Researchers in Vietnam have discovered why the H5N1 bird flu virus currently at large is so lethal to humans.

In a small study involving 18 patients infected with H5N1 and 8 with human flu, it was found that the virus replicates itself far more aggressively in people than common human flu viruses which could explain why it is so dangerous to human beings.

The study also found that the virus had infiltrated into the blood stream of many of the human victims it killed, which means it could have spread to other parts of the body.

Menno de Jong of the Oxford University Clinical Research Unit at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, says that people with bird flu had much higher levels of the virus in their throat than in their nose.

Cytokines are proteins in the immune system that fight off intruders such as bacteria and viruses and an unusually high viral load can trigger an intense immune system overreaction that can be fatal.

De Jong, says that with the H5N1 infection, the cytokine response appears to be very, very intense which can damage the body by working against the cells and organs.

The study which was carried out in 2004 and 2005 in Vietnam found far higher viral loads in the nose and throats of those infected with bird flu than human flu.

Thirteen patients with the H5N1 virus died and the virus was found in the blood of at least 9 of them, and also in the rectums of most of them, which suggests it could have spread from the respiratory tract through the blood stream into the gastrointestinal tract.

None of the patients with common flu died and none had the virus in their blood or rectum and the researchers say this leads them to believe the fatal outcome of the H5N1 infections is associated with high levels of replication of the virus and also the detection of the virus in the blood.

The team also found that levels of cytokines were much higher in H5N1 patients than in the human flu cases and again, the highest levels of cytokines were found in those who died of H5N1.

De Jong says the high levels of the virus triggered an overwhelming inflammatory response that contributed to lung dysfunction and eventual death and says there is a need to stop the virus replicating as soon as possible in order to prevent damage to the lungs and prevent the inflammatory response to the virus.

The researchers say the bird flu virus is more concentrated in the throat than the nose, which is the opposite of human flu and their finding may help doctors diagnose H5N1 bird flu earlier in people.

They note however that early diagnosis is a particular challenge in remote places where health services are often scant.

To date the virus has been responsible for 143 human deaths since 2003 worldwide, mainly in Asia and millions of birds have been killed or culled because of it.

It is of particular concern in poor and remote farming areas where families live in close contact with their poultry which is also an essential source of both food and income.

Health officials worldwide are closely monitoring the movement of the disease in poultry, wild birds and other animals as there has always been the worry that the virus will mutate into a form that could spreads easily from person to person, triggering a global pandemic.

At present the H5N1 virus remains a disease of birds which is only contracted by handling infected birds.

De Jong’s work was funded by the Wellcome Trust and is published in the latest issue of Nature Medicine.