Anthelmintics
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Direct answer
Anthelmintics exploit biochemical targets that mammalian hosts lack. Albendazole and mebendazole bind nematode beta-tubulin and starve worms of glucose; ivermectin opens invertebrate glutamate-gated chloride channels causing flaccid paralysis; praziquantel floods schistosomes and tapeworms with calcium; diethylcarbamazine clears microfilariae. Matching the worm to the drug — ivermectin for strongyloidiasis and onchocerciasis, praziquantel for schistosomiasis, albendazole for hydatid disease and neurocysticercosis — is the essence of this topic.
What you must remember
- Albendazole and mebendazole, benzimidazoles, block microtubule-mediated glucose uptake; a single four hundred milligram albendazole dose clears Ascaris, hookworm and pinworm, while albendazole's absorption (improved by fatty food) suits tissue disease — hydatid cyst months-long therapy and neurocysticercosis with corticosteroid cover.
- Benzimidazole toxicity is low — abdominal pain, transient transaminase rise — but they are avoided in the first trimester of pregnancy; albendazole is the preferred tissue-active agent.
- Ivermectin, an avermectin, is drug of choice for strongyloidiasis, onchocerciasis (microfilaricidal) and crusted scabies, and pairs with albendazole in lymphatic filariasis mass drug administration; killing microfilariae provokes the Mazzotti reaction of itching, fever and hypotension.
- Praziquantel raises tegumental calcium permeability, causing contraction and vacuolisation — drug of choice for all schistosomes, taeniasis, diphyllobothriasis and neurocysticercosis (with steroids); contraindicated in ocular cysticercosis where inflammatory destruction of the eye can blind.
- Diethylcarbamazine is the microfilaricidal drug for Wuchereria bancrofti and Brugia malayi, for Loa loa and tropical pulmonary eosinophilia; contraindicated in onchocerciasis where ivermectin is used, and capable of encephalopathy with heavy Loa loa microfilaraemia; DEC-fortified salt was a historic elimination strategy.
- Niclosamide, unabsorbed and safe, kills adult tapeworms in the lumen but not larval cysts; pyrantel pamoate single-dose therapy for pinworm and Ascaris agonises nematode nicotinic receptors; triclabendazole is the fascioliasis drug of choice.
- Avoid in pregnancy: albendazole, mebendazole, ivermectin and praziquantel where alternatives exist; pyrantel carries the best pregnancy profile among the narrow options.
Common confusion
The filarial drugs are forever swapped: diethylcarbamazine treats lymphatic filariasis but is contraindicated in onchocerciasis, where ivermectin rules — reversing the pair harms the eye. Equally, praziquantel and albendazole compete for neurocysticercosis: albendazole is generally preferred for better tissue levels, but praziquantel wins for taeniasis and schistosomiasis where albendazole is useless.
Exam-focused takeaway
Theory answers should group drugs by mechanism and then deliver the worm-to-drug map with doses where classical. Viva examiners ask why ocular cysticercosis excludes praziquantel and what the Mazzotti reaction signifies. MCQs test beta-tubulin binding, ivermectin's chloride channel, DEC contraindications, neurocysticercosis management and single-dose albendazole for soil-transmitted helminths.
Frequently asked questions
How do albendazole and mebendazole kill worms?
They bind nematode beta-tubulin selectively, disabling microtubule-dependent glucose uptake so the worm starves — host tubulin is far less susceptible.
Which drug is chosen for strongyloidiasis and onchocerciasis?
Ivermectin — it opens glutamate-gated chloride channels found only in invertebrates, paralysing the parasite; diethylcarbamazine is avoided in onchocerciasis.
Why is praziquantel contraindicated in ocular cysticercosis?
Dying intraocular cysts ignite inflammation that can destroy the eye, so lesion removal precedes any anthelmintic.
What is the Mazzotti reaction?
An allergic response to dying microfilariae after ivermectin or DEC — itching, urticaria, fever, lymphadenitis and hypotension — managed with antihistamines and sometimes corticosteroids.
Which anthelmintic treats hydatid disease?
Albendazole, given in prolonged courses peri-operatively or as standalone therapy for inoperable cysts, alongside meticulous surgical technique with scolicidal agents to prevent anaphylaxis from cyst spillage.