Anthelmintics: Drugs That Deworm
Worm infections affect more than a billion people — mostly through the gut, and mostly treatable with a single dose. The drugs work by a beautifully simple idea: paralyse the worm so the body can expel it. Two names cover most of what you'll ever prescribe.
Helminths are worms — proper multicellular animals, visible to the eye, that live in and off the human body. They fall into a few groups: the roundworms (nematodes) such as pinworm, hookworm, ascaris and the thread-like filariae; the flatworms or flukes (trematodes) such as schistosoma; and the tapeworms (cestodes). Unlike bacteria or protozoa, you usually can't kill a worm outright and hope the body absorbs it — the trick is to paralyse or starve it so it loosens its grip and is passed out or destroyed. Almost all of common practice is covered by just two drugs.
The two workhorses
Albendazole/mebendazole and ivermectin cover most worm infections. The benzimidazoles — albendazole and mebendazole — are the broad-spectrum backbone for the common intestinal roundworms (pinworm, ascaris, hookworm, whipworm). They work by starving the worm: they block its ability to take up glucose, so it runs out of energy, becomes paralysed and dies, and is passed in the stool. They're usually given as a short course or even a single dose, and are remarkably safe. The other pillar is ivermectin, which paralyses worms by opening chloride channels unique to the parasite's nerve and muscle cells — flooding them with negative charge so the worm goes limp. Ivermectin is the drug of choice for strongyloides and for the filarial diseases onchocerciasis (river blindness) and, in programmes, lymphatic filariasis. A third specialist drug, praziquantel, is the key agent for the flatworms — schistosomiasis (a major tropical disease of the bladder and gut) and tapeworms — where it causes a sudden, sustained muscle contraction that dislodges the worm.
Two practical points
First, pinworm (the itchy-bottom worm common in children) spreads easily within a household, so treatment usually means treating the whole family and repeating the dose after two weeks to catch newly hatched worms — and reinforcing handwashing. Second, ivermectin carries an important caution: in people heavily infected with a filarial worm called Loa loa, killing large numbers of microfilariae at once can trigger a dangerous brain reaction, so screening matters in endemic areas. Beyond that, the everyday reality is reassuring: most worm infections in general practice are a single dose of albendazole or mebendazole, one of the safest and most cost-effective treatments in all of medicine — which is why mass deworming programmes have improved the health of millions of children worldwide.
- Benzimidazoles (albendazole, mebendazole) = broad-spectrum for intestinal roundworms; starve the worm.
- Ivermectin paralyses worms (parasite chloride channels); first-line for strongyloides and onchocerciasis.
- Praziquantel is the key drug for flatworms — schistosomiasis and tapeworms.
- Pinworm: treat the whole household and repeat in 2 weeks; reinforce handwashing.
- Most worm infections = a single, very safe dose of albendazole/mebendazole.
Notice the recurring theme of selective toxicity, applied to a whole animal. Ivermectin is a perfect example: it opens chloride channels that exist in the worm's nerve and muscle cells but are shielded away in humans — behind the blood-brain barrier — so it paralyses the parasite while sparing us. It's the same principle you saw with antibiotics targeting the bacterial cell wall and antifungals targeting ergosterol: find a lock the parasite has and the host doesn't, and you have a safe drug. The bigger and more complex the parasite, the more elegant that selectivity has to be.
- Treating only the child for pinworm — treat the household and repeat in 2 weeks.
- Using a benzimidazole for schistosomiasis — flatworms need praziquantel.
- Giving ivermectin in Loa loa–endemic areas without screening — risk of severe reaction.
- Confusing worms with protozoa — entirely different drugs (e.g. not metronidazole).
A child has pinworm. Besides treating the child, what's essential?
- Roundworms: albendazole/mebendazole (starve the worm) — usually a single, very safe dose.
- Ivermectin (paralysis) for strongyloides and onchocerciasis; praziquantel for flatworms/tapeworms.
- Pinworm: treat the household, repeat in 2 weeks, reinforce hygiene.
- Selective toxicity again — the drugs hit worm machinery humans lack.
- Katzung BG. Basic & Clinical Pharmacology — Clinical Pharmacology of the Anthelmintic Drugs.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Anthelmintic agents.
- WHO — Preventive chemotherapy for soil-transmitted helminthiases & schistosomiasis.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Anthelmintic drugs.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Anthelmintic drugs.
