Wild ducks in Pennsylvania have tested positive for bird flu, but not the deadly Asian strain that has ravaged poultry and killed at least 141 people worldwide, the Agriculture Department said Saturday.

The mallards, which were sampled Aug. 28 in Crawford County, Pennsylvania, showed no signs of sickness — another indication that they were not exposed to the virulent Asian H5N1 strain.

The low-grade strain of H5N1 has been found many times in North American wild birds and poses no threat to people, but officials expect the deadlier strain to reach the continent this year.

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"It is possible that these birds were not infected with an H5N1 strain, but instead with two separate avian influenza viruses, one containing H5 and the other containing N1," the department said. Further testing is under way at the National Veterinary Services Laboratories in Ames, Iowa.

The ducks were sampled as part of the government's effort to test as many as 100,000 wild birds.

Any finding of highly pathogenic H5N1 in wild birds in the U.S. would prompt more intensive monitoring and extra security measures to protect commercial poultry flocks from infection.

Since 2003, the virulent H5N1 strain has been blamed for the death or destruction of millions of birds overseas. Nearly all the people who have been infected had close contact with sick birds or their droppings. However, scientists fear the virus could mutate into a form that is spread easily among humans.