Scabies

BASICS

Definition: Contagious parasitic skin infection caused by the mite Sarcoptes scabiei var. hominis.
Diagnosis: Primarily clinical (history and exam); may use dermoscopy or microscopy.
Systems affected: Skin/exocrine.

Incidence: Predominantly affects children, sexually active young adults, elderly.
Risk groups: Higher in resource-poor, overcrowded, and tropical settings.
WHO: Listed as neglected tropical disease (2017).
Transmission: Prolonged direct skin-to-skin contact; less commonly via fomites (bedding, clothing).

Pathogen: Sarcoptes scabiei var. hominis (obligate human parasite).
Mechanism: Female mite burrows into stratum corneum/epidermis and lays eggs.
Immunopathology: Itching is due to type IV delayed hypersensitivity to mite products.
Crusted/Norwegian scabies: Seen in immunocompromised, with massive mite burden.

Prolonged close contact (sexual, family, overcrowding, healthcare, homelessness)
Poor hygiene, malnutrition, poverty
Immunocompromised states (HIV/AIDS, leukemia, lymphoma, congenital deficiencies)
Hot/tropical climate
Geriatric patients (especially institutionalized)

Itching (often severe, worse at night)
Initial infection: asymptomatic for 3–4 weeks; reinfection: symptoms in 1–3 days
Identify contacts

Lesions: Erythematous, pruritic papules/vesicles; burrows (thin, curvy lines, 1–10 mm; pathognomonic; most common in hands/wrists)
Distribution: Finger webs, flexor wrists, elbows, axillae, buttocks, genitalia, feet/ankles (spares head/neck in adults)
Secondary: Excoriations, pustules if infected, nodular lesions in covered areas
Crusted scabies: Thickened, hyperkeratotic, psoriasiform plaques (immunocompromised/elderly)
Pediatrics: May involve palms, soles, scalp, head/neck

Confirmed: Microscopic identification of mite/eggs/fecal pellets or dermoscopy visualization
Clinical: Burrows, typical lesions on genitalia, or typical lesions in classic distribution + 2 history features (itch and contact)
Suspected: Typical lesions/distribution with 1 history feature, or atypical features with both itch and contact

Atopic/contact dermatitis, dermatitis herpetiformis, eczema, insect bites, psoriasis (esp. crusted), folliculitis, impetigo, pediculosis, papular urticaria, tinea corporis, seborrheic dermatitis, syphilis

Intensive: Permethrin 5% every 2–3 days for 1–2 weeks PLUS ivermectin PO on days 1, 2, 8, 9, 15 (additional doses if severe)

Crotamiton 10% cream (infants >3 months)
Precipitated sulfur 2–10% ointment (infants <2 months)
Lindane 1% lotion: Not preferred due to neurotoxicity (BBW), especially in children, elderly, <50 kg, immunocompromised
Topical benzyl benzoate, topical ivermectin 1% lotion (investigational, not FDA-approved)

Tea tree oil 5%: Demonstrated efficacy as a scabicide in vitro (not first-line, may cause local irritation)

Monitor if symptoms persist beyond 4 weeks; consider re-scraping, alternative diagnosis, reinfestation, or improper treatment.
Nodular scabies may persist and need intralesional steroids.
Consider dermatology referral if diagnosis/treatment uncertain.

Lesions regress 1–2 days after treatment, but pruritus/eczema may persist for weeks.
Crusted scabies requires aggressive treatment.

Nodular scabies (weeks–months after Rx)
Post-scabetic pruritus
Pyoderma, secondary bacterial infection (esp. in developing countries; may cause sepsis, glomerulonephritis, rheumatic heart disease)
Sleep disturbance, social stigma

Topical steroids may mask symptoms (scabies incognito).
Persistent pruritus after Rx is common—distinguish from treatment failure/reinfestation.
Treat all contacts; untreated index cases cause reinfection.