MARY LOUISE KELLY, HOST:
It was roughly a year ago that scientists believe bird flu first jumped into dairy cattle in the Texas Panhandle. More than 600 herds have been infected since then in more than a dozen states, and as NPR's Will Stone reports, scientists are concerned.
WILL STONE, BYLINE: For a time, Dr. Juergen Richt was optimistic the outbreak could be contained. His experiments with infected cattle indicated bird flu was primarily spreading through virus-laden milk, not as you'd expect from a typical respiratory virus.
JUERGEN RICHT: And I said, this is good news because it's not respiratory droplet transmitted.
STONE: Richt is a veterinary microbiologist at Kansas State University.
RICHT: We went out and said this is important for the industry to know. Control your milk contamination, maybe you can control the outbreak. It didn't happen, apparently.
STONE: The intervening months have made it painfully clear bird flu has a strong foothold in the nation's dairy supply. Not only that, it continues to sweep through large poultry operations, and it's still circulating in wild birds.
RICHT: This virus is not easy to get rid of. We will have to live with it.
STONE: For one, there are simply too many ways for the virus to spread in and around dairy farms. Gregory Gray is an epidemiologist at the University of Texas Medical Branch.
GREGORY GRAY: Short of a big vaccine program in the cattle, I just don't see how we're going to control it.
STONE: And as long as it's spreading in livestock and poultry, people working with those animals are at especially high risk. So far, there are more than 50 known human infections in the U.S., but the true number could be higher. For example, one study tested dairy workers in Michigan and Colorado who'd been at farms with outbreaks in cattle. About 7% of the people had evidence of a past infection.
GRAY: It's pretty clear, we're missing probably a lot of cases of H5N1 infections.
DEBORAH BIRX: We're making the same mistakes we made with COVID.
STONE: That's Dr. Deborah Birx. She helped oversee the pandemic response during the first Trump administration and is now a fellow at the Bush Institute. Without much more testing of cattle and people, she says there's no way to know the extent of the outbreak.
BIRX: The most important thing is to track where it is. And what have we learned over the last five years? Well, a lot of viruses spread asymptomatically.
STONE: What's been reassuring so far is that there's no compelling evidence the virus is spreading between people and that the human infections have largely been mild. An exception to that is a recent case in British Columbia. There, a teenager remains in critical condition. It's not clear how they caught the virus. Louise Moncla, a virologist at the University of Pennsylvania, says genetic sequencing indicates it came from wild birds, not cattle. But what's troubling is the virus appears to have evolved while in the teenager. It acquired mutations that might help the protein on the surface of the virus more easily infect humans.
LOUISE MONCLA: What this case in British Columbia shows us is that flu is always going to surprise us. It is very hard to predict which viruses are going to infect people and how they will change when they do.
STONE: Canadian health officials say it's possible these changes allow the virus to bind to receptors deep in the lungs and led to a more severe illness. There's no indication the teenager spread the virus to others. But scientists like Moncla are uneasy as flu season arrives. If a human were to catch bird flu and seasonal influenza at the same time, the two viruses could undergo a genetic mixing known as reassortment, and that could spawn a new, more dangerous virus.
MONCLA: Every past pandemic virus that we've had has been a reassortment event between a virus circulating in humans and a virus circulating in a different species. But translating that into a probability that we are close to a pandemic or that a pandemic will happen now...
STONE: That, she says, is impossible to predict.
Will Stone, NPR News. Transcript provided by NPR, Copyright NPR.
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