Why bird flu has been kept at bay? (BBC)

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Why bird flu has been kept at bay?

Scientists say they have identified a key reason why bird flu has so

far not posed a widespread menace to humans.

So far, the H5N1 strain has mainly infected birds and poultry workers,

but experts fear the virus could mutate to pass easily from human to

human.

However, Massachusetts Institute of Technology found that to enter

human respiratory cells the virus must first pick a very specific type

of lock.

The study appears in the journal Nature Biotechnology.

The researchers say their discovery may help scientists better monitor

changes in H5N1 - and find better ways to fight it.

Flu viruses attack by binding sugar chains, called glycans, that line

the airways and lungs.

Latching on

The chemical linkages between the sugar molecules in these chains

differ between humans and birds.

Until now it has been assumed that bird flu viruses would be adapt to

humans simply by acquiring mutations that enable them to attach to the

human types.

But Dr Ram Sasisekharan and colleagues found this step depends on the

shape assumed by the flexible sugar chains rather than the type of

linkage.

Bird flu viruses currently require cone-shaped glycans to infect

birds, so the umbrella shape found in humans has protected most of us

from avian flu.

This suggests that for the H5N1 bird flu virus to become pandemic it

must adapt so that it can latch onto the umbrella-shaped glycans of

the human upper respiratory tract.

Dr Berg of the National Institutes of Health which funded the

work said: " Sasisekharan's team has changed our view of flu viruses

and how they must adapt to infect us.

" The work may also improve our ability to monitor the evolution of the

H5N1 virus and thwart potential outbreaks. "

Professor Ian , professor of virology at the University of

Reading, said: " This new work shows that there are sublevels of sugar

that the virus prefers to use to get into cells and the authors

suggest this is a significant factor in why H5N1 has not yet spread to

humans.

" It provides a finer level of analysis than has been done so far but

it is likely that other factors, like the reduced temperate of the

human upper airway, also are involved. "

http://news.bbc.co.uk/2/hi/health/7170365.stm