An outbreak of distemper has been killing seal pups off the coast of Denmark, authorities said Saturday, warning that thousands of seals could die if the disease spreads to other northern European countries.

Since Tuesday, at least 41 harbor seal pups have been found dead on the small island of Anholt, midway between Denmark and Sweden, and tests indicate distemper, according to The Danish Forest and Nature Agency. The government agency normally finds around 30 dead seals a year.

"There is therefore reason to fear that we will see a large number of dead seals on Danish beaches in coming months," said Henrik Lykke Soerensen, an agency spokesman.

The agency said it would kill dying seals found on the shores of the island to try to prevent the spread of the disease, which does not affect humans.

The disease causes respiratory problems, fever and sometimes disorientation, while leaving the animal's immune system weakened and susceptible to other diseases, such as pneumonia.

Lykke Soerensen said it was still unclear which strain of virus authorities were dealing with, but that the agency was expecting to identify it in the next few days.

The seal population in Denmark has been hit by the disease twice before.

In 2002, it killed nearly half the harbor seal population along the coasts of Denmark, Sweden, Germany, the Netherlands, Britain and Ireland. Another outbreak, in 1988, wiped out nearly 60 percent of northern Europe's harbor seal population.

The cause of both outbreaks remains unknown.

The recent dead animals were pups born this year or in 2006. Older animals become immune to the virus, which is spread by direct contact with body fluids or by scratching, clawing or biting.

Anholt is a sanctuary for some of the estimated 12,000 harbor seals, also known as common seals, that live in the Baltic Sea waterway between the two Scandinavian countries.