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Mixed bird flu strain doesn't spread easily

Scientists combined genes from the notorious Asian bird flu with human flu but weren’t able to create a strain that could be easily spread.
/ Source: The Associated Press

Scientists combined genes from the notorious Asian bird flu with human flu but weren’t able to create a strain that could be easily spread.

Still, that doesn’t mean Mother Nature won’t find a way for the virus to create a pandemic.

While one leading expert called the test result a “small dose of reassurance,” that sentiment wasn’t shared by the head of the federal Centers for Disease Control and Prevention.

“Let’s not use the word reassuring,” Dr. Julie Gerberding said at a briefing on the study. “This virus is still out there, it’s still evolving.”

Viruses change to become easily spread in two main ways, Gerberding said. In one case they gradually evolve, which is probably what happened in the 1918 worldwide flu pandemic. In other cases they exchange genes with human flu strains, which she said happened in the pandemics of 1957 and 1968.

So the researchers mixed genes from human and bird flu viruses to see how easy it is to create a rapidly spreading strain.

It isn’t easy.

The importance of the research is in determining that the process is complex, said Dr. Jacqueline M. Katz of CDC, a co-author of the paper appearing in Tuesday’s issue of Proceedings of the National Academy of Sciences. In addition, Gerberding said, it provides a new tool to test viruses for ease of transmission.

The H5N1 bird flu virus has spread widely in Asia and parts of Europe, killing millions of chickens and other birds and raising fears of a worldwide epidemic if it were to spread between people. More than 130 people have died of the illness, but most cases were traced to contact with poultry rather than person-to-person transmission.

Researchers at CDC used ferrets to test the H5N1 bird virus that had exchanged genes with a common human flu known as H3N2. Ferrets and humans respond similarly to flu, including spreading it by coughing and sneezing.

Some combinations of genes resulted in viruses that were able to reproduce well but still could not be easily transmitted between animals. In other cases the mixed viruses had little ability to reproduce or transmit.

This test in ferrets can be a valuable tool in determining whether future mutations of the bird flu virus are easily transmitted, the researchers reported.

Katz said the research used a 1997 version of the bird flu virus and that further tests are ongoing using more recent versions. Tests are also planned in guinea pigs as well as ferrets, she said.

Dr. William Schaffner of Vanderbilt University agreed that this could be a way potential pandemic viruses could be tested rather quickly.

“For example, they could have taken that virus from that cluster in northern Indonesia and rushed it into the CDC laboratory and said: 'Do the ferret experiment.’ And I would not be surprised if this becomes one of the ways some of these candidate viruses — viruses we worry about — are tested,” said Schaffner, an influenza expert who advises the government on the disease.

Glimmer of good news
Schaffner, who was not part of the research team, found a further glimmer of good news.

This may help explain why the current bird flu virus, which has had plenty of chances to encounter human flu viruses and exchange genes, has not yet combined successfully to produce an efficiently transmitted human strain, he said.

“It is because it’s very complicated. The right combination of genes, we haven’t identified yet, and apparently the bird flu virus hasn’t been able to find yet either,” he said.

“So this may give some small dose of reassurance that perhaps this is not the bird flu viral strain that’s going to cause a pandemic. We should not go to sleep totally at ease, however. But if we’re looking for small doses of reassurance in a very troubled world ... maybe this is a small bit of good news,” he said in a telephone interview.

Dr. Wilbur Chen of the Center for Vaccine Development at the University of Maryland School of Medicine agreed that the study highlights the complexity of how the virus changes.

“I echo the authors’ endorsement of the ferret model as a valuable tool” for identifying if a virus is easily transmitted, added Chen, who was not part of the research team.