THE HAGUE, Netherlands (AP) — The Dutch government said Thursday it is ordering poultry farms to keep their birds indoors after six wild swans were found recently dead and tests on two of the birds came back positive for a highly pathogenic strain of bird flu.

The order was set to come into force at midnight Thursday, Agriculture Minister Carola Schouten said in a statement.

She said experts believe the H5N8 strain of avian influenza likely was brought to the Netherlands by birds migrating from Russia and Kazakhstan because the same strain circulated there over the summer.

The order to keep birds indoors on farms is intended to prevent the infection, which often kills affected birds, spreading from wild birds to farm poultry.

Hundreds of farmers in the Netherlands keep millions of chickens and other poultry to produce eggs and meat. Many of the birds are kept in indoor stalls.