If or when it strikes, the avian flu pandemic would likely kill victims by switching on an uncontrolled immune system response. That's according to researchers, who studied a 90-year-old flu virus in the hope of finding clues on how to fight a modern-day bird flu. VOA's Jessica Berman reports.

H5N1 bird flu virus

Researchers wanting to get a better idea why the dreaded H5N1 pandemic might be so deadly studied the 1918 Spanish flu, which killed 50 million people, many of whom were healthy, young adults.

Researchers altered the early 20th century virus at a biosafety lab in Winnipeg, Canada to make it genetically similar the H5N1 virus. Then scientists infected seven macaque monkeys with the altered virus.

The experiment was supposed to last three weeks. But after eight days, the monkeys become too sick to live.

"Monkeys were affected severely enough that required euthenization," he said.

Yoshihiro Kawaoka, a virologist at the University of Wisconsin, led the study which was published in Nature.

Unlike other avian flu viruses which are eventually controlled by the immune system, Kawaoka and colleagues found that their 1918 flu model stimulated an uncontrolled immune response.

"The severe illness found in patients in 1918 and animals in infected with 1918 virus was due to continued replication of the virus which triggered unusual immune response," said Kawaoka.

The investigators say that may help explain why so many young people died of influenza in 1918. Their healthy immune systems may have fueled the virus.

Co-author Michael Katze of the University of Washington says despite such research, no one can predict what would happen in the next flu pandemic.

"I think it's extremely critical to develop animal models with these highly pathogenic viruses so that should there be another epidemic or pandemic, we will have the models in place, we will have the drugs tested, we will have the vaccines tested, so that we will be better prepared. I think that's what this is all about," he said.