The H5N1 bird flu virus has been discovered in two locations in the US, but according to federal officials it is the low-pathogenic strain found previously in North America, not the deadly Asian version.
Both cases involved mallards, or wild ducks.
In the most recent discovery of the virus, ducks sampled on 28 August in Pennsylvania tested positive for the low-grade strain of H5N1.
The other case occurred in Maryland, where faecal samples collected by University researchers from wild birds on 2 August tested positive for the disease.
The US Department of Agriculture (USDA) officials said that in both cases the ducks appeared to be healthy, which would further suggest they were not carrying the high-pathogenic strain of the H5N1 avian influenza virus.
The results of confirmatory tests should be available in two weeks, officials from the Agriculture and Interior Departments said.
Veterinary expert Dr Sherrill Davison says the discovery of a low-pathogenic bird flu strain is no cause for alarm.
“A North American strain of avian influenza is unrelated to the Asian H5N1 highly pathogenic virus that has caused so much trouble overseas,” she said. “This finding reminds us of the continuing need for flock monitoring and biosecurity on poultry farms, but by itself is no threat to the commercial industry.”
The low-grade strain of H5N1 has been found many times in North American wild birds and poses no threat to people, but some commentators expect the deadlier strain to reach the continent this year.
However, the USDA fears that the distinction will be unclear to consumers, who may stop eating poultry meat and eggs. The Department has released an updated fact sheet outlining the differences between high- and low-pathogenic strains of bird flu, to help clarify the issue.