Nematodes !
- Parasites
Roundworms: gut worms caught from soil (soil-transmitted helminths), pinworms, and tissue worms such as filariae, Toxocara and Trichinella.
Updated 2026-11-01
What they are
Nematodes are cylindrical, unsegmented worms with separate sexes and a complete gut. Some live in the bowel and spread through soil or hand to mouth; others live in tissues and arrive through insects or undercooked meat.
What they share
- Cylindrical, unsegmented body with a cuticle.
- Separate sexes (except the parthenogenetic Strongyloides female).
- Soil-transmitted helminths: Ascaris, Trichuris, hookworms and Strongyloides.
- Several pass through the lungs (Ascaris, hookworms, Strongyloides).
- Filariae are insect-borne and many carry the bacterium Wolbachia.
Microorganisms in this group
- Ascaris lumbricoidesThe largest human gut worm. People swallow its eggs from soil; in children with many worms it can block the bowel.Exam priority: high
- Necator americanus / Ancylostoma duodenaleWorms that enter through bare feet, latch onto the gut and suck blood: they cause iron-deficiency anaemia, mostly in children.Exam priority: high
- Strongyloides stercoralisA worm that reinfects its host and can live in the body for decades. Under steroids it multiplies out of control and can cause fatal infection.Exam priority: high
- Trichuris trichiuraA whip-shaped colon worm, the commonest soil-transmitted helminth in Colombian schoolchildren; heavy loads cause dysentery and rectal prolapse.Exam priority: high
- Enterobius vermicularisA tiny white worm that lays eggs around the anus at night and causes itching. Very common in children; it spreads at home and at school.Exam priority: high
- Toxocara canis / Toxocara catiA roundworm of dogs and cats. A child who swallows its eggs from soil can have larvae wandering through liver, lung or eye, with no adult worm.Exam priority: medium
- Ancylostoma braziliense / Ancylostoma caninumA dog and cat hookworm. Its larva enters the skin on sandy beaches and leaves a red, winding, very itchy track.Exam priority: medium
- Wuchereria bancrofti / Brugia malayiA mosquito-borne worm that lives in lymph vessels. Over the years it causes massive leg swelling (elephantiasis) and hydrocele.Exam priority: medium
- Onchocerca volvulusA worm spread by a blackfly of fast rivers; it damages skin and eyes and can blind. Colombia was the first country in the world to eliminate it.Exam priority: medium
- Trichinella spiralisA tiny worm that comes with undercooked pork or game. Its larvae encyst in muscle and cause fever, muscle pain and swollen eyelids.Exam priority: medium
- Mansonella ozzardi / Mansonella perstansAn Amazonian filaria spread by biting midges. It rarely causes symptoms, but its microfilariae show up on thick smears and must be told apart.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Which intestinal nematodes pass through the lungs?
Ascaris lumbricoides, hookworms and Strongyloides stercoralis.
Which soil-transmitted helminths enter through the skin?
Hookworms and Strongyloides stercoralis, via filariform larvae in soil.
Which drugs are used in mass school deworming?
Benzimidazoles: albendazole or mebendazole.
Which filarial endosymbiont is targeted by doxycycline?
Wolbachia.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Soil-transmitted helminth infections (WHO, 2023); Geohelmintiasis (Ministerio de Salud y Protección Social de Colombia, 2014); Lymphatic filariasis (WHO, 2024); CDC - DPDx - Strongyloidiasis (CDC, 2019).
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Representative model, for educational purposes only.