Scabies: Transmission, Symptoms, Diagnosis & Treatment

Scabies is an infestation by the mite Sarcoptes scabiei var. hominis. It causes intense pruritus via delayed-type hypersensitivity to mite proteins/feces.

Transmission

• Prolonged skin-to-skin contact (10–15 min) – sexual, household, caregiving.
• Fomites (bedding, clothing) – minor role; mites survive 24–36 hrs off host.
• Crusted (Norwegian) scabies: highly contagious; thousands of mites; brief contact suffices.

Clinical Features

• Intense itch: Worse at night (circadian cortisol drop, increased skin temp).
• Burrows: Thin, grayish, serpiginous lines (3–10 mm) – pathognomonic. Sites: web spaces, wrists, elbows, axillae, beltline, genitalia, buttocks.
• Papules/nodules: Hypersensitivity reaction; persist after cure.
• Distribution: Spares face/scalp in adults (except infants/immunocompromised).

Diagnosis

• Clinical: History + burrows + distribution + contact history.
• Microscopy (gold standard): Skin scraping (mineral oil) → mites, eggs, scybala (feces).
• Dermoscopy: "Delta wing" (mite), "jet contrail" (burrow).
• Adhesive tape test: Applied to burrow → peel → microscopy.

Treatment

First-Line (All Household Contacts Simultaneously)

• Permethrin 5% cream: Neck to toes, 8–14 hrs, repeat day 7. Safe >2 mo, pregnancy.
• Ivermectin 200 mcg/kg PO: Single dose, repeat day 7–14. Preferred for crusted, institutional outbreaks, compliance issues. Not for <15 kg or pregnancy.

Adjunctive

• Antihistamines (sedating at night) for pruritus.
• Topical steroids for post-scabetic eczema.
• Antibiotics if secondary impetigo.

Environmental Decontamination

• Wash bedding/clothes in hot water (≥60°C) + high-heat dry.
• Seal non-washables in plastic bag × 72 hrs.
• Vacuum furniture/carpets.

Crusted Scabies: Special Protocol

• Ivermectin days 1, 2, 8, 9, 15 (or daily × 7) + permethrin daily × 7 then 2×/wk.
• Keratolytics (salicylic acid) to reduce scale.
• Isolation (contact precautions) until cured.

Follow-Up

• Itch may persist 2–4 weeks post-treatment (post-scabetic hypersensitivity).
• Retreat if new burrows or persistent lesions at 2–4 weeks.
• Screen/treat all contacts regardless of symptoms.