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 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.
