Anthelmintic Drugs

• Nematodes (roundworms) - intestinal (Ascaris, hookworms, pinworms, whipworms, Strongyloides) and tissue/filarial (Wuchereria, Onchocerca, Loa loa)
• Trematodes (flukes) - blood (Schistosoma), liver (Fasciola, Clonorchis), lung (Paragonimus)
• Cestodes (tapeworms) - Taenia solium/saginata, Diphyllobothrium, Echinococcus

I. Drugs for Nematodes (Roundworms)

1. Benzimidazoles - Mebendazole & Albendazole

• Indications: Whipworm (Trichuris), pinworm (Enterobius), hookworms (Necator, Ancylostoma), roundworm (Ascaris)
• Very poorly absorbed orally - acts locally in the gut
• ADRs: Abdominal pain, diarrhea; rare: convulsions in children <1 year; Stevens-Johnson syndrome with concurrent metronidazole
• Avoid in pregnancy (first trimester); may be used in 2nd/3rd trimester in MDA programs

• Broader spectrum - also used for hydatid disease (Echinococcus), cysticercosis, cutaneous larva migrans, visceral larva migrans, strongyloidiasis, trichinosis
• Undergoes rapid first-pass metabolism to the active metabolite albendazole sulfoxide; absorption is improved by a fatty meal
• Half-life ~8-12 hours; the sulfoxide penetrates cysts, CSF, and CNS well
• Preferred over praziquantel for neurocysticercosis (better CNS penetration, cheaper, not reduced by corticosteroids)
• ADRs (short courses): mild - GI upset, headache, dizziness. Long-term (hydatid disease): alopecia, elevated liver enzymes, pancytopenia - monitor LFTs and CBC
• Contraindicated in pregnancy; avoid in cirrhosis

2. Pyrantel Pamoate

• Indications: Pinworms, hookworms, Ascaris
• Mechanism: Depolarizing neuromuscular blocking agent - causes release of acetylcholine and inhibits cholinesterase, resulting in spastic paralysis and expulsion of the worm
• Poorly absorbed - acts locally in the intestine only; not effective against tissue infections
• A second dose is needed for pinworm (does not kill eggs)
• ADRs: Mild - nausea, vomiting, diarrhea

3. Ivermectin

• Indications: Drug of choice for strongyloidiasis (Strongyloides stercoralis), onchocerciasis (river blindness), cutaneous larva migrans. Also used for lymphatic filariasis (in combination), scabies, and head lice
• Mechanism: Binds glutamate-gated chloride channel receptors in invertebrate nerve/muscle cells → enhanced chloride influx → hyperpolarization → flaccid paralysis and death of the worm. Does not cross the human blood-brain barrier at therapeutic doses
• Note: In onchocerciasis, ivermectin kills microfilaria but not adult worms, so it is not curative; repeated dosing is needed
• Mazzotti reaction: Killing microfilaria can trigger fever, headache, dizziness, hypotension - severity correlates with parasite load; treat with antihistamines or steroids
• Avoid in pregnancy

4. Diethylcarbamazine (DEC)

• Indications: Lymphatic filariasis (Wuchereria bancrofti, Brugia spp.), loiasis (Loa loa)
• Used in MDA programs combined with albendazole (with or without ivermectin) for lymphatic filariasis
• Mechanism: Immobilizes microfilaria and enhances their clearance by the host immune system

5. Moxidectin

• A macrocyclic lactone (related to ivermectin) approved for onchocerciasis; generic only in the US
• Longer half-life than ivermectin

II. Drugs for Trematodes (Flukes)

Praziquantel

• Drug of choice for virtually all trematode and most cestode infections
• Indications: All Schistosoma species, liver flukes (Clonorchis, Opisthorchis), lung fluke (Paragonimus), intestinal flukes, tapeworms (Taenia, Diphyllobothrium, Hymenolepis)
• Mechanism: Increases permeability of the parasite cell membrane to calcium → tetanic contracture → paralysis and tegument disruption
• Rapidly absorbed orally; take with food; extensively metabolized; excreted renally
• ADRs: Dizziness, malaise, headache, GI upset (often from dying worms)
• Drug interactions: Strong CYP3A4 inducers (phenytoin, rifampin, carbamazepine) reduce praziquantel levels - co-administration is contraindicated
• Contraindicated in ocular cysticercosis (destruction of organisms in the eye can cause irreversible damage)
• Exception: Fascioliasis does NOT respond well to praziquantel - use triclabendazole

III. Drugs for Cestodes (Tapeworms)

IV. Drug of Choice Quick Reference (from Katzung, Table 53-1)

V. Pregnancy & Special Notes