|WASHINGTON--The 1918 flu that killed more than 20 million people may
have quietly percolated for several years, maybe even trading back and
forth between pigs and people, until suddenly growing strong enough to
become the world's worst pandemic.
That's the latest theory from the Armed Forces
Institute of Pathology, which reported Monday that researchers for the
first time have completely analyzed a critical gene from the killer influenza
Gene May Have Been "Sleeping"
The gene likely "was adapting in humans or
in swine for maybe several years before it broke out as a pandemic virus,"
said molecular biologist Ann Reid, lead author of the study published in
the Proceedings of the National Academy of Sciences
But "we can't tell whether it went from pigs
into humans or from humans into pigs," she said.
Different influenza strains circle the glove
annually. Usually, they're fairly similar to viruses people have
caught in the past. Every so often a strain tough enough to kill
millions emerges, and experts warn that the world is overdue for another
That's why understanding the 1918 flu's genes
are important. Scientists need to know what made that strain the
deadliest ever -- and why it struck down mostly young, healthy people --
to better react if similar killer flu emerges again.
Researchers Plot Virus' Path
Most experts believe that genetically stable
flu viruses reside harmlessly in birds, but that occasionally one of these
bird viruses infects pigs. The swine immune system attacks the virus,
forcing it to change genetically to survive. If it then spreads to
humans, the result can be devastating.
In two other pandemics -- the 1957 Asian flu
and 1968 Hong Kong flu -- viruses apparently made a fast jump from birds
to pigs to humans. That's because human flu genes from those pandemics
appear very similar to avian flu genes.
| But the new study finds no similarity
between those bird genes and a key gene in the 1918 flu.
1918 Virus Different
Reid studied lung tissue preserved from autopsies
of two soldiers who died from influenza, at Ft. Jackson, S.C. and Camp
Upton, N.Y., and from the frozen corpse of an Alaskan woman. Reid
fully mapped the hemagglutinin gene, which is key to influenza infection
She discovered that the hemagglutinin closely
resembles mammal genes.
So instead of making that fast bird-pigs-people
jump that scientists expect in a pandemic, the 1918 virus apparently evolved
in mammals -- either pigs or humans -- over many years before suddenly
mutating into a mass killer. It may have percolated in humans as
early as 1900, she said.
Viral Route Not Certain
But Reid can't tell if pigs developed the mutation
that turned the virus into a killer and gave it to people -- or if people
gave it to pigs.
Among the evidence: a huge wave of mild influenza
struck people during the spring of 1918, but no pigs were sick. Then
the flu struck again in the fall. This time it suddenly killed millions
of people, and this time pigs were sick, too -- but people who had the
mild spring flu were reported to be immune.
Regardless of which species evolved the killer
strain, the long incubation period has implications for predicting future
flu outbreaks. "We may have to expand our concept of where pandemics
come from," Reid said.
Institute scientists are analyzing other genes
from the 1918 virus, but Reid said the mystery so far is getting deeper.
"The more you study it, the more perplexing it becomes."