Sarcoptes scabiei

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

• Eczema
• Tinea
• Atopic dermatitis or contact dermatitis
• Papular urticaria
• Seborrheic dermatitis
• Bullous pemphigoid

Diagnosis

• Usually made clinically, since diagnostic testing has low sensitivity1
• If sending a skin scraping, use the leading edge of the burrow

Management

• 5% permethrin cream 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 clinical failure or more extensive infestations, ivermectin 200 μg/kg, repeated once at day 14
  • Treatments are not ovacidal
• For crusted scabies, consider combination topical plus oral
  • Permethrin applied daily for 7 days, repeated weekly for 2 weeks
  • Ivermectin 200 μg/kg p.o. on day 1, 2, 8, 9, and 15; can be extended to include days 22 and 29 for severe cases
• 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.