Intestinal helminths

Lesion

Worms living in the gut can block it, bleed it, cause anaemia or itching, and some invade the wall and the whole body.

Updated Nov 12, 2026

Key points

Ascaris: eggs from soil; larvae migrate through liver and lungs (pneumonitis), adults in the small bowel; masses of worms obstruct the bowel or bile ducts.
Strongyloides: larvae penetrate skin; autoinfection lets it persist for life; overwhelming hyperinfection with steroids or HTLV-1.
Hookworms (Necator, Ancylostoma): skin entry, attach to the duodenum and suck blood: iron deficiency anaemia.
Trichuris (whipworm) and Enterobius (pinworm) do not invade: heavy whipworm loads cause bloody diarrhoea and rectal prolapse in children; pinworms lay eggs on the perianal skin at night, causing itching (tape test).
Schistosoma: adults in mesenteric veins; eggs trapped in the wall cause granulomas, bleeding and fibrosis; also portal fibrosis.
Tapeworms (Diphyllobothrium, Taenia, Hymenolepis): stay in the lumen; fish tapeworm can cause B12 deficiency.

What it is

Parasitic infections affect more than half of the world's population on a chronic or recurrent basis. Soil-transmitted helminths (Ascaris, Strongyloides, hookworms and whipworm) are common where sanitation is poor, including rural Colombia. Worms that invade tissue provoke eosinophilia; those that stay in the lumen (pinworms, tapeworms) usually do not. Each worm leaves a characteristic lesion.

Intestinal worms: route, lesion and consequence

Worm	Route	Tissue reaction	Main consequence
Ascaris lumbricoides	Eggs swallowed	Eosinophil-rich inflammation; lung passage	Bowel or biliary obstruction, liver abscess
Strongyloides stercoralis	Larvae through skin; autoinfection	Eosinophilia, larvae in the mucosa	Lifelong infection, hyperinfection when immunosuppressed
Hookworms	Larvae through skin	Erosions, focal haemorrhage	Iron deficiency anaemia
Trichuris trichiura	Eggs swallowed	Little invasion	Bloody diarrhoea, rectal prolapse
Enterobius vermicularis	Eggs, fingers to mouth	None (luminal)	Perianal itching
Schistosoma (intestinal)	Cercariae through skin	Granulomas around trapped eggs	Bleeding, obstruction, portal fibrosis
Tapeworms	Raw or undercooked fish, beef, pork	None (luminal); no eosinophilia	Usually mild; fish tapeworm takes B12

Clinical notes

Before steroids or other immunosuppression, people from endemic regions should be screened or treated for Strongyloides.
Diagnosis is usually by eggs or larvae in stool; pinworm by perianal tape; tapeworm by proglottids.

Test yourself

Think of the answer, then tap the question to see it.

Which intestinal worm can complete its life cycle inside the human host, causing lifelong infection?

Strongyloides stercoralis, through autoinfection.

Which worm causes iron deficiency anaemia by attaching to the duodenal mucosa and sucking blood?

Hookworms (Necator americanus and Ancylostoma duodenale).

Which tapeworm can cause vitamin B12 deficiency?

Diphyllobothrium latum, the fish tapeworm, which competes for dietary B12.

Why do tapeworm infections rarely cause eosinophilia?

The worm stays in the lumen and does not invade tissue.

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Soil-transmitted helminth infections (World Health Organization, 2023); Tapeworm Infections and Cysticercosis (StatPearls (NCBI Bookshelf), 2025).

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