Foxes are tested each year, and samples taken last seek from a suspected infected fox in southwestern Sweden came back positive, the Swedish news agency TT reported Monday.
"This is very serious. It has never before been found in Sweden," said Carl Hard at the National Veterinary Institute.
Since tapeworm eggs can end up on berries and mushrooms through animal feces, Swedes may have to stop eating food picked fresh from the forest if more animals are found to be infected, authorities said.
While the parasite is relatively harmless for dogs, cats and foxes, it can be a serious health danger to humans, Hard said.
"It can form cysts in internal organs. If someone becomes infected, he or she will have to undergo lifelong anti-parasite treatment and survival is not certain," he said.
Since the worm has only been found in one animal, Hard estimated the infection has not been present in Sweden for very long.
"Now we have to start analyzing as many foxes and small rodents as we can from the area to see how widespread the infection may be," he said.