Warning: this article contains a medical photo of worms which some may consider graphic.
Sushi is often seen as a heathy lunch alternative, but doctors have warned that it may be causing parasitic infections, known as anisakiasis.
Writing in the British Medical Journal, doctors say the increasing popularity of eating raw or undercooked fish in the West may be to blame for a growing number of people experiencing anisakiasis.
The warning comes after they treated a 32-year-old man who had had severe upper gut pain, vomiting and fever for a week.
A blood test indicated mild inflammation and the area below his ribs was tender.
But it was only when the man revealed that he had recently eaten sushi that the doctors suspected he might have anisakiasis, then found worms in his gut.
Anisakiasis is caused by eating raw or undercooked fish or seafood infected with worm parasites of the species Anisakis.
When doctors put an endoscope – a long tube with a camera on the end – down the 32-year-old’s throat to look into his stomach, they found the larva of the parasite firmly attached to an area of swollen and inflamed gut lining.
After the larva was removed with a special kind of net, the man’s symptoms cleared up straight away.
Most of the reported cases to date of anisakiasis have been in Japan, where a raw fish diet is very common, said the authors.
“However, it has been increasingly recognised in Western countries,” they added, and advised clinicians to consider the condition in patients with pain, nausea, vomiting and other complications, such as bowel obstruction and bleeding, who have recently eaten raw or undercooked fish.
In response to the findings, a Food Standards Agency (FSA) spokesperson told HuffPost UK: “Occasionally raw fish, for example salmon, contains parasitic larvae, such as Anisakis, which can make you ill. These parasites may cause health problems in people if they eat raw or undercooked fish infected with their larvae.
“Food business operators must place safe food on the market. Under European food hygiene legislation, certain fishery products destined to be eaten raw in dishes (such as sushi) need to be frozen before use. Freezing will kill these parasites.
“Otherwise, the FSA’s advice is that meat and fish should always be cooked properly before eating, following manufacturer’s instructions.”
They added that is you choose to make your own sushi from fish at home, you should ensure you follow a reputable recipe.
“If wild fish are to be eaten raw or lightly cooked, ensure that all parts, especially the thickest part, have been frozen for at least four days in a domestic freezer at -15°C or colder,” they said.
“This will ensure that any undetected Anisakis larvae are killed.”