Past outbreaks of the virus have decimated seal populations.
Tests were carried out on a dead seal found at Vejbystrand in southern Sweden, said Tero Harkonen, marine biology specialist at the country’s natural history museum.
Authorities detected the virus in seals at a reserve on the island of Anholt between Denmark and Sweden in late June.
Previous outbreaks decimated the seal population at the Anholt reserve in 1988 and 2002, when 60 percent and 30 percent of Denmark’s seal population died respectively. It also spread to other northern European countries.
Some 10,000 seals died of the virus in 2002 in Sweden, amounting to about 50 percent of the total population.
Harkonen said that some seals that had contracted the virus and survived during the 2002 outbreak had been immunized, which could help limit the number of deaths.