Ectoparasiticides and Special Populations

Crusted (Norwegian) scabies occurs in immunocompromised patients and harbors thousands to millions of mites (vs. ~15 in ordinary scabies). Topical permethrin alone fails because the thick hyperkeratotic crust prevents drug penetration. Standard approach: oral ivermectin (200 μg/kg on days 1, 2, 8, 9, 15) combined with daily or twice-weekly topical 5% permethrin cream, plus keratolytics (5–10% salicylic acid) to reduce crust thickness and improve drug penetration. All close contacts and household members must be treated simultaneously, and bedding and clothing must be laundered. Post-treatment itch for 2–4 weeks is a normal hypersensitivity response to dead mite antigens and should not prompt retreatment without confirmed live infestation.

Six interaction pairs to know by class. Rifampin + praziquantel: potent CYP3A4 induction reduces praziquantel AUC by ~85% — therapeutically ineffective; never co-administer. Metronidazole + warfarin: CYP2C9 inhibition increases warfarin anticoagulant effect — monitor INR at 3–5 days and reduce warfarin dose. Metronidazole + alcohol: inhibits aldehyde dehydrogenase → acetaldehyde accumulation → disulfiram-like reaction; avoid alcohol for 48 hours after last dose. Ivermectin + P-glycoprotein inhibitors (ritonavir, verapamil, quinidine): increase CNS penetration — encephalopathy risk. Pyrimethamine + other myelosuppressants: additive bone marrow suppression — always co-prescribe folinic acid, not folic acid. Artemether-lumefantrine and other ACT combinations containing lumefantrine or piperaquine: prolong QTc — avoid concurrent QTc-prolonging drugs (fluoroquinolones, macrolides, antipsychotics).

Three organizing principles unite the antiparasitic chapter. First, selectivity follows biochemical difference: most antiparasitic drugs exploit a biochemical target absent or distinct in the parasite — ferredoxin (metronidazole), GluCl channels (ivermectin), trypanothione (nifurtimox, antimonials), helminth β-tubulin (benzimidazoles), ergosterol analogs (polyenes vs. Leishmania). Second, stage matters: some drugs kill only blood-stage parasites while others reach liver or tissue stages; treatment failures often trace to a stage mismatch. Third, pregnancy always requires treatment of malaria — no antimalarial drug is as dangerous as untreated malaria in a pregnant patient, and the decision to withhold must never be made on grounds of fetal drug concern alone.