But the Swedish fishing industry has kept a tight lid on information about the growing numbers, according to a report in newspaper Dagens Nyheter (DN).
"The worry is that it'll have very negative consequences, and that people will stop eating fish. So we want more information on the subject and so far this has not been a financial priority," Sven-Gunnar Lunneryd, an expert on seals, told the newspaper.
Though the symptoms usually disappear within two weeks, severe cases can see the cod worm eating its way through the stomach and showing up in other places in the body.
According to the DN report, one unlucky man in the United States suffered from a truly stubborn cod worm after eating sushi.
The parasite crawled up his esophagus, and from there proceeded to eat its way through into the man's neck.
Another incident reported by the paper was one sufferer who coughed up live worms after experiencing a ticklish and achy throat.
But when a similar revelation was made in Germany in the 1980's it caused a significant drop in fish consumption and authorities are fearing the same will happen in Sweden, Lunneryd told the paper.
"There aren't any reports about this being common in the Baltic, so I got interested and looked through 2,000 fishes collected in the area. From these we can see that the parasite is quite common."
According to Lunneryd, this is well known among his co-workers at the Swedish Board of Fisheries (Fiskeriverket, now Havs- och vattenmyndigheten).
But even so, he says no official investigations have been made, nor have any reports been written. In fact, Lunneryd's study was made in his own free time.
One person who's seen the cod worm up close is Harry Löfgren, who was just about to eat his newly-caught cod on Friday when he received a nasty surprise.
"I noticed something moving in my sauce," the 9-year-old told DN.
Despite his father cooking the fresh caught cod, the worms, which were "yellow and about a centimeter long", were very much alive, according to the newspaper.
"Next time we will freeze the fillets before we cook them," the boy told the paper.