Scabies

Overview

• Fitzpatrick's Dermatology, p. 3303

The Mite: Biology and Life Cycle

• S. scabiei belongs to family Sarcoptidae, class Arachnida
• The adult female is 0.4 × 0.3 mm - pearl-like, translucent, white, eyeless, oval, with 4 pairs of short legs (slightly too small to see with the naked eye)
• The female excavates a sloping burrow at 0.5-5 mm/day in the stratum corneum, reaching the stratum granulosum boundary
• She lays 0-4 eggs/day, up to 50 eggs over a 30-day lifespan
• Eggs hatch in 10-12 days; larvae mature on the skin surface
• A mite can survive 3 days off-host (7 days in mineral oil)
• Mites cannot fly or jump
• A typical host harbors 3-50 mites (immunocompetent); crusted scabies patients harbor millions
• Fitzpatrick's Dermatology, p. 3304

Transmission

• Close personal contact is the primary route (including non-sexual contact among children and family members)
• Fomites are a secondary route, especially significant in crusted scabies
• In crusted scabies: 6,000 mites/g of debris have been found on sheets, floors, curtains, and nearby furniture - even passersby in the vicinity are at risk

Clinical Presentation

Incubation and Symptoms

• First infestation: intense pruritus begins 4-6 weeks after exposure (up to 3 months in some; some are never sensitized)
• Re-infestation: symptoms develop within 2-3 days
• Pruritus is characteristically nocturnal and worst at night
• Some individuals remain asymptomatic carriers despite infestation

Classic Distribution (adults)

• Interdigital webs of fingers
• Sides of fingers, volar wrists, lateral palms
• Elbows, axillae
• Scrotum, penis (pruritic nodules here are highly suggestive)
• Labia, areolae in women
• Umbilicus, buttocks, subungual skin
• Head and neck are spared in healthy adults

Infants and Young Children

• Face, scalp, palms, and soles may be involved
• Lesions are often papules and vesicopustules
• Indurated, crusted nodules in intertriginous areas and trunk

Primary Lesion - The Burrow

• A burrow (2-5 mm gray-white, serpiginous line) is pathognomonic of scabies
• Best site to find mites on microscopy
• Also seen: papules, pustules, vesicles, and excoriations

Crusted (Norwegian) Scabies

• Thick, hyperkeratotic, grayish-white plaques with granular, sand-like scales
• Distributed over palms, soles, elbows, knees; may be generalized
• Nail thickening and dystrophy
• Surprisingly non-pruritic despite enormous mite burden (thousands to millions of mites)
• Head and neck are commonly involved (unlike classic scabies)
• Highly contagious - can cause major outbreaks in healthcare facilities
• Risk of bacterial superinfection and fatal septicemia in immunocompromised hosts

• Fitzpatrick's Dermatology, p. 3304; Dermatology 5e, p. 1650

Diagnosis

1. Characteristic history (nocturnal pruritus, household contacts affected)
2. Typical distribution of lesions
3. Epidemiologic exposure

Confirmatory Tests

Differential Diagnosis

• Atopic dermatitis
• Contact dermatitis
• Pityriasis rosea (symmetric maculopapular rash)
• Papular urticaria
• Secondary syphilis
• Folliculitis
• Insect bite reactions
• Varicella / miliaria

• Dermatitis herpetiformis
• Lichen planus (violaceous polygonal papules)
• Bullous pemphigoid
• Psoriasis
• Drug eruption
• Delusions of parasitosis

Treatment

First-Line: Permethrin 5% Cream

• Apply from neck down, covering all areas: under nails, umbilicus, around nipples, genitals
• Include face and scalp in infants, young children, and immunocompromised
• Leave on 8-14 hours (usually overnight), then wash off
• Repeat in 1-2 weeks to increase cure rate
• Pregnancy category B
• Note: permethrin resistance is increasing

Oral Ivermectin

• 200 µg/kg orally on days 1 and 8
• For crusted scabies: days 1, 2, 8, 9, and 15
• Not recommended for children <15 kg, pregnant or lactating women
• Highly effective with good safety profile

Full Treatment Table (Fitzpatrick's)

• Fitzpatrick's Dermatology, p. 3306

Symptomatic Relief

• Oral antihistamines and mid-potency topical corticosteroids for itch
• Emollients (avoid harsh soaps)
• Important: pruritus may persist weeks to months after successful treatment - the dead mites and eggs retain antigenic properties

Environmental Decontamination (Mandatory)

• Machine-wash clothing, bed linens, towels in hot water (60°C/140°F); dry in hot dryer
• Items that cannot be washed: dry clean, heat in hot dryer ×15 min, or seal in plastic bag for 1 month (2 weeks per Fitzpatrick's)
• Treat ALL household members and close contacts simultaneously - failure to do this causes recurrences

Special Populations

Complications

• Bacterial superinfection (impetiginization from scratching) - most common
• Septicemia - in immunocompromised hosts
• Post-streptococcal glomerulonephritis - from secondary streptococcal infection
• Institutional outbreaks - especially in nursing homes (crusted scabies)