Write a short note on scabies.

Definition & Aetiology

Scabies is a contagious skin infestation caused by the mite Sarcoptes scabiei var. hominis. The fertilised female mite burrows into the stratum corneum, laying 2–3 eggs daily for about 4–6 weeks. An affected person typically carries only 10–15 adult mites, which is why lesions are sparse despite intense symptoms.

Transmission is by prolonged skin-to-skin contact — within families, in crowded housing, hostels and institutions, and through sexual contact in adults. Fomite transmission through clothing and bedding is possible but much less important, except in crusted scabies. It is common in India wherever overcrowding and limited water supply coexist.

Pathogenesis of Itch

The itch is a delayed type IV hypersensitivity reaction to mite proteins, saliva and faeces, not to the mite's movement. This explains two clinically important facts: symptoms appear only 4–6 weeks after first infestation (but within 1–2 days on re-infestation, when the patient is already sensitised), and itch persists for 2–4 weeks after successful treatment, which must be explained to the patient or they will assume treatment failure.

Clinical Features

• Intense itching, characteristically worse at night and in the warmth of bed — the cardinal symptom.
• Burrows — short, wavy, grey-white threadlike lines with a vesicle at one end; the pathognomonic lesion, best sought in finger webs and wrist flexures.
• Distribution: finger web spaces, flexor wrists, elbows, axillary folds, periumbilical skin, waistband area, buttocks, genitalia (scrotal and penile nodules are highly characteristic in men), and breasts and areolae in women.
• The head, face and scalp are spared in adults but are involved in infants and the elderly, along with palms and soles — an important exception.
• Itching in multiple family members is a strong diagnostic pointer; always ask.
• Secondary lesions: excoriations, eczematisation, and secondary bacterial infection with impetiginisation, which in India may lead to post-streptococcal glomerulonephritis.

Variants

• Crusted (Norwegian) scabies: occurs in the immunosuppressed — HIV, corticosteroid therapy, leprosy, malnutrition, learning disability — with thick hyperkeratotic crusts harbouring thousands to millions of mites. Itching may be minimal or absent, and it is extremely contagious, causing institutional outbreaks.
• Nodular scabies: persistent itchy nodules on the genitalia and axillae, representing a hypersensitivity response; persists after mites are eradicated.
• Scabies incognito: modified by topical corticosteroids, with atypical distribution and loss of classical features.

Diagnosis

Largely clinical. Confirmatory methods: skin scraping from a burrow examined in potassium hydroxide for mites, eggs or faecal pellets; dermoscopy showing the "delta wing jet" or "hang glider" sign (the mite's pigmented head with the burrow trailing); and the burrow ink test.

Management

General measures — as important as the drug:

• Treat all household and close contacts simultaneously, whether symptomatic or not. Failure to do this is the commonest reason for recurrence.
• Apply topical treatment to the whole body from the neck down in adults (including the scalp and face in infants and the elderly), paying attention to web spaces, umbilicus, genitalia and under the nails; leave overnight and wash off.
• Wash clothing, bedding and towels in hot water and dry in sunlight; items that cannot be washed should be sealed in a bag for a week, since mites die within 2–3 days away from the host.

Drugs:

• Permethrin 5% cream — first-line; applied overnight and repeated after one week.
• Oral ivermectin — useful for institutional outbreaks, crusted scabies and treatment failure; repeated after one week because it is not ovicidal.
• Benzyl benzoate and sulphur ointment — cheaper alternatives; sulphur is safe in infants and pregnancy, where permethrin choices are restricted.
• Symptomatic: oral antihistamines and topical corticosteroids for post-scabetic itch; antibiotics for secondary bacterial infection.
• Crusted scabies: combined oral ivermectin (multiple doses) with topical scabicide and a keratolytic to remove crusts; isolate the patient.