Intensely pruritic infestation by Sarcoptes scabiei mite — burrows in web spaces and intensely itchy nocturnal rash; household contacts affected.
Also known as: scabies, Sarcoptes scabiei, crusted scabies, Norwegian scabies
Overview
Cutaneous infestation by the human itch mite Sarcoptes scabiei var. hominis, transmitted by prolonged skin-to-skin contact and (less commonly) fomites. Classic disease produces intense pruritus and burrows in characteristic distribution; crusted (Norwegian) scabies is a severe hyperinfestation in immunocompromised hosts.
Epidemiology
Affects ~200 million people globally; >400 million annual incident cases. WHO classifies scabies as a neglected tropical disease. Outbreaks common in long-term care facilities, prisons, refugee camps, daycare. All ages and socioeconomic strata.
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Immunosuppression (HIV, transplant, lymphoma) — risk of crusted scabies
Elderly in nursing homes; cognitive impairment (delayed scratching response)
Healthcare workers caring for infested patients
Pathophysiology
Female mite burrows into stratum corneum, lays 2-3 eggs/day for ~30-day lifespan. Larvae hatch in 3-4 days; mature mites emerge in 10-14 days. Pruritus and rash are primarily a delayed type IV hypersensitivity reaction to mite proteins and feces (scybala); takes 4-6 weeks for initial sensitization, 1-3 days on reinfestation. Total mite burden in classic scabies is typically 10-15 mites; crusted scabies hosts thousands to millions.
Clinical presentation
Symptoms
Intense generalized pruritus, worse at night, disrupting sleep
Pruritus in close contacts/household members
Onset 4-6 weeks after primary exposure (sensitization period); 1-3 days on reinfestation
Crusted scabies: paradoxically LESS itchy due to impaired immune response
Dyshidrotic eczema — Deep-seated vesicles on palms/soles/lateral fingers
Dermatitis herpetiformis — Symmetric grouped vesicles on elbows/knees/buttocks; gluten sensitivity, DIF granular IgA
Contact dermatitis — Pattern matches contactant; no burrows
Pediculosis — Lice/nits visible; specific body sites (head, body, pubic)
Senile / xerotic pruritus — Elderly, dry skin, no rash or burrows; emollients improve
Diagnostic workup
Diagnostic criteria
IACS 2020 consensus: confirmed (mite/eggs/scybala visualized), clinical (burrows + classic distribution + 1 history feature), or suspected (typical lesions + history).
Labs
Skin scraping with mineral oil from burrow or papule, examined under microscopy: mites, eggs, or fecal pellets (scybala) — sensitivity only 30-50%, so negative scraping does not exclude disease
Dermoscopy: 'jet with contrail' sign — dark triangular mite head with linear burrow trailing behind
PCR or videodermoscopy in research/specialty settings
Empiric treatment commonly used when clinical suspicion high
Ivermectin days 1,2,8,9,15 + permethrin daily; keratolytic; isolate
Scabies variants and first-line therapy.
Treatment
First-line
Topical permethrin 5% cream — apply from neck down (include scalp/face in infants and elderly) overnight (8-14 hours), wash off in morning; REPEAT in 7 days (kills newly hatched larvae); first-line for most patients ≥2 months old
Oral ivermectin 200 mcg/kg PO on day 0 and day 7-14 — first-line alternative; preferred for institutional outbreaks and crusted scabies; safe in adults and children >15 kg (avoid in pregnancy and infants)
Treat ALL close contacts simultaneously regardless of symptoms — sensitization takes weeks, so asymptomatic carriers are common
Decontamination: wash all clothing, bedding, towels used in past 4 days in hot water and dry on hot setting; items that cannot be washed bagged 72 hours (mites die without host)
Crusted (Norwegian) scabies
Combination therapy: oral ivermectin 200 mcg/kg on days 1, 2, 8, 9, 15 (± 22, 29) PLUS topical permethrin 5% daily × 7 days then 2x/week × 2 weeks
Keratolytic (5-10% salicylic acid or lactic acid) to debulk crusts so scabicide can penetrate
Contact isolation; treat all healthcare workers and contacts
Manage underlying immunosuppression
Pregnancy / infants <2 months
Topical permethrin 5% (pregnancy category B) is preferred
Avoid ivermectin in pregnancy and infants <15 kg
Sulfur 5-10% precipitated in petrolatum × 3 nights — old but safe alternative for neonates
Second-line / adjunct
Topical benzyl benzoate 10-25% (not available in US)
Topical malathion 0.5% lotion
Counsel: pruritus may persist 2-4 weeks AFTER successful treatment due to residual antigen — not a failure; treat with topical steroid + oral antihistamine, NOT repeat scabicide unless mites confirmed
Complications
Secondary bacterial infection (S. aureus, S. pyogenes) → impetigo, cellulitis, sepsis
Post-streptococcal glomerulonephritis (high incidence in endemic populations)
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