The influenza virus that wreaked havoc worldwide in 1918-1919 founded a viral dynasty that persists to this day, according to scientists from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.
In an article published online on June 29 by the New England Journal of Medicine, authors Anthony S. Fauci, M.D., Jeffery K. Taubenberger, M.D., Ph.D., and David M. Morens, M.D., argue that we have lived in an influenza pandemic era since 1918, and they describe how the novel 2009 H1N1 virus now circling the globe is yet another manifestation of this enduring viral family.
"The 1918-1919 influenza pandemic was a defining event in the history of public health," says NIAID Director Dr. Fauci. "The legacy of that pandemic lives on in many ways, including the fact that the descendants of the 1918 virus have continued to circulate for nine decades."
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Influenza viruses have eight genes, two of which code for virus surface proteins--hemagglutinin (H) and neuraminidase (N)--that allow the virus to enter a host cell and spread from cell to cell. There are 16 H subtypes and 9 N subtypes, and therefore, 144 possible HN combinations. However, only three (H1N1, H2N2 and H3N2) have ever been found in influenza viruses that are fully adapted to infect humans. Other combinations, such as avian influenza H5N1, occasionally infect people, but they are bird viruses, not human viruses.
"The eight influenza genes can be thought of as players on a team: certain combinations of players may arise through chance and endow the virus with new abilities, such as the ability to infect a new type of host," says Dr. Morens, Senior Advisor to the NIAID Director. That is likely what happened to spark the 1918 pandemic, he adds. Scientists have shown that the founding virus was an avian-like virus. The virus had a novel set of eight genes and through still-unknown mechanisms, gained the ability to infect people and spread readily from person to person.
Not only did the 1918 H1N1 virus set off an explosive pandemic in which tens of millions died, it continues to evolve to this day.
"Ever since 1918, this tenacious virus has drawn on a bag of evolutionary tricks to survive in one form or another and to spawn a host of novel progeny viruses with novel gene constellations, through the periodic importation or exportation of viral genes," write the NIAID authors.
"All human-adapted influenza A viruses of today--both seasonal variations and those that caused more dramatic pandemics--are descendants, direct or indirect, of that founding virus," notes Dr. Taubenberger, Senior Investigator in NIAID's Laboratory of Infectious Diseases. "Thus we can be said to be living in a pandemic era that began in 1918."