Scientists uncover why Spanish flu was so deadly

LONDON - The 1918 Spanish flu that killed up to 50 million people worldwide caused a severe immune response which may help to explain why it was so deadly, American scientists have said.

The pandemic was one of the worst in recorded history and killed more people than World War I. But researchers did not understand what made it so lethal.

By infecting mice with a reconstructed version of the 1918 virus and monitoring their response, a team of scientists believe they have found some clues to solve the puzzle as well as a possible new way to fight pandemic flu.

"What we think is happening is that the host's inflammatory response is being highly activated by the virus, and that response is making the virus much more damaging to the host," said Dr John Kash, of the University of Washington in Seattle, who headed the research team.

"It is an overblown inflammatory response," he said in an interview, adding that it could have caused a similar immune response in humans.

Kash and his team, whose research is reported online in the journal Nature, believe targeting the patient's immune system response against the infection, as well as the virus itself, could provide a two-pronged attack against pandemic flu.

Scientists fear the next pandemic could occur if the H5N1 bird flu virus that has killed more than 145 people since 2003 mutates into a strain that becomes highly infectious in humans.

The Spanish flu pandemic was caused by the H1N1 influenza strain. Unlike other flu viruses that afflict mainly the elderly and children, the 1918 pandemic struck young adults and people without immune system problems.

One theory to explain its deadly impact was that a secondary infection may have attacked Spanish flu sufferers whose immune systems were already weakened.

But Kash and his colleagues discovered that the reconstructed virus activated immune system genes in the rodents and caused serious lung damage and death.

When they looked more closely at the animals' response, they found several genes had been activated including those that are linked to cell death. A second group of mice infected with a benign flu virus had a less serious immune response and none died.

"When the body responds to infection, there are components of the immune system that can be beneficial and those that can be harmful," said Dr Christopher Basler, of the Mount Sinai School of Medicine in New York, who is a co-author of the study.

The scientists are planning further studies to try to understand why the immune system reacts so strongly to the virus.

- REUTERS