Scabies

Overview

Scabies is an infestation of the epidermis by the mite Sarcoptes scabiei var. hominis (class Arachnida, family Sarcoptidae). It affects over 100 million persons annually worldwide, crossing all ages, races, and socioeconomic levels. Prevalence in some underdeveloped regions reaches 4–100% of the population.

The Mite

Pearl-like, translucent, white, oval, eyeless; 4 pairs of short stubby legs
Adult female: ~0.4 × 0.3 mm (just below naked-eye visibility); male slightly smaller
Cannot fly or jump
Survives 3 days off-host in air; up to 7 days in mineral oil
Entire life cycle completed on human skin

Life cycle: The female excavates a burrow of 0.5–5 mm/day in the stratum corneum (up to 1 cm long), laying 0–4 eggs/day (~50 total over her 30-day lifespan). Eggs hatch in 10–12 days → larvae → nymphs (on skin surface) → adults.

A normal host harbors 3–50 mites. Crusted (Norwegian) scabies hosts may harbor millions.

Transmission

Primary route: Close, prolonged skin-to-skin contact (sexual and non-sexual)
Fomites: Bedding, clothing, upholstered furniture — especially relevant in crusted scabies (6,000 mites/g of debris documented)
Occurs more commonly in winter months

Clinical Features

Incubation: 4–6 weeks after primary infestation (symptoms can be delayed up to 3 months). On reinfection, symptoms develop within 2–3 days.

Cardinal symptom: Intense nocturnal pruritus

Distribution (adults):

Interdigital web spaces (hands)
Sides of fingers, volar wrists, lateral palms
Elbows, axillae
Scrotum, penis, labia, areolae
Periumbilical skin, buttocks
Head and neck spared in healthy adults

In infants/young children: Face, scalp, palms, soles involved; lesions are often papules and vesicopustules; nodular lesions in intertriginous areas

Pathognomonic lesion: The burrow — a thin, thread-like, linear or J-shaped tunnel, 1–10 mm long, in the stratum corneum. Best seen in interdigital webs and wrists.

Scabies burrow at 15x magnification showing a serpiginous, whitish, translucent tunnel with discrete entry points

Scabies burrow at 15× magnification showing the serpentine tunnel in the stratum corneum with the "delta sign" mite at the leading edge.

SEM of Sarcoptes scabiei female mite in a cutaneous burrow showing oval body, transverse ridges, spines, and fecal pellets (scybala)

SEM (×300) of an adult female S. scabiei within a cutaneous burrow. Fecal pellets (scybala) are visible alongside the mite.

Crusted (Norwegian) Scabies

Occurs in immunosuppressed patients (HIV with CD4 <200/mm³), elderly, those with neuropathy or dementia, leprosy, paraplegia
Hyperkeratotic, grayish-white plaques — palmar/plantar predominance; nail thickening and dystrophy
Paradoxically minimal or absent pruritus despite millions of mites
Highly contagious — nosocomial outbreaks documented
Head and neck commonly affected (unlike classic scabies)
Can mimic seborrheic dermatitis, psoriasis, drug eruptions, eczema

Diagnosis

Clinical: Based on characteristic pruritus, distribution, and exposure history.

Definitive (laboratory):

Mineral oil preparation of skin scrapings from a burrow → microscopic identification of mites, eggs, or fecal pellets (scybala)
Dermoscopy: "Delta sign" / "jet sign" — triangular dark structure at the leading end of the burrow
KOH preparation of skin scrapings
PCR for S. scabiei DNA (emerging technique)

Burrow identification tip: Ink test (apply ink over suspected burrow, wipe off — ink tracks into the burrow channel).

Differential Diagnosis

Most Likely	Also Consider
Atopic dermatitis	Dermatitis herpetiformis
Dyshidrotic eczema	Psoriasis
Contact dermatitis	Bullous pemphigoid
Insect bite reactions	Linear IgA bullous dermatosis
Pyoderma / Id reaction	Drug eruption
Varicella, miliaria	Delusions of parasitosis
Pityriasis rosea	Secondary syphilis

Treatment

All household members and close contacts must be treated simultaneously, regardless of symptoms (asymptomatic carriers are common).

Drug	Dose	Notes
Permethrin 5% cream (first-line)	Apply neck-down, 8–14 h, wash off; repeat in 7 days. Crusted: daily × 7 days then twice weekly until cured	Pregnancy category B; most effective topical agent; tolerance emerging
Ivermectin 200 µg/kg oral	Days 1 and 8; crusted: days 1, 2, 8, 9, 15	Highly effective; avoid in <15 kg children, pregnancy, lactation
Crotamiton 10% cream	Apply days 1, 2, 3, and 8	Also antipruritic; marginal efficacy
Precipitated sulfur 5–10%	Apply days 1, 2, and 3	Safe in neonates and pregnancy; limited data
Benzyl benzoate 10%	Apply 24 h	Not available in USA
~~Lindane 1%~~	(avoid)	FDA black-box warning (neurotoxicity, seizures); banned in California; resistance documented

Crusted scabies: Combination of topical permethrin + repeated oral ivermectin is recommended. Add keratolytic agents to facilitate scale removal.

HIV-associated crusted scabies: Treat with combined permethrin + multi-dose ivermectin; watch for bacterial superinfection and septicemia.

Adjunct measures

Oral antihistamines and emollients for pruritus (avoid harsh soaps)
Environmental decontamination: Wash sheets and clothing at ≥60°C (140°F) and tumble-dry on high heat
Items that cannot be washed: seal in a plastic bag for 2 weeks

Post-treatment note

Pruritus may persist for 2–4 weeks after successful eradication due to ongoing hypersensitivity reaction — this does not indicate treatment failure.

Complications

Secondary bacterial infection (impetigo, cellulitis) — especially in children
Post-streptococcal glomerulonephritis (in endemic areas)
Fatal septicemia in immunocompromised hosts with crusted scabies

Sources: Fitzpatrick's Dermatology (8e), Rosen's Emergency Medicine (10e), Dermatology 2-Volume Set (5e) — Goldman-Cecil Medicine (2e)

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