Antimalarial, Antiamoebic & Anthelmintic Drugs Guide

Antimalarials kill Plasmodium parasites in the blood and liver, antiamoebic drugs destroy Entamoeba histolytica in the gut wall and liver, and anthelmintics paralyse or starve parasitic worms so the body expels them. Parasites are eukaryotes like us, so these drugs must exploit the few biological differences available: the malaria parasite's digestion of haemoglobin, the amoeba's anaerobic metabolism, and the worm's nerve and muscle channels.

This matters enormously in India, where malaria still hospitalises lakhs each year, amoebic dysentery spreads through contaminated water, and intestinal worms stunt the growth of millions of children.

Key Concepts

• Plasmodium species: falciparum (deadly, drug-resistant), vivax and ovale (relapse from liver hypnozoites), malariae (mild).
• Chloroquine: concentrates in the parasite's food vacuole and blocks haem detoxification; still works for sensitive vivax malaria but falciparum is widely resistant.
• Artemisinin combination therapy (ACT): artemether plus lumefantrine; the artemisinin kills parasites within hours and the partner drug mops up survivors over days; first-line for falciparum malaria.
• Primaquine: the only drug killing liver hypnozoites (radical cure of vivax and ovale); mandatory G6PD testing first because it causes haemolysis in G6PD-deficient patients.
• Metronidazole: kills anaerobic protozoa including Entamoeba in tissues and the liver; it does not clear gut lumen cysts.
• Luminal agents: paromomycin or diloxanide furoate, given after metronidazole to wipe out intestinal cysts and prevent relapse.
• Albendazole and mebendazole: block worm microtubules, starving roundworms, hookworms and whipworms.
• Ivermectin: opens worm chloride channels, paralysing Strongyloides and scabies mites; praziquantel is the drug for schistosomiasis and tapeworms.

Formulas

• Chloroquine total dose: 25 mg base per kg over 3 days: 10 mg/kg on day 1, 10 mg/kg on day 2, 5 mg/kg on day 3.
• Primaquine radical cure: 0.25 to 0.5 mg per kg per day for 14 days (after G6PD testing).
• Ivermectin: 200 micrograms per kg as a single dose.

Worked Example

A 60 kg adult has chloroquine-sensitive vivax malaria. Step 1: total chloroquine base needed = 25 mg/kg times 60 kg = 1,500 mg. Step 2: day 1 dose = 10 mg/kg times 60 = 600 mg; day 2 = another 600 mg; day 3 = 5 mg/kg times 60 = 300 mg. Step 3: 600 plus 600 plus 300 = 1,500 mg, matching the target. Step 4: after the blood parasites clear, check G6PD; if normal, give primaquine 0.25 mg/kg/day times 60 kg = 15 mg daily for 14 days to kill liver hypnozoites and prevent relapse months later. Skipping step 4 is the classic error: the patient feels cured, then relapses from the liver reservoir.

Common Misconceptions

• "Every fever in the rainy season is malaria, so take antimalarials." Dengue, typhoid and viral fevers mimic malaria; treatment must follow a blood smear or rapid test, because blind chloroquine use drives resistance.
• "One deworming tablet protects forever." Reinfection from soil is constant where sanitation is poor, so mass deworming repeats every 6 to 12 months.
• "Metronidazole alone fully cures amoebiasis." It kills tissue invaders but leaves gut cysts untouched; without a luminal agent like paromomycin, the patient keeps shedding cysts and relapses.

Real-World Applications

• National malaria treatment guidelines: ACT for falciparum, chloroquine plus primaquine for vivax.
• School-based mass deworming with single-dose albendazole 400 mg.
• Amoebic liver abscess: metronidazole followed by a luminal agent, with drainage only for large or threatening abscesses.
• Scabies outbreaks: ivermectin plus topical permethrin for cases and contacts.

Antimalarials clear blood parasites and, with primaquine, the liver reservoir; metronidazole plus a luminal agent fully cures amoebiasis; and weight-based anthelmintics expel worms. Doses are calculated per kilogram, G6PD must be checked before primaquine, and diagnosis must precede treatment. These rules turn ancient parasitic scourges into routinely curable conditions.