Expectant mothers infected with avian influenza can pass on the deadly bird flu virus to their unborn children, fuelling fears that a pandemic could be much more dangerous than previously thought.

In a finding that has completely taken the scientific community by storm, Chinese scientists have reported that the H5N1 virus strain could infect a fetus by passing through the placenta.

Professor Jiang Gu from Peking University, Beijing, and his team examined post-mortem tissue samples of a 24-year-old woman from Anhui province, who was four months pregnant when she died after handling infected birds.

To their surprise, the scientists found traces of H5N1 in the placenta, as well as in the lungs, liver and immune system of her unborn baby. The scientists also found that the H5N1 virus affected the respiratory system, gastrointestinal tract, immune and central nervous systems.

"Initially, the virus seemed to be limited to the lungs, but it seems H5N1 could spread all over. The finding helps us understand H5N1's high fatality rate. It will also help us better develop diagnostic tests, drugs and vaccines," the authors said.

Dr S C Dubey, one of India's top influenza scientists, said, "This finding is extremely interesting. Normal flu does not pass from mother to her fetus." Professor Jiang explained, "This study has now shown the capacity for human vertical transmission of the H5N1 virus. This warrants careful investigation, since maternal infections with common human influenza virus are generally thought not to infect the fetus."

The research revealed that the fetus's lungs contained many infected cells, but did not sustain the severe damage seen in the lungs of the pregnant woman. The scientists said the lack of damage was likely due to the fact that the immune system of the fetus was not developed and therefore the virus didn't trigger production of the immune system chemicals - cytokines and chemokines - that swamp the lungs of adult H5N1 patients and which may exacerbate their lung damage.

The findings, published in Lancet, raise fears that the virus, which has infected more than 300 people in 12 countries, killing around 200, is more potent than first realised. Conservative estimates say up to 700,000 lives could be lost if the H5N1 strain of bird flu mutates into a form that spreads quickly and easily among humans.

Scientists fear the virus will mutate sometime, unleashing a pandemic similar to the 1918 outbreak that killed at least 20 million people.