Anthelmintic Drugs

Anthelmintics are drugs that either kill (vermicidal) or expel (vermifuge) parasitic worms (helminths) from the host. For NEET PG, the highest-yield content is the mechanism of action of each class and the drug of choice (DOC) for each specific helminthic infection. Helminths are broadly grouped into nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms).

Classification of Helminths

Knowing which worm belongs to which class anchors the drug-of-choice questions, because the choice is largely class-dependent.

Class	Common name	Important members
Nematodes	Roundworms	Ascaris lumbricoides, hookworm (Ancylostoma/Necator), Enterobius vermicularis (pinworm/threadworm), Trichuris (whipworm), Strongyloides, filarial worms (Wuchereria, Brugia), Trichinella, Dracunculus
Cestodes	Tapeworms	Taenia solium, Taenia saginata, Diphyllobothrium latum, Echinococcus (hydatid), Hymenolepis nana
Trematodes	Flukes	Schistosoma (blood fluke), Clonorchis, Paragonimus (lung fluke), Fasciola (liver fluke)

High-yield: Nematodes (round, non-segmented) → benzimidazoles, pyrantel, ivermectin, DEC. Cestodes & trematodes (flat) → praziquantel is the broad workhorse (except Echinococcus and Fasciola).

Major Drug Classes and Mechanisms

1. Benzimidazoles — Albendazole, Mebendazole, Thiabendazole

These are the most widely used broad-spectrum anthelmintics.

• Mechanism: Bind selectively to β-tubulin of the parasite and inhibit microtubule polymerisation. This blocks glucose uptake, depletes glycogen stores, reduces ATP, and the worm is immobilised and dies. The selectivity arises because parasite β-tubulin has higher affinity than mammalian tubulin.
• Albendazole is better absorbed (especially with a fatty meal) and is converted to the active metabolite albendazole sulfoxide in the liver — this systemic absorption makes it the agent of choice for tissue-dwelling infections (hydatid disease, neurocysticercosis).
• Mebendazole is poorly absorbed → acts mainly in the gut lumen → ideal for intestinal nematodes.
• Thiabendazole is largely obsolete (hepatotoxicity, dizziness) but historically used in strongyloidiasis and cutaneous larva migrans (now replaced by ivermectin/albendazole).

High-yield: Benzimidazoles inhibit microtubule (tubulin) polymerisation → block glucose uptake. They are embryotoxic and teratogenic → avoid in pregnancy (especially first trimester).

High-yield: Albendazole + corticosteroids (± antiepileptics) is used in neurocysticercosis; steroids prevent the inflammatory reaction from dying larvae.

2. Pyrantel Pamoate

• Mechanism: A depolarising neuromuscular blocker — acts as a nicotinic acetylcholine receptor agonist, causing persistent depolarisation → spastic (rigid) paralysis of the worm, which is then expelled.
• Poorly absorbed → acts in the gut → good for ascariasis, enterobiasis, hookworm.
• Pharmacological antagonism note: Pyrantel (spastic paralysis) and piperazine (flaccid paralysis) are mutually antagonistic — never combine them.

3. Piperazine (largely historical)

• Mechanism: GABA agonist at the worm's neuromuscular junction → hyperpolarisation → flaccid paralysis; the live but paralysed worm is expelled by peristalsis.
• Was used for ascariasis and enterobiasis; now superseded by safer benzimidazoles.

4. Ivermectin

• Mechanism: Binds and activates glutamate-gated chloride channels (and to a lesser extent GABA-gated chloride channels) in invertebrate nerve and muscle. Increased chloride conductance → hyperpolarisation → tonic flaccid paralysis and death of the parasite.
• Selectivity: These glutamate-gated chloride channels are found only in invertebrates; in mammals ivermectin normally cannot cross the blood–brain barrier (P-glycoprotein keeps it out), so GABA channels in the human CNS are spared.
• Uses: Drug of choice for onchocerciasis ("river blindness"), strongyloidiasis, cutaneous larva migrans, scabies, and pediculosis.

High-yield: Ivermectin acts on glutamate-gated chloride channels unique to invertebrates → flaccid paralysis. It is microfilaricidal (kills microfilariae, not the adult worm) in onchocerciasis. Avoid in children < 15 kg and in pregnancy.

5. Diethylcarbamazine (DEC)

• Mechanism: Not fully defined; it immobilises microfilariae and alters their surface membrane so they are more easily destroyed by the host's immune system (opsonisation). It also interferes with arachidonic acid metabolism in the filariae.
• Uses: Drug of choice for lymphatic filariasis (Wuchereria bancrofti, Brugia malayi), tropical pulmonary eosinophilia, and loiasis.
• Toxicity: The Mazzotti reaction — fever, itching, lymphadenopathy, headache — results from the antigen release of dying microfilariae. This reaction is severe in onchocerciasis, which is why DEC is contraindicated in onchocerciasis (risk of severe ocular damage/blindness); ivermectin is used instead.

High-yield: DEC = DOC for lymphatic filariasis. Contraindicated in onchocerciasis (severe Mazzotti reaction → blindness). Ivermectin is preferred there.

6. Praziquantel

• Mechanism: Increases the cell membrane permeability to calcium (Ca²⁺ influx) in the worm → sustained muscle contraction and spastic paralysis, plus vacuolisation and disintegration of the tegument, exposing parasite antigens to host immunity.
• Spectrum: Broad activity against trematodes and cestodes. Drug of choice for all schistosomiasis (Schistosoma spp.) and most fluke and tapeworm infections.

High-yield: Praziquantel → Ca²⁺ influx → tegument damage + spastic paralysis. DOC for schistosomiasis and intestinal tapeworms. Notable exception: NOT effective against Fasciola hepatica (liver fluke) → use triclabendazole instead.

7. Other Agents

• Triclabendazole: Benzimidazole derivative; DOC for fascioliasis (Fasciola hepatica) and paragonimiasis.
• Niclosamide: Inhibits oxidative phosphorylation (uncouples) and anaerobic ATP generation in cestodes; an older alternative for tapeworms. Not larvicidal — risk of cysticercosis with T. solium if scolex digested (theoretical).
• Oxamniquine: Effective only against Schistosoma mansoni.
• Metrifonate: Organophosphate, only against S. haematobium (now obsolete).
• Bithionol: Old DOC for fascioliasis before triclabendazole.

Mechanism Summary Table

Drug	Mechanism	Net effect on worm	Key use / DOC
Albendazole / Mebendazole	Inhibit β-tubulin polymerisation; block glucose uptake	Energy depletion, death	Most intestinal nematodes; albendazole for hydatid & cysticercosis
Pyrantel	Nicotinic agonist (depolarising NM block)	Spastic paralysis	Ascaris, hookworm, pinworm
Piperazine	GABA agonist	Flaccid paralysis	Ascaris (obsolete)
Ivermectin	Activates glutamate-gated Cl⁻ channels	Flaccid paralysis	Onchocerciasis, strongyloidiasis, scabies
DEC	Immobilises microfilariae + immune opsonisation	Microfilaricidal	Lymphatic filariasis
Praziquantel	Ca²⁺ influx → tegument damage	Spastic paralysis + tegument lysis	Schistosomiasis, tapeworms
Triclabendazole	β-tubulin inhibition	Death	Fasciola, Paragonimus

Drug of Choice — Infection-wise (the highest-yield table)

Infection	Causative worm	Drug of Choice
Ascariasis (roundworm)	Ascaris lumbricoides	Albendazole (or mebendazole, pyrantel)
Hookworm	Ancylostoma duodenale / Necator	Albendazole (or mebendazole)
Enterobiasis (pinworm)	Enterobius vermicularis	Albendazole / mebendazole (repeat dose at 2 weeks; treat family)
Trichuriasis (whipworm)	Trichuris trichiura	Albendazole / mebendazole (± ivermectin)
Strongyloidiasis	Strongyloides stercoralis	Ivermectin
Cutaneous larva migrans	Ancylostoma braziliense	Albendazole / ivermectin
Trichinellosis	Trichinella spiralis	Albendazole (+ steroids)
Lymphatic filariasis	Wuchereria / Brugia	DEC (± albendazole, ivermectin)
Onchocerciasis (river blindness)	Onchocerca volvulus	Ivermectin
Loiasis	Loa loa	DEC
Hydatid disease	Echinococcus granulosus	Albendazole (+ PAIR/surgery)
Neurocysticercosis	Taenia solium larvae	Albendazole (+ steroids ± praziquantel)
Intestinal taeniasis	Taenia spp., Diphyllobothrium	Praziquantel (or niclosamide)
Schistosomiasis (all)	Schistosoma spp.	Praziquantel
Fascioliasis (liver fluke)	Fasciola hepatica	Triclabendazole (NOT praziquantel)
Paragonimiasis (lung fluke)	Paragonimus	Praziquantel / triclabendazole
Clonorchiasis	Clonorchis sinensis	Praziquantel

High-yield exceptions to memorise: Strongyloides → ivermectin (not benzimidazole). Fasciola → triclabendazole (the one fluke praziquantel does not cover). Onchocerciasis → ivermectin, never DEC.

Stepwise Approach to a "Drug of Choice" Question

When a stem describes a worm or clinical picture, work through it:

Step 1 — Classify the worm: nematode vs cestode vs trematode. → Step 2 — Intestinal lumen or tissue? Luminal nematode → mebendazole/albendazole/pyrantel; tissue larva (hydatid, cysticercus, trichinella) → albendazole (systemic). → Step 3 — Filarial worm? Lymphatic/Loa → DEC; Onchocerca → ivermectin. → Step 4 — Flatworm? Almost always praziquantel; remember the two exceptions: Echinococcus → albendazole, Fasciola → triclabendazole.

Clinical Features (pointers that flag the worm)

• Ascaris: Loeffler syndrome (transient pulmonary eosinophilic infiltrate during larval lung migration), intestinal/biliary obstruction; large white worm.
• Hookworm: Iron-deficiency anaemia (chronic blood loss), ground itch at entry site.
• Enterobius: Nocturnal perianal pruritus, diagnosed by NIH cellophane tape test for eggs.
• Strongyloides: Autoinfection → hyperinfection syndrome in immunosuppressed (steroids, HTLV-1); larva currens rash.
• Lymphatic filariasis: Lymphoedema, elephantiasis, hydrocele; nocturnal periodicity of microfilariae in blood.
• Onchocerca: Subcutaneous nodules, dermatitis, river blindness.
• Echinococcus: Liver/lung hydatid cyst (eggshell calcification, daughter cysts).
• Neurocysticercosis: Adult-onset seizures in endemic areas; ring-enhancing lesion on CT/MRI.
• Schistosomiasis: Haematuria (S. haematobium, linked to squamous cell bladder carcinoma), portal hypertension (S. mansoni/japonicum).

Diagnosis / Investigation of Choice

• Intestinal worms: Stool microscopy for ova and parasites (multiple samples).
• Enterobius: Cellophane/Scotch tape test (perianal), not stool.
• Filariasis: Nocturnal peripheral blood smear for microfilariae; antigen detection (ICT card test) is more sensitive.
• Onchocerciasis: Skin snip for microfilariae; Mazzotti test (historical).
• Hydatid: Imaging (USG/CT) + serology; avoid diagnostic aspiration (anaphylaxis risk — but PAIR is now done under albendazole cover).
• Schistosomiasis: Eggs in urine (haematobium) or stool; serology.

Complications & Adverse Effects of the Drugs

Drug	Key adverse effects / cautions
Albendazole/Mebendazole	Abdominal pain, hepatotoxicity, reversible bone-marrow suppression (prolonged high-dose), teratogenic — avoid in pregnancy
Ivermectin	Mazzotti-type reaction, dizziness; avoid < 15 kg, pregnancy; caution with Loa loa (encephalopathy)
DEC	Mazzotti reaction; contraindicated in onchocerciasis
Praziquantel	Dizziness, headache, abdominal discomfort; avoid in ocular cysticercosis (inflammatory damage to eye)
Pyrantel	GI upset; antagonised by piperazine
Triclabendazole	Biliary colic (dying flukes), transient transaminitis

High-yield: Praziquantel is contraindicated in ocular and spinal cysticercosis because the inflammatory response to dying larvae can cause irreversible damage. In neurocysticercosis it is given with corticosteroids.

Key Differentials / Tricky Comparisons

• Spastic vs flaccid paralysis: Pyrantel and praziquantel → spastic (contraction); ivermectin and piperazine → flaccid (relaxation). A frequent one-liner MCQ.
• DEC vs ivermectin in filariasis: DEC for lymphatic filariasis; ivermectin for onchocerciasis (DEC contraindicated there).
• Albendazole vs mebendazole: Albendazole = better absorbed → tissue infections; mebendazole = luminal only.
• Praziquantel vs triclabendazole for flukes: All flukes → praziquantel except Fasciola → triclabendazole.

Mnemonics & Memory Hooks

• "PIPER goes Flat, PYRANTEL stays Rigid" — Piperazine → flaccid; Pyrantel → spastic/rigid.
• "Praziquantel = Ca²⁺ in, Pieces out" — calcium influx and tegument disintegration.
• "DON'T give DEC for ONcho" — DEC contraindicated in Onchocerciasis.
• "BenzimidaZOLE = tubuLE" — benzimidazoles hit tubulin/microtubules.
• "Ivermectin Invertebrate Chloride" — glutamate-gated Cl⁻ channels of invertebrates.

Recently asked / exam angle

• Mechanism matching is the single most common format: "Activates glutamate-gated chloride channels" → ivermectin; "Increases Ca²⁺ permeability of tegument" → praziquantel; "Inhibits microtubule polymerisation" → benzimidazoles; "Depolarising nicotinic agonist causing spastic paralysis" → pyrantel.
• DOC questions: Onchocerciasis (ivermectin), lymphatic filariasis (DEC), schistosomiasis (praziquantel), hydatid (albendazole), strongyloidiasis (ivermectin), fascioliasis (triclabendazole). These recur almost every cycle.
• Contraindication MCQs: DEC in onchocerciasis; benzimidazoles in pregnancy; praziquantel in ocular cysticercosis.
• Spastic vs flaccid paralysis classification and the pyrantel–piperazine antagonism.
• The Mazzotti reaction and its association with DEC/ivermectin in filarial infections.
• Albendazole + steroids in neurocysticercosis is a recurring management vignette.

Rapid revision

1. Benzimidazoles (albendazole/mebendazole) → inhibit β-tubulin polymerisation, block glucose uptake; teratogenic.
2. Albendazole is systemically absorbed (fatty meal) → DOC for hydatid and neurocysticercosis; mebendazole acts only in gut lumen.
3. Pyrantel = depolarising nicotinic agonist → spastic paralysis; antagonised by piperazine.
4. Piperazine = GABA agonist → flaccid paralysis (now obsolete).
5. Ivermectin activates glutamate-gated chloride channels (invertebrate-specific) → flaccid paralysis; DOC for onchocerciasis and strongyloidiasis.
6. DEC = DOC for lymphatic filariasis and loiasis; contraindicated in onchocerciasis (severe Mazzotti reaction).
7. Praziquantel = Ca²⁺ influx → tegument lysis + spastic paralysis; DOC for schistosomiasis and tapeworms.
8. Triclabendazole = DOC for Fasciola hepatica — the one fluke praziquantel misses.
9. Hookworm → iron-deficiency anaemia; Enterobius → perianal itch diagnosed by cellophane tape test.
10. Praziquantel + steroids in neurocysticercosis; contraindicated in ocular cysticercosis.
11. Onchocerciasis = ivermectin (microfilaricidal); never DEC. Loiasis = DEC, watch for encephalopathy with ivermectin.
12. S. haematobium → haematuria and squamous cell carcinoma of bladder; treated with praziquantel.