Scabicides

Scabicides treat scabies, the infestation by Sarcoptes scabiei var. hominis that causes intense itch, burrows and secondary bacterial infection. Treatment requires eradication of mites in the patient and all close contacts at the same time, because itching persists for weeks and mites can reinfest.

Mechanism. Permethrin, a synthetic pyrethroid, keeps sodium channels open in mite nerve cells, causing paralysis and death. Benzyl benzoate and malathion are neurotoxic to mites. Ivermectin binds glutamate-gated chloride channels, causing paralysis. Sulphur acts as a keratolytic and acaricide.

Pharmacokinetics. Topical permethrin has under 2 percent absorption. Oral ivermectin is absorbed better with food (about 2.5-fold) and has a half-life of about 18 hours; it kills mites but has no ovicidal effect, so a second dose is needed.

• Permethrin 5 percent cream: first-line topical.
• Benzyl benzoate 25 percent lotion (12.5 percent in children, 6.25 percent in infants): alternative; irritant.
• Malathion 0.5 percent aqueous lotion: alternative.
• Ivermectin oral: for crusted scabies, outbreaks, institutions, and when topical therapy is not feasible or fails.
• Sulphur 5 to 10 percent in petrolatum: infants and pregnancy; messy and odorous.
• Crotamiton 10 percent: less effective.

Adult regimens.

• Permethrin 5 percent cream: apply to the whole body from neck to soles (and to scalp, face and ears in infants, older adults and immunocompromised patients), including under nails and skin folds; leave for 8 to 12 hours, then wash off. Repeat after 7 days.
• Benzyl benzoate 25 percent: apply to the whole body from the neck down and leave for 24 hours before washing; repeat on the next day or at 7 days.
• Malathion 0.5 percent: apply for 24 hours, repeat after 7 days.
• Ivermectin 200 micrograms/kg orally with food as a single dose, repeated after 7 to 14 days.
• Crusted scabies: ivermectin 200 micrograms/kg on days 1, 2 and 8 (additional doses on days 9, 15, 22 and 29 for severe disease) with daily permethrin and a keratolytic (salicylic acid or urea) for the first week, then twice weekly.
• Children: permethrin from 2 months; benzyl benzoate 12.5 percent for children and 6.25 percent for infants; ivermectin is generally given from 15 kg (5 kg in some regions).

Selection and household measures. Treat all household members and close contacts the same day, even without symptoms. Wash clothing, bedding and towels at 50 to 60 degrees Celsius or seal in a plastic bag for 72 hours. Itch can persist 2 to 4 weeks after cure (post-scabetic itch); manage with emollients, a mild steroid or a sedating antihistamine, and avoid retreatment without evidence of live mites.

Contraindications. Hypersensitivity; ivermectin in pregnancy, breastfeeding (first week) and children under 15 kg unless benefits clearly outweigh risk; ivermectin in patients co-infected with Loa loa at high microfilarial load (risk of encephalopathy in endemic areas).

Effects. Permethrin: burning, itching, rash. Benzyl benzoate: marked irritation. Ivermectin: nausea, dizziness, transient itch; Mazzotti-type reactions in onchocerciasis.

Interactions. Ivermectin with warfarin (rare INR rise); topical agents have none clinically relevant.

Monitoring. Review at 2 weeks; look for new burrows; consider treatment failure if new lesions appear after 4 weeks. Treat secondary impetigo.

Special groups. Pregnancy: permethrin first line; sulphur is an alternative. Breastfeeding: permethrin acceptable. Region: scabies is common in crowded housing, schools, prisons and refugee settings in East Africa; secondary skin infection drives glomerulonephritis and rheumatic heart disease, so treat both the infestation and the infection. Community-wide treatment may be needed in outbreaks.