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Researchers have suggested why the H5N1 bird flu virus has so far been inefficient at infecting people and unable to spread between them.

In papers published tomorrow (23 March) by Nature and Science, they say the virus may be physically unable to reach vulnerable cells deep inside human lungs.

Although H5N1 is very good at spreading through large populations of birds, it has infected fewer than 200 people since 2003.

The virus has killed about half of those infected and could spark a devastating human flu pandemic if it mutates to spread easily between people.

For this to happen, it would need to be able to attach to, infect and replicate in human cells. After multiplying, coughs and sneezes would spread the virus to other people.

But this week's findings show that the virus is rarely able to attach to cells in the upper respiratory tract.

What's more, it seems that mucus could be trapping the virus, which is then expelled before it can replicate, says Thijs Kuiken of the Erasmus Medical Centre in the Netherlands.

While H5N1 cannot enter cells close to the nose and mouth, both Kuiken's team and another led by Yoshihiro Kawaoka, of the University of Wisconsin-Madison in the United States, found cells deep inside the human lungs that the H5N1 virus can bind to — if it is able to get that far.

This fits neatly with observations made during autopsies of people killed by the virus: that most damage was deep in lung tissue.

Kuiken's team found that while H5N1 also attaches to cells deep in cat and ferret lungs, it tends to bind to cells higher up the respiratory tracts of mice.

This, they say, suggests that cat and ferrets would make better research subjects than mice in studies of how H5N1 may infect and cause disease in people.

Link to full paper by Kuiken et al. in Science

Reference: Science doi: 10.11126/science.1125548

Link to full paper by Kawaoka et al. in Nature

Reference: Nature 440, 435 (2006)

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