Scabies

Definition
Scabies is a highly contagious skin infestation caused by the mite Sarcoptes scabiei var. hominis, resulting in intense itching and a characteristic rash due to the body’s allergic reaction to the mite, its eggs, and faeces.

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Cause

• Infestation by Sarcoptes scabiei mite

• Spread through prolonged skin-to-skin contact or sharing of contaminated clothing, bedding, or towels

• Mites burrow into the stratum corneum, laying eggs that hatch and perpetuate the cycle

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Risk Factors

• Crowded living conditions (nursing homes, prisons, refugee camps)

• Close contact with an infected person

• Immunocompromised individuals (higher risk for crusted scabies)

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Pathophysiology

• Mite burrows trigger a delayed type IV hypersensitivity reaction

• Itching may take 2–6 weeks to develop after first exposure but appears within 1–4 days on re-exposure

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Clinical Features

• Itching: intense, worse at night

• Rash: small red papules, vesicles, and linear burrows (thin, wavy, grey-white lines)

• Common sites: finger webs, wrists, elbows, armpits, waist, buttocks, genitalia, breasts (in women), scalp and face in infants/elderly

• Crusted scabies: thick crusts with high mite load, minimal itching (seen in immunocompromised)

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Diagnosis

• Clinical history and examination

• Dermatoscopy: visualisation of mite (“delta wing jet” sign)

• Skin scraping microscopy to confirm mite, eggs, or faecal pellets

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Treatment

First-Line

• Permethrin 5% cream:

  • Apply to the whole body from the neck down (include under nails, between toes/fingers); in infants/elderly, apply to scalp, face, and ears (avoid eyes/mouth)

  • Leave on for 8–12 hours, then wash off

  • Repeat after 7 days

• Dose: Quantity varies — adult usually needs 30–60 g per application

Alternative (if permethrin unsuitable)

• Oral ivermectin (200 micrograms/kg as a single dose, repeated after 7–14 days) — useful in crusted scabies or outbreaks

Supportive Measures

• Treat all close contacts simultaneously (even if asymptomatic)

• Wash clothing, bedding, and towels used in the last 3 days at ≥60°C; seal non-washable items in a plastic bag for ≥72 hours

• Antihistamines or mild topical steroids for itch (after starting treatment)

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Complications

• Secondary bacterial infection (impetigo, cellulitis)

• Eczema-like dermatitis

• Post-scabetic itch (can persist for weeks after successful treatment)

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Quick-Reference Clinical Chart — Scabies

Feature	Details
Definition	Skin infestation by Sarcoptes scabiei mite
Transmission	Prolonged skin contact, shared clothing/bedding
Incubation	2–6 weeks (primary); 1–4 days (re-exposure)
Key Symptoms	Severe nocturnal itch, papules, vesicles, burrows
Common Sites	Finger webs, wrists, axillae, waist, buttocks, genitals, breasts; scalp/face in infants/elderly
Diagnosis	Clinical + dermatoscopy or microscopy
First-Line Treatment	Permethrin 5% cream, whole body, repeat after 7 days
Alternative	Oral ivermectin (200 mcg/kg, repeat after 7–14 days)
Prevention	Treat contacts, wash/decontaminate fabrics
Complications	Secondary bacterial infection, post-scabetic itch
Prognosis	Excellent with correct treatment and contact management