Killer Flu Strains Lurk & Mutate for Years Before They Go Pandemic

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July 14th, 2009 5:34 PM

Genetic “pieces” of the 1918 flu virus, which killed between 50 and 100 million people worldwide, were likely circulating between pigs and people two to 15 years before the pandemic struck, according to a new study published in Proceedings of the National Academy of Sciences.

Catch two different flu viruses at once and a new one can emerge, something scientists call reassortment. Birds are the ultimate origin of influenza viruses, but because pigs can catch both bird and human flu strains, they’ve long been recognized as a species mixing vessel [AP]. The research shows that lethal flu strains may be the result of such reassortment of pre-existing strains, not a sudden genetic “jump.” It’s a cautionary tale for those studying the current swine flu outbreak, say researchers, as the findings suggest that the swine flu virus could evolve slowly over many years into a more dangerous form.

The analysis found that the 1918 epidemic was most likely created by interactions between human seasonal influenza and a flu strain circulating in pigs, which may have originated in birds. It had [previously] been thought that the 1918 virus emerged quickly, directly from a bird form [USA Today]. To come to the new conclusion, scientists used a computer program to construct flu strains’ evolutionary trees and find their common ancestors. They entered the genetic information of all known strains, including those that infect people, pigs, and birds. The program worked backward from genetic relationships and estimated dates to find where and when bits and pieces of deadly strains arose.

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