How Did Bird Flu Kill 162 Baby Seals?

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The science of tracing the deaths back to avian flu, and what it might mean for humans

seal-615.jpgMike Baird/Flickr

It's pretty much the saddest thing one can possibly read: baby harbor seals are dying, one by one. They began washing up on the beaches of New England last fall, apparently the victims of a viral disease researchers identified this week as bird flu. To be precise, a genetic mutation of avian flu known as H3N8. 

Although the chances are small, some epidemiologists worry that the flu virus' leap from waterfowl to harbor seals could someday repeat itself, endangering humans in the process.

That raises the question: how exactly does a virus make the jump from one species to the next? We've seen it happen with HIV, SARS, and West Nile virus. Pretty much all forms of influenza that can make humans sick came from birds. What's the mechanism behind cross-species infection, and how do scientists know what to look for?

The first step is to narrow down the range of possibilities. Dr. Simon Anthony, a postdoctoral researcher at Columbia University's Center for Infection and Immunity and the lead author of the headline-making seal flu study published in mBio, said that in a different time, scientists might have deliberately exposed test animals to various strains of a virus in an attempt to pinpoint the right one. But with the advance of research technology, scientists can now use molecular analysis to find out what they're dealing with.

Here's how it works: after a pathologist has performed an autopsy on a diseased animal -- in this case, a baby seal -- epidemiologists come up with a list of likely suspects that may have contributed to the creature's death. Then comes a battery of genetic tests designed to eliminate the wrong culprits. With a bit of synthetic DNA called a fluorescent probe, scientists can screen a sample of infected tissue for genes that serve as identifiers of specific viruses or bacteria.

These probes work a bit like litmus tests: they change color when they come into contact with the right illness gene. But the probes are programmed beforehand only to respond to a particular gene. This allows researchers to rule out diseases as they go.

Once the researchers have struck some options from the list, things get easier. "When we identify influenza," Anthony told me, "the next thing to do is sequence the entire genome [of the virus] and identify where the virus likely originated. And what we do is, we compare the sequence of the virus we found with a large database of influenza viruses, and we look to see what is the closest related to it."

As it happens, the virus Anthony and his team discovered in the baby seals shares a 96 percent genetic match with H3N8, an avian flu that affects North American ducks. Because seals and ducks have been known to interact with one another on shorelines, the researchers think that's how the germ was passed from one species to the other -- although they still aren't sure when the spillover happened, or how severe it was.

"For example, were the birds constantly shedding the virus and randomly, one day, for some reason, a seal became affected? Were there lots of spillover events? We don't know any of that," said Anthony.

It's also unclear what's going on with the other four percent of the virus' genetic sequence. While Anthony is "very confident" that they've identified the right strain of flu, it's possible, he said, that the discrepancy reflects mutations the bug has undergone in order to adapt to its new hosts.

"We've found many mutations within the genome that separate it from the avian close relative -- the avian strain," said Anthony. "It could be that any one of those mutations, or all of those mutations, are necessary for increased virulence. If the virus persists over time, it may accumulate more mutations and the virulence may increase or decrease. We really don't know."

The virus' constant evolution has scientists calling for further surveillance -- both of the bug itself as well as of seal and duck populations in New England. There's a slight potential for a human spillover of the virus, Anthony admits. The gene sequence has one mutation that in previous viruses has been linked to higher virulence. But for the moment, scientists don't know whether that mutation does the same thing in the seal virus as it's done in others. 

What is for sure: when the lives of baby seals are at stake, the rest of us start paying attention.

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Brian Fung is the technology writer at National Journal. He was previously an associate editor at The Atlantic and has written for Foreign Policy and The Washington Post.

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