Scabies

Scabies is a pruritic infestation by the mite Sarcoptes scabiei var. hominis. Transmission is primarily via prolonged skin-to-skin contact; fomites can spread crusted scabies. Classic scabies presents with nocturnal itch and burrows/papules in characteristic locations. Crusted (Norwegian) scabies occurs in immunocompromised or neurologically impaired patients with hyperkeratotic plaques and heavy mite burden. Diagnosis is clinical, supported by dermoscopy or microscopy. Treatment includes topical permethrin 5% or oral ivermectin, decontamination of clothing/bedding, and simultaneous treatment of close contacts. Post-scabietic itch may persist despite cure.

Epidemiology and Risk Factors

• Affects all ages; outbreaks in households, long-term care facilities, prisons, refugee settings.
• Risk factors: close living quarters, sexual contact, immunosuppression, neurological conditions (reduced scratching), institutional environments.

Clinical Features

• Intense pruritus, worse at night.
• Lesions: erythematous papules, excoriations, nodules; pathognomonic burrows (thin, wavy, grayish lines).
• Distribution (classic): finger webs, flexor wrists, anterior axillary folds, areolae, umbilicus, waistline, buttocks, penis/scrotum; infants/elderly may have scalp, face, palms, soles involvement.
• Crusted scabies: thick hyperkeratotic plaques, widespread scale/crust, minimal itch; highly contagious.

Differential Diagnosis

• Atopic/contact dermatitis, arthropod bites, prurigo nodularis, folliculitis, dermatitis herpetiformis, papular urticaria, scabies surrepticius (steroid-modified).

Diagnosis

• Clinical; confirm with:
  • Dermoscopy: “delta wing” sign (triangular mite) with trailing burrow.
  • Skin scraping from burrow for light microscopy to identify mites/eggs/scybala.
  • Adhesive tape test or burrow ink test.
• In crusted scabies, mite detection is easier due to high burden.

Management

1. First-line therapies

• Permethrin 5% cream:
  • Apply from neck down (include scalp in infants/elderly/crusted cases), under nails, and genital folds; leave 8–12 hours then wash off.
  • Repeat in 7–14 days to kill newly hatched mites.
• Oral ivermectin:
  • 200 µg/kg single dose, repeat in 7–14 days; take with food to enhance absorption.
  • Useful for outbreaks, compliance issues, or crusted scabies.
  • Not recommended in pregnancy and children <15 kg in many guidelines; weigh risks/benefits.

2. Crusted scabies (severe)

• Combined regimen: ivermectin (e.g., days 1, 2, 8, 9, and 15; add days 22, 29 if severe) plus daily permethrin 5% to entire body (including scalp) until crusts resolve.
• Keratolytics (e.g., salicylic acid, urea) to reduce scale and improve penetration.
• Isolation/contact precautions and environmental decontamination; screen and treat contacts aggressively.

3. Contacts and environmental control

• Treat all household/close contacts simultaneously, regardless of symptoms.
• Wash clothing, bedding, towels used in the prior 3 days in hot water and dry on high heat; items that cannot be laundered: seal in plastic bags for ≥72 hours (mites survive only 2–3 days off host).
• Vacuum upholstered furniture and mattresses.

4. Symptom control and post-scabietic itch

• Antihistamines for nocturnal itch; medium-potency topical steroids for eczematous dermatitis; emollients.
• Post-scabietic pruritus may last 2–6 weeks despite eradication; reassure patients.
• Persistent papules/nodules (especially scrotal nodules) can be treated with topical/intralesional steroids or calcineurin inhibitors.

5. Special populations

• Pregnancy: permethrin 5% is preferred and considered safe; avoid ivermectin.
• Infants/young children: permethrin safe from 2 months; treat scalp/face avoiding eyes/mouth.
• Elderly and immunosuppressed: high suspicion for crusted scabies; consider oral ivermectin combinations.
• Institutional outbreaks: coordinate mass treatment, staff education, and environmental measures.

Treatment failure considerations

• Inadequate application (missed areas), not treating contacts, reinfestation, or resistance (rare; consider switching class or combination therapy).

References (recent guidelines and key reviews)

• WHO and dermatology society scabies control guidelines, 2022–2024.
• Trials and reviews comparing permethrin vs ivermectin and combination regimens for crusted scabies, 2021–2024.
• Outbreak management in long-term care facilities, 2022–2024.