Doctors have warned of the dangers of ingesting parasites from sushi.
They said its growing popularity in the West has been linked to a rise in parasitic infections.
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Writing in British Medical Journal Case Reports, experts from Portugal told how they treated a 32-year-old man who was found to have larvae from a parasite attached to his gut lining.
He had been suffering stomach pain, vomiting and fever for a week.
A blood test showed mild inflammation, and the area below his ribs was found to be tender.
But it was only when the man said he had recently eaten sushi that doctors suspected he might have anisakiasis.
Anisakiasis is a parasitic disease caused by anisakid nematodes (worms) that can invade the stomach wall or intestine of humans.
It occurs when infected larvae are ingested from undercooked or raw fish or squid.
Doctors performed an endoscopy on the man, inserting a long tube with a camera into the stomach.
They found the larvae of a worm-like parasite firmly attached to an area of swollen and inflamed gut lining.
They added that most of cases of anisakiasis to date had been reported in Japan but warned: "However, it has been increasingly recognised in Western countries.