Crusted Scabies and Scabies Therapy
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Direct answer
Crusted scabies, also called Norwegian scabies, is a hyperinfestation of the skin with thick, hyperkeratotic crusts teeming with very large numbers of Sarcoptes mites. It occurs mainly in immunosuppressed people, including those with HIV, lymphoma or on corticosteroids, and in people with neurological disease. Because the patient itches little yet is highly contagious, management combines systemic ivermectin with topical scabicides and strict treatment of contacts.
What you must remember
- Hyperinfestation: crusted scabies carries very large mite numbers, so the patient is far more infectious than the mild itch suggests.
- Risk groups: HIV infection, corticosteroid therapy, lymphoma, organ transplantation and neurological disease such as paraplegia predispose to the crusted form.
- Appearance: thick, grey, hyperkeratotic crusts over the palms, soles, scalp and nails, often with surprisingly little itch.
- Systemic backbone: oral ivermectin 200 micrograms/kg given as several doses (for example days 1, 2 and 8, with more in severe cases) is essential because crusts shield mites from topical agents.
- Topical standard: permethrin 5% cream applied from the neck downwards, left for 8-14 hours and repeated after a week; benzyl benzoate is a practical alternative.
- Special groups: permethrin is usually the choice in pregnancy and infancy, while ivermectin is avoided in pregnancy and in children under 15 kg.
- Lindane: avoided because of neurotoxicity, especially in infants and in patients with seizures or broken skin.
A treatment pathway for crusted disease
A 58-year-old man on long-term prednisolone presents with thick crusts over both palms and a family history of mild itch. The approach balances confirmation, isolation and combined therapy, with each step covering one element of the plan.
- Confirm and isolate: Skin scraping from the crusts shows heavy mite burden, and the patient is isolated from vulnerable contacts until treatment is under way.
- Soften the crusts: A keratolytic such as salicylic acid preparation is applied to thin the crust so that scabicides reach the mites beneath. This step is usually completed before the topical scabicide is started.
- Give systemic ivermectin: Oral ivermectin 200 micrograms/kg is given as several doses (for example days 1, 2 and 8, with extra doses in severe disease), reaching mites that topical agents cannot. The exact schedule follows specialist advice.
- Apply topical permethrin: Permethrin 5% is applied from the neck downwards and left for 8-14 hours, with the regimen repeated after a week. The entire body, including the scalp and the nail folds, is covered.
- Treat contacts and the environment: Household and close contacts receive treatment at the same time, and clothing and bedding are laundered in hot water and dried on a hot cycle. Mites survive only briefly away from human skin, so this removes the reservoir.
- Review and reassess: Clinical review and repeat scrapings check whether the mite burden is falling, with retreatment considered if new burrows or itch appear.
Where the exam tries to catch you
Crusted scabies questions test recognition of a hyperinfested patient and the logic of combination therapy.
- Trap: treating crusted scabies with topical permethrin alone. Correct: crusts protect the mites from topical agents, so oral ivermectin is added as standard systemic therapy.
- Trap: reassuring the family because the patient itches little. Correct: hyperinfestation is highly contagious, so isolation and contact treatment remain mandatory despite mild symptoms.
- Trap: giving ivermectin to a pregnant woman or a small child. Correct: permethrin is preferred in pregnancy and infancy, while ivermectin is reserved for eligible patients who meet the weight and pregnancy criteria.
- Trap: using lindane as a routine first choice. Correct: its neurotoxicity makes it avoidable, so permethrin or benzyl benzoate is preferred.
Frequently asked questions
What distinguishes crusted scabies from classic scabies?
Crusted scabies shows thick, hyperkeratotic crusts teeming with very large numbers of mites, while classic scabies has few mites and an itchy papular rash. Crusted scabies is far more contagious and typically affects immunosuppressed patients, so it needs combined systemic and topical therapy.
Why is oral ivermectin added in crusted scabies?
Oral ivermectin is added in crusted scabies because thick crusts shield mites from topical scabicides, which cannot reach all of them. Ivermectin acts systemically after oral dosing, so it complements topical permethrin. Several doses are needed in crusted disease, for example on days 1, 2 and 8.
Which scabicide is preferred in pregnancy?
Permethrin 5% cream is the usual preferred scabicide in pregnancy and infancy. Ivermectin is avoided in pregnant women, and sulphur preparations are an older alternative that can be smelly and stain clothing, so the choice should follow current local guidance.
Why must clothing and bedding be managed in household scabies?
Scabies mites survive only briefly away from human skin, so laundering clothes and bedding in hot water and drying them on a hot cycle removes the mites. Items that cannot be washed are sealed away for several days.
Why is lindane avoided in scabies therapy?
Lindane is avoided in scabies because it causes neurotoxicity, particularly in infants, the elderly and patients with seizures or broken skin. Permethrin or benzyl benzoate is preferred as the topical scabicide in its place.