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How Swine Flu Can Become Pandemic

A new influenza virus that can spread from pigs to people has put some public health officials and state-fair goers on edge this season, even though the virus so far has not posed a major threat.
/ Source: Discovery Channel

A new influenza virus that can spread from pigs to people has put some public health officials and state-fair goers on edge this season, even though the virus so far has not posed a major threat.

The fear is that a random mutation could turn the virus into an infecting machine, leading to another swine flu pandemic, like the 2009 H1N1 outbreak that may have killed more than half a million people around the world in its first year of circulation.

A new study illustrates one way that the next flu pandemic might begin. Focusing on a variant of swine flu circulating in Korea, researchers discovered a gene mutation that makes the virus especially virulent.

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It's not this particular virus that should cause concern at this point. Instead, the research represents an incremental step towards understanding what it is that turns a mild virus into a devastating one. By compiling a more complete library of such mutations, the hope is to better predict which animal viruses we should be most worried about and better prevent major outbreaks.

"This is not: Oh my gosh, we've got to run for the hills, this virus is coming," said Richard Webby, an influenza virologist at St. Jude Children's Research Hospital in Memphis, and one of the new study's authors. "We really want to be able to try to assign some sort of risk to viruses we find in animal populations, and to know which out of a huge diversity of viruses we in human health should be concerned about. It's very hard to do that at this stage."

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Influenza viruses are constantly moving targets, said Daniel Perez, a virologist who works with influenza at the University of Maryland, College Park. Regularly and without warning, this group of viruses acquire new mutations that affect how they behave.

To better understand what makes some flu viruses more threatening than others, Webby and colleagues in both the United States and Korea looked at several viruses that are currently circulating only in Korean pigs but are closely related to strains found in North America. To see how those strains might affect people, the researchers used them to infect ferrets, which respond to flu viruses much like we do.

Most of the viruses caused only mild infections. But one virus, called Sw/1204, killed the animals within 10 days. And it spread from ferret to ferret through droplets that entered the air as the animals breathed.

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Genetic analysis isolated two mutations that made the Sw/1204 virus more infectious than others. One mutation, called HA-225, had already been well studied, Webby said. It is known to affect the part of the virus that sticks to host cells, allowing infection to occur.

The other mutation, called NA-315, hadn't been implicated in influenza infections before. This mutation seems to influence the virus' ability to leave the host cell after it has replicated and go on to infect other cells.

It wouldn't be surprising if the same mutation appeared in other parts of the world, Perez said, because the Korean viruses used in the study are very similar to viruses circulating elsewhere.

Still, the two mutations discovered in the study are far from the only gene changes that can cause outbreaks.

Through more studies like this one, scientists hope to build up a database of mutations that give flu viruses the power to cause pandemics, giving experts clues to look for when assessing viruses circulating through animal populations.

There is still a long way to go.

"It will take a long time for us to really figure out whether our database is truly a good predictor of what these viruses can do in the future," Perez said. "We are just scratching the surface when it comes to predicting the ability of these viruses to become more virulent."