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Bird flu might cause such severe disease and kill so many people because it makes the immune system 'overreact', say researchers.

They say that suppressing the immune response could be a way to treat infection by the H5N1 virus, which has killed 64 people in Asia — about half of all confirmed cases.

In a study published online today (11 November) by Respiratory Research, the researchers showed that lung cells infected with the virus produce considerably more 'messenger' chemicals than cells infected with a normal human flu virus.

These chemical messengers alert the immune system to send white blood cells to attack the source of infection.

The production of so many messenger molecules could explain why H5N1-infected people's lungs are "full" of a kind of white blood cell called macrophages, says lead researcher Malik Peiris of the University of Hong Kong.

His team previously showed that white blood cells release a chemical that, if present in abundance, can damage lung tissue.

Peiris says that if the disease and deaths H5N1 causes can be proven to be due to such changes to the immune system, it might be possible to treat infection by targeting the messenger chemicals produced by infected cells.

A drug that blocks the messenger chemical or limits its production might protect infected patients from their own immune system while they receive flu drugs that can kill the virus itself.

But Peiris points out that the research needed would take "not months, but years".

Experts fear the world could face a human flu pandemic if the H5N1 virus became able to spread from person to person – and most scientists believe this is a case of when, not if.

Peiris says that historical records suggest "there is no reason to believe we are not due another pandemic within the next ten years, irrespective of H5N1".

But he adds that if H5N1 sparked a pandemic, the situation could be dire because at present the virus seems to be much more dangerous to people than the viruses that caused the 1957 and 1968 flu pandemics.

Although H5N1 might also become less virulent if it adapts to spread easily between people, he says, there is no guarantee that this will definitely occur in the short term.   

Link to full paper in Respiratory Research

Reference: Respiratory Research 6, 135 (2005)

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