LEILA FADEL, HOST:
More than 28 million poultry birds, like chickens and turkeys, have been lost in the U.S. because of a new bird flu. The virus either made the birds sick or they were culled to prevent its spread. Unlike previous bird flus, this one is also affecting a lot of wild birds. As NPR's Nell Greenfieldboyce explains, that could keep the virus in circulation for a long time.
NELL GREENFIELDBOYCE, BYLINE: The last time a deadly new bird flu showed up here - seven years ago - it really hit poultry farms.
BRYAN RICHARDS: In 2014, 2015, we saw, I think, somewhere in the neighborhood of 50 million domestic poultry affected.
GREENFIELDBOYCE: Bryan Richards says that virus didn't infect many wild birds. This time, it's different.
RICHARDS: We've got wild bird detections in 32 states.
GREENFIELDBOYCE: Richards is the emerging disease coordinator at the U.S. Geological Survey's National Wildlife Health Center. He says this virus came across the Atlantic a few months ago, probably carried by migratory birds.
RICHARDS: It can kill some waterfowl, but I think there's pretty clear evidence that some waterfowl likely are not affected by it. And therefore, they're perfect transport mechanisms for taking it very long distances.
GREENFIELDBOYCE: Since this virus arrived, it's killed birds that belong to more than 40 species - mostly ducks and geese, but also scavengers like black vultures and bald eagles that presumably eat the carcasses of birds killed by the virus.
David Stallknecht is a bird flu researcher with the University of Georgia. He says there have been large die-offs of ducks in Florida and snow geese in the Midwest.
DAVID STALLKNECHT: This outbreak in the wild bird population is a lot more extensive than we saw in 2014, 2015 - just a lot more birds appear to be affected.
GREENFIELDBOYCE: So far, he hasn't seen indications that any species will lose so many birds that it will become threatened. But the spread of this virus in wild birds suggests that this outbreak may not burn itself out like the last one did. Ron Fouchier is a flu expert at Erasmus Medical Center in the Netherlands.
RON FOUCHIER: There's a chance that the virus will stick around, and this will become a long-term problem.
GREENFIELDBOYCE: He says that's what this virus has done in Europe, where it arrived a few years ago and never left. It's been causing massive die-offs in wild birds and continues to strike poultry flocks, resulting in the deaths of more than 17 million poultry birds since December. Fouchier says there's only been one known human infection - a farmer in the United Kingdom who lived in close quarters with ducks that got this flu. That person tested positive but didn't have any symptoms.
FOUCHIER: We haven't seen any other farmers or veterinarians or other people being infected.
GREENFIELDBOYCE: Still, since this bird flu arrived in the United States, the Centers for Disease Control and Prevention has been keeping a close watch. Todd Davis works on animal-to-human diseases at the agency. He says bird flu viruses related to this one have sickened and even killed people during past outbreaks in other countries. That's why public health officials here have been monitoring the health of more than 500 people in 25 states who have had contact with sick or dead birds.
C TODD DAVIS: Because humans have no prior immunity to these viruses typically, if they were to be infected and spread the virus to other humans, then we could have another pandemic virus on our hands. And so that's our primary concern.
GREENFIELDBOYCE: Besides testing any people who show flu-like symptoms, they're also closely tracking genetic changes in the virus, looking for anything that would suggest it might become more of a threat to people.
Nell Greenfieldboyce, NPR News.
(SOUNDBITE OF LYMBYC SYSTYM'S "BIRDS") Transcript provided by NPR, Copyright NPR.
