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.
Try two board-style Scabies questions
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Question 1Infectious DiseaseMedium
A 55-year-old male has 6 months of worsening diffuse pruritus especially at night with multiple linear excoriations, small papules, and vesicles on the finger webs, wrists, and belt line. His wife has similar pruritus. Which of the following is the most likely diagnosis?
AAtopic dermatitis
BDermatitis herpetiformis
CScabies
DContact dermatitis
EPediculosis corporis
Reveal answer & full explanation
Correct answer: C — Scabies
AAtopic dermatitis
BDermatitis herpetiformis
CScabies✓
DContact dermatitis
EPediculosis corporis
Why Scabies is correct
Severely pruritic linear excoriations, papules, and vesicles in the finger webs, wrists, and belt line with household contact involvement (wife has similar symptoms) is classic
Caused by Sarcoptes scabiei var. hominis — the female mite burrows into the stratum corneum and lays eggs
Transmitted via prolonged skin-to-skin contact
Pathognomonic finding: S-shaped burrows at the finger webs
Severe nocturnal pruritus reflects a hypersensitivity reaction to mite proteins, eggs, and feces
Household and sexual contacts must all be treated simultaneously
Why the others are wrong
Dermatitis herpetiformis — has IgA deposits and is associated with celiac disease; not spread via contact and does not characteristically involve the finger webs (confused-with a pruritic vesicular eruption)
Atopic dermatitis — flexural distribution without burrows and does not spread to household contacts (right-concept-wrong-setting for a chronically pruritic dermatosis)
Contact dermatitis — localized to the site of allergen/irritant exposure and not transmitted between contacts (anchoring on excoriations/vesicles without the contagious pattern)
Pediculosis corporis — body lice also spread among close contacts and cause intense pruritus with excoriations around the waist and belt line where clothing seams rest, but the lice and nits live in clothing seams rather than burrowing into skin, and they do not produce vesicles in the finger webs and wrists (confused-with another contagious pruritic infestation)
Additional high-yield points
Treatment: (1) permethrin 5% cream — apply to the entire body from neck to toes, leave 8-14 hours, repeat in 1 week; (2) ivermectin 200 mcg/kg PO — for crusted scabies, permethrin-resistant cases, or large outbreaks; give 2 doses 2 weeks apart; (3) lindane 1% — alternative limited by neurotoxicity
Pruritus may persist 2-4 weeks after successful treatment — treat residual symptoms with antihistamines and topical steroids
Classic locations: finger webs, wrists, belt line, genitals, areolae; spares the face (except in infants and the immunocompromised)
Question 2DermatologyMedium
A 7-year-old girl presents with two weeks of intense itching that is worse at night. She lives in a crowded household where her two siblings and her mother have all developed similar itching over the past month. On exam she has excoriated papules and thin, serpiginous gray lines in the finger webs and on the flexor wrists. Which of the following is the most likely diagnosis?
AContact dermatitis
BTinea corporis
CScabies infestation
DAtopic dermatitis
EPapular urticaria
Reveal answer & full explanation
Correct answer: C — Scabies infestation
AContact dermatitis
BTinea corporis
CScabies infestation✓
DAtopic dermatitis
EPapular urticaria
Why Scabies infestation is correct
This child has scabies.
It is caused by the Sarcoptes scabiei mite and spreads through prolonged skin-to-skin contact, so crowding and clustering of pruritus among household members are the strongest epidemiologic risk signals; the hallmark findings are intense nocturnal itching and thin serpiginous burrows in the finger webs, wrists, and other intertriginous sites, all present here.
Why the others are wrong
Atopic dermatitis — produces itchy, oozing, eczematous plaques in the flexural creases and is associated with a personal or family history of atopy, but it is not contagious and does not produce burrows or cause simultaneous itching in multiple household contacts.
Tinea corporis — is a dermatophyte infection producing annular scaly plaques with central clearing, often after animal contact, and does not cause burrows or the household-wide itch pattern seen here.
Papular urticaria — is a hypersensitivity reaction to insect bites (fleas, mosquitoes, bedbugs) that causes crops of pruritic papules on exposed skin, typically only in the sensitized child even when others share the exposure, and it does not produce burrows in the finger webs or on the wrists.
Contact dermatitis — a new soap, detergent, or other shared household product can make several family members itch at the same time, but contact dermatitis produces eczematous plaques at the sites of contact and does not form the thin serpiginous burrows in the finger webs and on the flexor wrists seen here.
Multiple itchy contacts in one crowded household, plus burrows in classic locations, is the epidemiologic and morphologic fingerprint of scabies.
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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 (mites, eggs, or feces on light microscopy of skin samples or on a high-powered imaging device, or mite seen on dermoscopy); clinical (burrows, OR typical lesions on the male genitalia, OR typical lesions in a typical distribution plus 2 history features: itch and positive contact history); suspected (typical lesions in a typical distribution plus 1 history feature, OR atypical lesions or distribution plus 2 history features).
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
Pregnancy: topical permethrin 5% is preferred; infants <2 months: permethrin is FDA-approved only from 2 months of age, so precipitated sulfur is used
Avoid ivermectin in pregnancy and infants <15 kg
Sulfur 5-10% precipitated in petrolatum × 3 nights — the usual choice for infants <2 months (safe in neonates)
Second-line / adjunct
Topical benzyl benzoate 10-25% (not available in US)
Topical malathion 0.5% lotion (not FDA-approved for scabies; limited data)
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
Topical spinosad 0.9% suspension (FDA-approved 2021 for age ≥4 years); crotamiton 10% (FDA-approved for adults only; frequent treatment failures)
Complications
Secondary bacterial infection (S. aureus, S. pyogenes) → impetigo, cellulitis, sepsis
Post-streptococcal glomerulonephritis (high incidence in endemic populations)
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