If more animals are found to be infected by the Echinococcus multilocularis parasite, Swedes may have to stop eating fresh berries picked right from the forest.
"This is very serious. It has never before been found in Sweden," Carl Hård af Segerstad, division head at the National Veterinary Institute (Statens veterinärmedicinska anstalt, SVA), told news agency TT late on Sunday.
Up to 400 foxes are tested each year in an effort to detect the parasite. Last week, samples were taken from a suspected infected fox in northern Bohuslän in southwestern Sweden. On Friday, the infection was confirmed.
Tapeworm eggs can end up on berries and mushrooms through animal faeces. For dogs, cats and foxes, the parasite is relatively harmless. However, people must take precautions to prevent infection.
"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," said Hård af Segerstad.
The worm is one of the reasons that dogs and cats entering Sweden must be dewormed. The parasite is found in many parts of Europe and is most common in the Alps, where people are discouraged from picking and eating mushrooms and berries in the countryside.
"If we ingest the eggs that the fox has crapped out in the woods, there is a risk that we'll get infected. As a result, we may need to change our habits in the forest, for example not picking berries from a stem and eating them," said Hård af Segerstad.
He emphasised that the worm has only been found in one animal. As such, he estimated that the infection has not been present here for very long.
"Now we have to start analysing as many foxes and small rodents as we can from the area to see how widespread the infection may be. There is no risk of infection now. There is nothing in the woods that one can put in one's mouth right now," he said.
A number of agencies, including the National Board of Health and Welfare (Socialstyrelsen) and the Swedish Institute for Communicable Disease Control (Smittskyddsinstitutet), are expected to meet soon to develop an action plan for how to manage the parasite's entry into Sweden.