TORONTO – A new analysis of swine flu viruses suggests the first cases in humans probably started several months before authorities recognized there was an outbreak.
The study, the second to try to pinpoint from genetic data when this strain of viruses emerged, also said the viruses may been circulating in swine, possibly in Asia, for about 10 years before making the jump into people.
First author Gavin Smith, a virologist and evolutionary biologist at the University of Hong Kong, said there are still too many missing links in the genetic data to draw firm conclusions, but the viruses may have jumped into people in late 2008 or early 2009, or even as far back as last August.
“But basically the underlying message is the amount of missing data means we can’t answer most of these questions,” Smith said from Hong Kong.
“And it’s really going to take more (genetic) sequence data, particularly from North America and probably from quite a number of years before we can fully answer these questions.”
Whether those data exist is not yet clear, he admitted. But scientists with unstudied swine flu viruses in their lab freezers have been urged to dig out their neglected collections and look for viruses that could fill in the gaps in this virus’s family tree.
Smith said he and his co-authors found the nearest relatives of the viruses uncovered to date, 15 H1N2 swine flu viruses isolated in Hong Kong. The new pandemic virus is an H1N1 virus, which shares seven of its eight genes with those H1N2s.
They reported the finding and their analysis in a letter to the journal Nature which was published electronically on Thursday.
Smith said all the various pieces of the H1N1 genetic puzzle – the previously unseen constellation of genes – have not been found in North America.
The swine flu virus is what is called a reassortant, a virus containing genes swapped among several previous viruses. It is made up of flu genes from pigs, birds and people and the swine flu genes come from viruses circulating in North America and in Eurasia.
“The only place where you’ve got the necessary (genetic) diversity so far from what’s being described basically has been in Asia,” he said. “It raises the possibility that the reassortant was generated here.”
The lack of evidence of these different genes in swine viruses in North America may mean they didn’t or don’t all exist here. Or it may simply mean that there hasn’t been enough surveillance for flu viruses in North American pigs to pick up viruses containing these genes.
The international team of authors said the work highlights the need for ongoing and systematic surveillance of influenza in swine herds.
And they suggested movement of live pigs between Eurasia and North America seems to have facilitated the merging of a variety of flu viruses, culminating in the new pandemic strain.
Smith said that even though the Hong Kong viruses are closer to the outbreak virus than others spotted so far, “they’re still very, very, very divergent.”
The authors also said their findings do not support an earlier hypothesis that the virus might have been engineered in a laboratory and released by mistake. That theory was also dismissed by the World Health Organization and a number of leading influenza experts.