An analysis of more than 300 samples of the H5N1 virus taken from humans and birds has revealed that its family tree has started to branch out in a way that could make it more threatening to people.

Before 2005 every known human case of avian flu had been caused by a particular subtype of the H5N1 virus, which affected Vietnam, Cambodia and Thailand.

The H5N1 virus that started to infect people in Indonesia last year, however, has now been found to have subtle genetic differences, which scientists likened to those between human cousins. While this mutation has not in itself made the H5N1 virus any more readily transmissible from person to person — the key step if it is to start a pandemic — the changes are worrying because they show that the virus is increasing in genetic diversity.

This is important because a wider gene pool creates more opportunities for H5N1 to acquire the characteristics it needs to infect humans with ease, though it remains uncertain whether or not this will happen. It will also make it more difficult for scientists to monitor the way that the virus is changing, so as to track potentially dangerous mutations.

“As the virus continues its geographic expansion, it is also undergoing genetic diversity expansion,” said Rebecca Garten, of the US Centres for Disease Control and Surveillance (CDC) in Atlanta, Georgia, who led the study.

“Back in 2003 we only had one genetically distinct population of H5N1 with the potential to cause a human pandemic. Now we have two.”

John Wood, a virologist at the National Institute for Biological Standards and Control in Hertfordshire, agreed that the changes added to the pandemic risk from the virus, although there are no indications that the necessary mutations have happened yet. He said: “The more mutations that are seen, the more the virus is able to mutate, the greater the chances of the virus changing.”

Influenza viruses are separated into strains, which are labelled according to the particular versions of two proteins that they carry — haemagluttinin (H) and neuraminidase (N).

These strains are then subdivided into genetic groups called genotypes, and some genotypes can be separated further into smaller units called clades. A vaccine that protects against one type of bird flu may not be effective against another.

www.timesonline.co.uk/birdflu
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