By Maggie Fox, Health and Science Editor

WASHINGTON, Sept 27 (Reuters) - The H5N1 bird flu virus can pass through a pregnant woman's placenta to infect the fetus, researchers reported on Thursday.

They also found evidence of what doctors had long suspected -- that the virus not only affects the lungs, but passes throughout the body into the gastrointestinal tract, the brain, liver and blood cells.

"The work helps us to understand H5N1's high fatality rate, as well as serving as model for global collaboration in the field of emerging infectious diseases," said Dr. Ian Lipkin of Columbia University in New York, who directed the study.

Lipkin and a team at Peking University in Beijing studied tissue taken from two people killed by H5N1 in China -- a 24-year-old pregnant woman and a 35-year-old man.

The study is the first to come out of the Infectious Disease Center at Peking University in Beijing, established after the epidemic of severe acute respiratory syndrome or SARS, a new virus that spread out of China in 2003, killing 800 people and infecting 8,000 before it was stopped.

The center is now looking at victims of H5N1 avian influenza. The virus mostly infects birds, but occasionally infects people and has killed 200 out of 328 infected since 2003. Because experts fear it could cause a pandemic that would kill millions, they are studying it in great detail.

Jiang Gu and colleagues at Peking University looked at tissue samples from throughout the bodies of the victims.

They found genetic material from the virus in the lungs, as expected, but also in the brain, the placenta, the intestines, and in immune system cells in the blood and the liver.

The four-month-old fetus, which died with its mother, was also infected, the researchers reported in the Lancet medical journal.

Their findings support the theory of a "cytokine storm" -- the idea that the immune system overreacts to the virus in some cases, and sends out an overwhelming swarm of signaling chemicals that end up killing the patient.

"Many people have talked about cytokine storm," Lipkin said in a telephone interview.

"Here the lung findings are that the amount of damage appears to be disproportional to the number of cells that were infected. This supports the hypothesis that there might be indirect methods of damage."

They also found evidence the virus had damaged immune cells including macrophages, which they said suggests the virus not only overstimulates parts of the immune system but can also suppress other parts.

Previous studies of H5N1 victims have produced evidence the virus may have evaded their immune systems' defenses by suppressing them.

The researchers noted that no one had thought human influenza could cross the placenta and affect unborn babies. "But there just isn't that much information," Lipkin said.

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