Dead swans found recently in northeast Japan carried the deadly H5N1 strain of avian influenza, the government said Tuesday. One dying and three dead swans were discovered April 21 near Lake Towada - a popular tourist spot - in Akita prefecture. Officials had determined earlier that they were infected with the bird flu virus, but further testing by the National Institute of Animal Health confirmed the strain involved. The incident is Japan's first outbreak of H5N1 since March 2007, when researchers found the virus in an eagle on the southern island of Kyushu. Authorities in Akita prefecture plan to spend Wednesday and Thursday inspecting 15 farms within a 30-kilometer radius of where the swans were discovered. They have also issued a warning to local farmers to keep wild birds from entering their property and to immediately report chickens showing abnormal symptoms. Neighboring Aomori prefecture has released a similar directive to its residents. Highly pathogenic, H5N1 has killed tens of millions of birds since the first cases were reported in Hong Kong in 1997.