Scabies
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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Risk factors
- Crowded living conditions, institutional settings
- Prolonged skin-to-skin contact (household, sexual)
- 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
Signs / physical exam
- Burrows: thin gray-white serpiginous lines 2-15 mm — pathognomonic; finger web spaces, flexor wrists, elbows, anterior axillary folds, areolae, periumbilical, waistline, penis/scrotum, buttocks
- Erythematous papules, excoriations, secondary nodules (especially scrotum, penis, axillae — postscabietic nodules)
- Infants and children: vesiculopustular lesions on palms, soles, scalp, face (typically spared in adults)
- Crusted (Norwegian) scabies: thick hyperkeratotic crusted plaques on hands, feet, scalp, sometimes generalized; minimal pruritus; severely contagious; immunocompromised host
Classic findings
Burrows in finger web spaces, flexor wrists, genitalia; intense nocturnal pruritus; multiple affected family members.
Differential diagnosis
- Atopic dermatitis — Personal/family atopy, flexural; no burrows; household contacts not affected
- Bedbug bites — Linear breakfast-lunch-dinner clusters on exposed skin; nocturnal exposure
- Insect bites / arthropod assault — Discrete punctate lesions, exposed areas, NO burrows
- 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
Imaging
- Not indicated
Diagnostic algorithm
| Variant | Clinical Hallmark | Therapy |
|---|---|---|
| Classic scabies | Burrows + nocturnal itch + household spread | Permethrin 5% (days 0 + 7) OR ivermectin 200 mcg/kg (days 0 + 7-14) |
| Infant scabies | Vesicles on palms/soles/scalp | Permethrin 5% neck-to-feet INCLUDING head; sulfur if <2 mo |
| Nodular | Itchy nodules on scrotum/axillae | Treat scabies + intralesional steroid for residual nodules |
| Crusted (Norwegian) | Hyperkeratotic plaques, minimal itch, immunocompromised | Ivermectin days 1,2,8,9,15 + permethrin daily; keratolytic; isolate |
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)
- Postscabietic nodules — persistent itchy nodules on scrotum, penis, axillae lasting months
- Crusted scabies → secondary bacteremia, sepsis, mortality up to 20% if untreated
- Outbreaks in healthcare facilities and refugee settings
- Psychiatric: 'delusions of parasitosis' developing after legitimate scabies
PANCE pearls
- Intense itching with normal-appearing affected family members = scabies until proven otherwise.
- Burrows are pathognomonic — examine finger webs, wrists, areolae, genitalia with magnification or dermoscopy.
- Treat all household and sexual contacts SIMULTANEOUSLY — even asymptomatic ones — to prevent reinfestation ping-pong.
- Persistent itching after treatment is the rule, not the exception — do not retreat unless new burrows or mites confirmed.
- Crusted scabies in an HIV patient = highly contagious medical emergency requiring isolation and combined ivermectin + permethrin therapy.
References
- IDSA / CDC — CDC Scabies Resources for Health Professionals — Diagnosis, Treatment, Outbreak Management
- IACS 2020 — International Alliance for the Control of Scabies Consensus Criteria for the Diagnosis of Scabies (Engelman et al., Br J Dermatol 2020)
- Cochrane 2018 — Interventions for Treating Scabies (Rosumeck et al., Cochrane Database Syst Rev 2018)
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