A study published recently in Nature1 reconstructs the origins of influenza A virus and traces its evolution and flow through different animal hosts over two centuries. The virus that caused the 1918 influenza pandemic probably sprang from North American domestic and wild birds, not from the mixing of human and swine viruses.
“The methods we’ve been using for years and years, and which are crucial to figuring out the origins of gene sequences and the timing of those events, are all flawed,” says lead author Michael Worobey, an ecologist and evolutionary biologist at the University of Arizona in Tucson.
Worobey and his colleagues analysed more than 80,000 gene sequences from flu viruses isolated from humans, birds, horses, pigs and bats using a model they developed to map evolutionary relationships between viruses from different host species. The branched tree that resulted showed that the genes of the deadly 1918 pandemic virus are of avian origin.
Birds have been implicated in the deadly strain’s origins before. A 2005 genetic analysis of the 1918 pandemic virus pulled from a victim’s preserved tissue concluded that it most closely matched viruses of avian origin2. But a 2009 study3 found instead that the viral genes circulated in humans and swine for at least 2 to 15 years before the pandemic and combined to make the lethal virus.
Gavin Smith, an evolutionary biologist at Duke-NUS Graduate Medical School at the National University of Singapore, calls the current study “an important contribution to how we analyse data”. Smith, a co-author of the 2009 study, notes that it identified an avian relationship for two genes in the 1918 virus, but not for six genes, as the latest study has done.
Worobey's study is highly persuasive, says Oliver Pybus, an evolutionary biologist at the University of Oxford, UK. “It shows the evidence for a pig origin is a lot weaker, but it’s almost impossible to completely shut the door on that.”