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Why are traces of bird flu showing up in the milk supply?


What's the best information we have about an outbreak of bird flu? This disease is in dairy cattle. And just this week, federal health officials revealed they're finding traces of the virus in the commercial milk supply. NPR's Will Stone asked how much we need to worry.

WILL STONE, BYLINE: It sounds unsettling, but so far, there's no evidence that the milk on grocery store shelves contains infectious virus. Instead, what's being found are viral fragments. Federal scientists stressed this point to reporters yesterday. Here's Don Prater from the FDA.


DONALD PRATER: We've seen nothing that would change our assessment that the commercial milk supply is safe.

STONE: He says pasteurizing milk - heating it up for long enough to knock out bacteria and viruses - can still leave behind traces of those germs.


PRATER: Pasteurization is very likely to effectively inactivate heat-sensitive viruses like H5N1 in milk from cows.

STONE: The messaging here is a bit tricky, because there are no studies specifically looking at the effect of pasteurization on cow milk and this strain of bird flu. But scientists who study food and viruses don't currently see this as a threat to human health for a number of reasons. Past research has shown this virus doesn't stand up well to high temperatures. For example, it's inactivated when you pasteurize eggs. Lee-Ann Jaykus is a food microbiologist at North Carolina State University.

LEE-ANN JAYKUS: It's really important to sort of not get out of control about fear of consuming milk.

STONE: That's because all that's been found so far are small pieces of genetic material. But she says even if scientists did find some infectious virus...

JAYKUS: There really isn't evidence that you can get flu by consuming a food that has virus on it.

STONE: Especially because it doesn't appear that the virus spreading in cows has adapted to easily infect humans. Richard Webby studies avian influenza at St. Jude Children's Research Hospital. He's been looking at samples of milk with this viral residue from bird flu and seeing if it will grow in the lab.

RICHARD WEBBY: If that represented actual replicating virus, we would have expected to see virus growing, and we didn't. So I think it's pretty good evidence.

STONE: This is also what federal health officials have noted, although there have only been a small number of samples tested so far. As part of his work, Webby picked up some milk, thinking he could compare the positive samples he'd been sent to this virus-free one from his local supermarket.

WEBBY: Well, it turned out to be positive.

STONE: Positive for viral genetic material. But that did not stop Webby from bringing it home and cooking with it.

WEBBY: It's still in my fridge at home, and I made some chicken spaghetti the other day with it.

STONE: If anything, he says, this just underscores that the outbreak among dairy cattle is probably much more widespread than the official numbers show. Webby says it's still not clear exactly how it's spreading, but scientists have suspicions.

WEBBY: It's in the milk. It's in the udder, probably getting transferred from one cow to the other by something that sort of goes from udder to udder.

STONE: Milk from sick cows is supposed to be thrown away. A big question is to what extent the virus is circulating in cows without obvious symptoms. Dr. Tom Inglesby directs the Johns Hopkins Center for Health Security.

TOM INGLESBY: That could mean that the extent of this outbreak is larger than we're seeing - could be substantially larger. And to get to the bottom of that, we would need to do surveillance testing in places that don't already have clear outbreaks.

STONE: Federal health officials just announced that dairy cows will have to test negative for bird flu before they can be moved across state lines. Alexis Thompson will be handling some of these tests. She's a veterinarian at Texas A&M Veterinary Medical Diagnostic Laboratory.

ALEXIS THOMPSON: I have every confidence that the diagnostic labs are going to do their best. But this is going to be very resource-intensive, and I don't know how long term sustainable that will be.

STONE: It was early March when Thompson first started hearing reports of an illness affecting cattle near her. She says no one expected it to be avian flu. Now, starting Monday, she'll have her hands full trying to keep it from spreading further. Will Stone, NPR News.

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Will Stone
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