  • Common worldwide
  • More common in:
    • Crowded conditions
    • Institutional setting
    • Children less than 2 years of age
      • School transmission unlikely given contact is not typically close enough
    • Winter time in Northern hemisphere
    • Periods of civil unrest
  • In the case of crusted scabies, may also have transmission via fomites

Pathophysiology

  • Male and female crawl around on the skin and mate either on the skin or in a burrow
  • Female, once fertiziled, burrows into epidermis and lays 2-3 eggs daily along the path of the burrow
    • Lays up to 20-25 eggs before she dies
    • Moults about every 5 days
  • Eggs hatch into larvae in 3 to 4 days
  • Mites can live off of human host for 24 to 36 hours (longer if cold)

Clinical Manifestations

Classical Scabies

  • Low burden (5 to 15 mites)
  • Only symptom is intense pruritis, which is caused by delayed type IV hypersensitivity reaction
    • Usually appears after after 4 to 6 weeks on first infection, but within 24 to 48 hours on repeat infestation
    • Often worse at night
  • Can see burrows, but not always
    • Classically in webs of fingers
    • They can be subtle and atypical

Crusted Scabies

  • Sometimes called Norwegian scabies
  • Seen in cellular immunocompromise, including advanced HIV, leprosy, lymphoma, and the elderly
  • Starts as areas of erythema anywhere on the body (most commonly scalp, hands, and feet)
  • Develops into crusted or scaly areas
  • No or minimal itching, due to lack of cell-mediated hypersensitivity
  • Heavy mite burden, with thousands

Atypical Scabies

  • Scalp, seen in infants and young children
  • More generalized lesions, in infants and children
  • Nodular scabies (violacenous, pruritic nodules)
    • Typically on scrotum or genetalia, or breasts

Differential Diagnosis

Diagnosis

  • Usually made clinically
  • If sending a skin scraping, use the leading edge of the burrow

Management

  • 5% permethrin applied from hairline of neck down entire body, left for 12 hours, then washed off
    • Must be reapplied on any areas (like hands) that are washed during the 12 hours
  • For crusted scabies, ivermectin 200 μg/kg, repeated once at day 14
    • Treatments are not ovacidal
  • Alternatives include lindane 1% lotion, benzyl benzoate 10-25%, and others
  • Household management
    • All clothes/beddings must either be washed in 50+ ºC water, ironed, dry-cleaned, or placed in a sealed plastic bag for 3 to 7 days
    • Household contacts should be treated as well
  • Itching may continue for 4 to 6 weeks due to the cell-mediated hypersensitivity reaction to dead mites and eggs

Prevention

Infection Prevention and Control

  • For crusted scabies, strict contact precautions
  • Contacts should be treated empirically, including family, patient care staff, etc.