An outbreak of distemper has killed at least 41 seal pups, whose carcasses have washed up on a Danish island.
It is not yet clear which strain of the virus is killing seals
The Danish Forest and Nature Agency is investigating the scale of the outbreak among harbour seals off Anholt island, between Denmark and Sweden.
There are fears that thousands of seals could die if the virus spreads.
A distemper outbreak in 2002 killed about 30% of seals off Denmark, but the virus killed nearly 60% of seals in the area in 1988, the agency says.
The virus causes laboured breathing, fever and neurological problems. It does not affect humans.