DANIELLE KURTZLEBEN, HOST:
Does the U.S. have bird flu under control? It's hard to tell, even though it's been close to three months since the virus was first spotted in dairy cattle. New cases are still cropping up, most recently in Wyoming and Texas. This week, federal health agencies released several new assessments of the situation. NPR's Will Stone is here to tell us the latest. Hey, Will.
WILL STONE, BYLINE: Hey there.
KURTZLEBEN: So what did we learn from these latest reports?
STONE: Well, we got a better sense of how bird flu is spreading between different dairy farms. The virus has been detected in more than 90 herds in a dozen states since late March. Many of these cases were in Michigan. The USDA drilled down into what's happening there. They found it's mainly being transmitted from her to herd in three ways. So first, through shared equipment and vehicles, second, people are carrying the virus inadvertently on their clothing or boots. And finally, some animals that are infected are moving between farms.
There had been concern that wild birds could be repeatedly reintroducing the virus into dairy cattle. So far, federal officials say, the evidence does not support that. Based on genetic sequencing of viral samples, it appears there was a single spillover event when a wild bird spread it into cattle in Texas sometime last year, and that sparked this unprecedented nationwide outbreak in dairy cattle.
KURTZLEBEN: Right. Well, beyond animals, though, I know a few humans have contracted this flu, which has some folks concerned about this sparking a pandemic. Do we have any better sense of the risk to people here?
STONE: So right now, federal health officials say the risk to the general public remains low. If you work with dairy cattle, obviously, the risk level is higher. There's still have only been three human cases reported. The most recent one was in a Michigan dairy worker. That was more concerning because the person actually had respiratory symptoms. I asked the Centers for Disease Control and Prevention this week whether there were any alarming mutations in the samples of the virus they collected from that person. Here's what doctor Nirav Shah with the CDC told me.
NIRAV SHAH: We didn't see any signs in the virus's fingerprints that suggests that it's more able to transmit between humans or from animals to humans.
STONE: So clearly, that is reassuring, but without a doubt, there are people in public health who are deeply worried about the situation; the fact that we don't have more visibility around this outbreak.
KURTZLEBEN: Are there unknowns that have public health officials concerned?
STONE: There's a lot. For starters, exactly, how many cattle are infected right now, we don't know. I asked the CDC if we even have some sense of the trajectory here. Have we hit the peak of cases in cattle? The answer was also, we don't know. Andrew Pekosz brought up another big concern when I spoke to him, which is better surveillance on the human side of things. Pekosz is a virologist at Johns Hopkins Bloomberg School of Public Health.
ANDREW PEKOSZ: That's really the critical thing that we're now worried about, is if this virus gets into humans, then the virus can adapt to humans directly. This has been going on long enough so that there should be antibody signatures in the blood of people who have gotten exposed, and that would really give us a sense of how much low-level exposure, perhaps mild disease, has been in this population.
STONE: So basically, figuring out how many cases we might have missed by looking for evidence of a past infection, that can help us gauge the overall risk to humans. Pekosz says it still seems that within a herd, the virus is primarily spreading in the process of milking. It could be the virus just keeps doing that and doesn't end up adapting to spread from human to human if we can keep it from infecting other people. But that's optimistic and not something we can bank on.
KURTZLEBEN: That's NPR health correspondent Will Stone. Thanks, Will.
STONE: Thank you. Transcript provided by NPR, Copyright NPR.
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