Sushi lovers are being warned of the dangers of ingesting parasites from raw and undercooked fish.
Doctors writing in British Medical Journal Case Reports said sushi's growing popularity in the West could be linked to a rise in parasitic infections.
Experts treated a 32-year-old man, in Lisbon, who was found to have parasite larvae on 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.
A special kind of net was used to remove the larvae "and the patient's symptoms resolved immediately", the team from a central Lisbon hospital said.
They added that most cases of anisakiasis to date had been reported in Japan, but warned: "However, it has been increasingly recognised in Western countries."
Atlantic salmon and sea trout caught at sea or in UK rivers are also known to be at risk of being infected with same species of parasite, NHS Choices says.
The advice is to remove the guts of the fish, freeze it for at least four days and then cook thoroughly before eating.