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A single bird flu vaccine would not be able to control a human flu pandemic triggered by the H5N1 virus because it is evolving into genetically distinct forms, say researchers.

Their study, published this week by Proceedings of the National Academy of Sciences, also includes new evidence that migratory birds are spreading the virus over large distances.

It says, however, that the epidemic is being sustained in Asia by the transport of infected poultry.

After analysing the genes of H5N1 viruses from different parts of Asia, the team, led by Robert Webster of the US-based St Jude Children's Research Hospital, concluded that the virus had originated in southern China at least ten years ago, and has since has evolved into distinct regional forms.

This means that a single vaccine could not control all of the emerging viral forms, they add, and future vaccines will need to adapt to the virus as it continues to change.

The researchers showed that H5N1 does not always kill the birds it infects, and that infected ducks could carry the virus over long distances because they can continue to spread the virus for up to seven days.

Last year, an H5N1 outbreak at Lake Qinghai in north-central China last year was linked to previous outbreaks nearly 2,000 kilometres away in the southeast of the country (see China: migrant birds 'open flight path for bird flu').

The new research confirms this link, and suggests that migratory ducks carried the virus to Qinghai after getting infected by poultry while they spent the winter at Poyang Lake in southeast China.

The study also suggests that the virus has survived in southern China and that this source must be contained to help prevent a pandemic. From here, the virus has spread several times into both neighbouring and distant countries.

Webster's team concludes that the movement of locally infected poultry is keeping the virus circulating in each region — if migratory birds were to blame, they would be continually reintroducing different viral lines, making it unlikely that the regions would have genetically distinct bird flu viruses.

Link to abstract of paper in Proceedings of the National Academy of Sciences

Reference: Proceedings of the National Academy of Sciences doi 10.1073/pnas.0511120103 (2006)

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