A green-winged teal shot by a hunter in northern Washington state has tested positive for a strain H5N1 bird flu — but it's only a distant relative of the virus that's infected nearly 700 people globally and killed 400 of them.
It's the first case of highly pathogenic H5N1 avian influenza ever seen in the U.S. The good news is that the strain is genetically different from the strain that's circulated in China, across Asia and most recently in Egypt. It doesn't appear to have infected any people or even domestic poultry.
But it shows that wild migrating birds can carry dangerous viruses to the U.S., says Hon Ip, an expert on wildlife pathogens at the U.S. Geological Survey in Madison, Wisconsin.
"Unlike the Asian H5N1 strain that has been found in Asia, Europe, and Africa, this Washington state strain has only been found in wild waterfowl and has not been associated with human illness, nor has this new Washington state strain been found in domestic poultry," USGS said in a statement.
The U.S. Department of Agriculture will need to keep a close eye out for other birds with H5N1, Ip said.
"USDA and the Department of the Interior are going to have to spend time to understand the implication of this introduction," Ip told NBC News. "At the moment, we do not know of a single human case with our H5."
Flu is a mutation-prone virus and there are hundreds of strains. Even with precise-sounding names such as H5N1, there can be a large degree of genetic variation. Scientists are watching for mutations and other genetic changes that can make a virus strain more likely to infect people and poultry.
The H5N1 that most people are worried about has killed or forced the destruction of hundreds of millions of chickens around the world. It occasionally infects people and the fear is it will mutate into a form that makes it pass easily from person to person. That would spark a pandemic that could kill millions.
Since 2006, Ip's lab has been watching for H5N1 and other strains of bird flu in wild migrating birds along the U.S. west coast, as well as in Alaska and Maine. They've found many different types — most recently a pathogenic strain of H5N8, as well as many non-pathogenic strains that don't appear to make birds sick.
To make matters more complicated, this strain of H5N1 found in the teal appears to be a mix of H5N1 and the H5N8 found in Washington state and elsewhere in the U.S. as well as in Europe, South Korea, Japan and Taiwan.
That makes it of concern to poultry farmers but not to people — yet, Ip says. "But because the H5 comes from the Asian H5N1, I would say the risk is not zero," Ip adds.
A wild bird can infect domestic flocks, which is why agriculture officials are on high alert. And infected domestic birds can infect people. Wild birds can infect hunters and other people who come into contact with them.
"Whenever avian influenza viruses are circulating in poultry, sporadic infections or small clusters of human cases are possible in people exposed to infected poultry or contaminated environments, especially in households," the World Health Organization says. "Human infections remain rare and these influenza A (H5N1) viruses do not currently appear to transmit easily among people. As such, the risk of community-level spread of these viruses remains low."
But they are highly deadly, because the human body doesn't have any defenses against these strains.
The other strain of avian influenza that has officials worried is the H7N9 strain, which has infected close to 500 people, almost all of them in China. Like H5N1, it doesn't pass easily from person to person and most patients appear to have had some contact with birds.