Ivermectin

Ivermectin is an oral macrocyclic lactone with broad activity against onchocerciasis, strongyloidiasis, lymphatic filariasis, scabies, head lice and several other parasites. It is a core drug in mass administration programmes. It must not be given in areas where loiasis is endemic without testing, because heavy Loa loa infection can cause serious brain illness.

Class. Antiparasitic; macrocyclic lactone.

Mechanism. Binds glutamate-gated chloride channels in invertebrate nerve and muscle cells, causing paralysis and death of the parasite. It has little effect in humans because these channels are absent.

Pharmacokinetics. Oral absorption increased by food. Half life about 18 hours. Metabolised by CYP3A4; excreted in faeces. It is a P-glycoprotein substrate.

Strongyloidiasis. 200 micrograms/kg orally once daily for 2 days, repeated after 2 weeks; hyperinfection or disseminated disease: daily until stool and sputum are negative for 2 weeks, with albendazole.

Onchocerciasis. 150 micrograms/kg as a single dose every 6 or 12 months for 10 to 15 years.

Scabies. 200 micrograms/kg orally, repeated after 7 days; crusted scabies: 200 micrograms/kg on days 1, 2, 8, 9 and 15 (and days 22 and 29 if severe), with a topical keratolytic and scabicide.

Lymphatic filariasis (mass drug administration). 200 micrograms/kg once yearly with albendazole 400 mg.

Head lice. 200 micrograms/kg, repeated after 7 days.

Loading dose, maintenance dose and titration. Not applicable. Maximum: 200 micrograms/kg in a single dose (about 12 mg in a 60 kg adult).

Main alternatives. Albendazole (strongyloides), permethrin (scabies), diethylcarbamazine (filariasis).

Contraindications. Hypersensitivity; high Loa loa microfilaraemia (over 20,000 microfilariae/mL) because of risk of encephalopathy; children under 15 kg (limited data).

Serious adverse effects. Encephalopathy in Loa loa co-infection, severe Mazzotti-type reactions in onchocerciasis (fever, itching, hypotension, eye inflammation), and hepatotoxicity (rare).

Common adverse effects. Itching, rash, fever, headache, arthralgia, dizziness and swollen lymph nodes (from killed parasites).

Interactions. Warfarin effect may rise; P-glycoprotein inhibitors (ciclosporin, verapamil) raise levels; no important interactions with antiretrovirals.

Common prescribing errors. Giving in loiasis-endemic areas without testing, a single dose in hyperinfection, and treating only the patient and not close contacts in scabies.

Special groups. - Pregnancy: avoid unless benefit is clear. - Breastfeeding: compatible after the first week. - Children: from 15 kg (5 years and over in mass programmes). - Older adults: usual dose. - Kidney impairment: no adjustment. - Liver impairment: caution in severe disease.

Regional points. Loiasis is endemic in parts of Central Africa and its eastern fringe, so test-and-treat is used before mass ivermectin there; onchocerciasis and strongyloidiasis are common in parts of Uganda and Tanzania, and scabies is common among crowded and displaced populations in East Africa.

Formulations. Tablets 3 mg and 6 mg; topical cream (not covered here).

Administration. Single dose on an empty stomach with water, at least 2 hours before a meal; take with food in scabies treatment if advised.

Baseline. Species identification, Loa loa microfilarial test where endemic, pregnancy status, HIV status in strongyloidiasis.

Monitoring. Symptoms for 3 days after the first dose in onchocerciasis; stool or sputum clearance in strongyloidiasis; itch and lesions at 2 to 4 weeks in scabies.

Stopping thresholds. Stop for encephalopathy, severe hypotension or severe skin reaction.

Referral and escalation. Admission for confusion, drowsiness or collapse after treatment in endemic areas; specialist review for hyperinfection syndrome.