Spanish Flu pandemic arose from bird flu virus
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The influenza virus that caused the 1918 ‘Spanish Flu’ pandemic was probably a highly virulent, entirely bird-flu type that adapted to humans, scientists have shown.
The periodic occurrence of flu pandemics has raised concerns that a new pandemic may be in the offing, possibly with a strain as aggressive as the one that caused 50 million deaths in 1918.
Previous studies have shown that the 1957 and 1968 influenza pandemics were due to human viruses that had acquired two or three genes from bird flu virus strains. The Spanish Flu was apparently an avian influenza to begin with.
In the research journal Nature, Dr. Jeffery K. Taubenberger and colleagues at the Armed Forces Institute of Pathology in Rockville, Maryland, report their findings from mapping certain genes from the 1918 influenza virus.
The team found that only 10 amino acid positions differed between the 1918 and subsequent human influenza viral proteins and avian influenza proteins.
Moreover, some of these differences are shared by some H5N1 bird flu viruses currently circulating in Asia, and which have infected humans.
“This suggests,” Taubenberger said in a statement, “that these H5N1 viruses might be acquiring the ability to adapt to humans, increasing their pandemic risk,”
Meanwhile, in a paper in the journal Science, Dr. Terrence M. Tumpey, from the Centers for Disease Control and Prevention in Atlanta, Georgia, and colleagues say that they generated recombinant viruses containing coding sequences of the eight viral segments from the 1918 virus, in order to try to figure out that made the virus so virulent.
“We felt we had to recreate the virus and run these experiments to understand the biological properties that made the 1918 virus so exceptionally deadly,” said Tumpey.
The experiment, in which the virus was recreated employing a process called reverse genetics using preserved samples of the 1918 virus, allowed the researchers to test it in the laboratory and in several animals.
They found that the complete 1918 virus and a recombinant influenza virus bearing only one of the 1918 segments could replicate without a component, trypsin, that’s usually necessary for flu viruses to reproduce through multiple cycles.
Also, virus containing all eight genes was “an exceptionally virulent virus” compared with other human influenza viruses tested, the team reports.
“We identified a number of virus proteins that were essential for the development of severe pulmonary (lung) disease,” Tumpey said.
In particular, he said, a protein called hemagglutinin—the “H” in flu names—was key. When the 1918 hemagglutinin was replaced with a modern influenza hemagglutinin, the resulting virus was not very deadly at all.
Tumpey’s group points out that the antiviral drugs oseltamivir and amantadine are effective against viruses carrying 1918 genes. Also, a recently developed vaccine containing two 1918 components was protective in mice.
The 1918 flu was an H1N1 flu and very different from H5N1, the researchers stress.
And today’s human flu viruses are all descendants of the 1918 flu, which means people have some immunity to them, CDC director Dr. Julie Gerberding said. What is frightening about H5N1 is that people do not have any immunity to it.
If not H5N1, then some other influenza virus is certain to cause a pandemic that could be much worse than the 1918 flu, Gerberding said.
“Most experts agree it is not a question of if—it is a question of when,” she said.
SOURCE: Nature, October 6, 2005; and Science, October 7, 2005.
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