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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Question 1DermatologyMedium
A 23-year-old has intense nocturnal pruritus and burrows in the finger webs. Which of the following is the most appropriate treatment?
ATopical mupirocin for the patient only
BPermethrin for patient and contacts
COral cephalexin for the patient only
DTopical clobetasol for the patient only
Reveal answer & full explanation
Correct answer: B — Permethrin for patient and contacts
ATopical mupirocin for the patient only
BPermethrin for patient and contacts✓
COral cephalexin for the patient only
DTopical clobetasol for the patient only
Why Permethrin for patient and contacts is correct
Burrows in the finger webs with nocturnal pruritus indicate scabies from Sarcoptes scabiei infestation.
Topical permethrin is first-line, and all close contacts must be treated simultaneously to prevent reinfestation.
Bedding and clothing should also be laundered or sealed because mites survive briefly off the host.
Why the others are wrong
Topical mupirocin for the patient only — Mupirocin targets bacteria such as impetigo and has no effect on mites, and treating only the patient leaves contacts to reinfest.
Oral cephalexin for the patient only — A systemic antibiotic addresses secondary bacterial superinfection, not the underlying mite infestation driving the itch.
Topical clobetasol for the patient only — A potent steroid may blunt the itch transiently but never eradicates the mites, so the infestation persists.
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?
AScabies infestation
BAtopic dermatitis
CTinea corporis
DBullous impetigo
Reveal answer & full explanation
Correct answer: A — Scabies infestation
AScabies infestation✓
BAtopic dermatitis
CTinea corporis
DBullous impetigo
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.
Bullous impetigo — is a superficial bacterial infection presenting with honey-colored crusting around the nose and mouth, not the pruritic burrows of a mite infestation.
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 (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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