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Federal officials are tracking the outbreak of avian influenza in dairy cattle

STEVE INSKEEP, HOST:

Federal officials are tracking an outbreak of bird flu in dairy cattle. It's been detected in herds in at least half a dozen states, which scientists are watching because they're alert for any potential pandemic. NPR's Will Stone reports.

WILL STONE, BYLINE: Cows are just the latest mammal to end up with this kind of bird flu. There have been red foxes in the Netherlands, sea lions in South America, bears in Canada, and dozens more. Avian influenza generally does not spread well between mammals. One reason it's appearing more often now is probably because the outbreak is so huge. Millions of wild and domestic birds have been infected around the world.

NICHOLA HILL: It's got the largest geographic footprint we've ever seen.

STONE: That's Nichola Hill, a disease ecologist at the University of Massachusetts Boston.

HILL: So it's creating all of these new transmission pathways that we didn't know existed.

STONE: It's still not clear exactly how the virus arrived in dairy cattle in Texas. Hill says so far, genetic sequencing of the virus in cows suggests it jumped from wild birds.

HILL: There are certainly mutations in those bird flu viruses that are now in cattle, but there's no evidence of it mutating in the direction of efficient human-to-human transmission, so that's really good news.

STONE: Of course, the fear is that could change the more that the H5N1 virus spreads in mammals that are in close contact with humans. The genetic data from the Texas dairy worker who was recently infected did show a mutation that's typically seen when bird flu gets into mammals. Anice Lowen, an influenza virologist at Emory University, says this is the first giveaway that it's evolving to better replicate.

ANICE LOWEN: So we're not surprised by it. It's not a good sign, but we also know that it's not enough to really create pandemic potential.

STONE: What would be concerning? Changes that allow avian influenza to easily infect humans. That could happen if the virus adapts to lock onto the receptors in the upper respiratory tract of humans.

LOWEN: Different receptors are used by influenza viruses in different species, so we're not seeing a lot of evolution in that function of the virus.

STONE: At least not when there's been spillover into mammals so far. Lowen says luckily, cows are not hosts for other influenza A viruses that could mix with this bird flu and spawn new, dangerous versions. She'd be much more worried if it were spreading in pigs.

LOWEN: Because influenza A viruses are widespread in pigs, and those viruses that circulate in pigs are quite similar to the viruses that circulate in humans.

STONE: Scientists have also used lab experiments with ferrets to model how this version of bird flu spreads. A study from the CDC looked at the virus isolated from a severe human case. It had a high capacity to replicate in human cells, but required direct contact between the animals to spread. Darwyn Kobasa led his own study at Canada's National Microbiology Laboratory, this looking at virus found in wild animals, including a hawk.

DARWYN KOBASA: It very rapidly transmitted by contact from infected to uninfected animals, and in the originally uninfected animals, it caused lethal infection.

STONE: He says there was also preliminary evidence suggesting it may be able to spread from one animal to another in the air, but that did not lead to severe illness, and there are many caveats. Ferrets are not the same as humans.

KOBASA: We certainly don't see any changes that would suggest that there's any way to support efficient airborne transmission.

STONE: When it comes to dairy cattle, it's not really looking like a respiratory illness. There's not much virus in the nasal swabs. Instead, it's mostly in the milk.

DAVID SWAYNE: A lot of virus in the milk.

STONE: David Swayne is a veterinarian who spent decades studying avian influenza.

SWAYNE: At least some of the hypotheses are is that the transmission is actually occurring in a very unique and defined environment.

STONE: Maybe in the milking parlor, maybe as the cattle are being transported. It's still not clear. Overall, Swayne is not alarmed.

SWAYNE: It is different from previous outbreaks 'cause we do have wild mammals infected, and now we have dairy cows, but the details tell us that we're not to a point where we have a high level of concern.

STONE: After all, he says, only a handful of human cases have been reported worldwide.

Will Stone, NPR News.

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