Northern pintail birds in Ohio have tested positive for a low-pathogenic strain of the H5N1 bird flu virus, the U.S. government said on Saturday, adding to recent cases in Pennsylvania, Maryland and Michigan.

A strain of the H5N1 avian influenza virus was found in "apparently healthy" wild birds sampled October 8 in Ottawa County, located on Lake Erie about 15 miles southeast of Toledo, the departments of Agriculture and Interior said.

"Initial tests confirm that these wild bird samples do not contain the highly pathogenic H5N1 strain that has spread through birds in Asia, Europe and Africa," the USDA said in a statement.

The government said it was conducting additional tests to determine, in part, if the ducks had H5N1 or two separate strains with one virus contributing H5 and the other N1. A second round of tests could take up to 21 days to confirm whether it was the low-pathogenic H5N1 bird flu.

The virus also was found in Pennsylvania in September and in Michigan and Maryland in August.

The Agriculture and Interior departments are working with states to collect between 75,000 and 100,000 wild bird samples in addition to more than 50,000 environmental tests throughout the United States.

A low-pathogenic strain, which produces less disease and mortality in birds than does a high-pathogenic version, poses no threat to humans. It is common for mild and low pathogenic strains of bird flu to appear in the United States and other countries.

The latest H5N1 bird flu strain in Asia, Europe and Africa is known to have killed at least 141 people and forced hundreds of millions of birds to be destroyed.