U.S. officials say they have found a possible new case of bird flu in the country, but say it is not a serious threat to humans or animals.

A spokesman, Ron DeHaven, for the Department of Agriculture said Monday the virus was found in two wild mute swans in the northern state of Michigan.

He said the strain is a mild form of the H5N1 avian flu virus that often causes minor or no symptoms of illness in animals. He added it is very different from the lethal H5N1 strain found in Asia.

Officials say the swans found with the virus were not sick. Further tests on the virus should be completed in two weeks.

The World Health Organization says the lethal strain of bird flu has killed at least 139 people worldwide since reappearing in Asia in 2003.