Anthelmintic Drugs

Anthelmintic Drugs (including Ivermectin)

1. Definition, terminology & the helminth problem

• Anthelmintics are drugs that either kill (vermicide/vermicidal) or expel (vermifuge) the worms infesting the host; they are also called deworming agents (KDT 8e Ch.63, p.906; Padmaja 7e Ch.55, p.651).
• Helminthiasis is globally prevalent — roughly one-third of the world's population harbours worms — and is far more common in developing countries with poorer personal and environmental hygiene; multiple (mixed) infestations in one individual are not infrequent (KDT 8e Ch.63, p.906; Katzung Ch.53, p.990).
• More than 1 billion people are infected with intestinal nematodes, and many millions with filarial nematodes, flukes and tapeworms; through WHO mass drug administration (MDA), ≈1.24 billion people received ≥1 anthelmintic in 2019 (Katzung Ch.53, p.990; G&G 14e Ch.68, p.1325).
• Worms harm the host by depriving him of food, causing blood loss, organ injury, intestinal/lymphatic obstruction and by secreting toxins; helminthiasis is rarely fatal but a major cause of ill health/morbidity (KDT 8e Ch.63, p.906).
• Classification of helminths by morphology and habitat (G&G 14e Ch.68, p.1325):
  • Platyhelminths (flatworms) —
  • Flukes (trematodes): lung flukes (Paragonimus spp.), liver flukes (Fasciola spp., Clonorchis sinensis, Opisthorchis spp.), intestinal flukes, blood flukes (Schistosoma spp.).
  • Tapeworms (cestodes): Taenia solium, T. saginata, Diphyllobothrium latum, Hymenolepis nana, Echinococcus spp.
  • Nematodes (roundworms) —
  • Soil-transmitted helminths (STHs): Ascaris lumbricoides, Trichuris trichiura, Necator americanus, Ancylostoma duodenale, Strongyloides stercoralis.
  • Filarial worms: lymphatic filariasis (Wuchereria bancrofti, Brugia malayi, B. timori); subcutaneous filariasis (Loa loa, Mansonella streptocerca, Onchocerca volvulus); serous-cavity filariasis (Mansonella perstans, M. ozzardi).
• Sexual biology: blood flukes and nematodes are bisexual, whereas all other flukes and the tapeworms infecting humans are hermaphroditic; all helminths pass through egg → larval (juvenile) → adult stages, but life cycles differ greatly (G&G 14e Ch.68, p.1325).
• Principles of drug choice — efficacy is only one factor; selection also weighs lack of side effects/toxicity, ease of administration (single dose preferred), and low cost; for most worms a parasite should be identified before treatment (KDT 8e Ch.63, p.906; Katzung Ch.53, p.990).
• In clinical use of anthelmintics, resistance has historically not been a problem in humans, in contrast to its emergence in veterinary/farming settings (KDT 8e Ch.63, p.906).