From ProMed:

Contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico, federal agriculture officials now believe that it most likely emerged in pigs in Asia, but then traveled to North America in a human. But they emphasized that there was no way to prove their theory and only sketchy data underpinning it.

There is no evidence that this new virus, which combines Eurasian and North American genes, has ever circulated in North American pigs, while there is tantalizing evidence that a closely related “sister virus” has circulated in Asia.

American breeding pigs, possibly carrying North American swine flu, are frequently exported to Asia, where the flu could have combined with Asian strains. But because of disease quarantines that make it hard to import Asian pigs, experts said, it is unlikely that a pig brought the new strain back West. “The most likely scenario is that it came over in the mammalian species that moves most freely around the world,” said Dr. Amy L. Vincent, a swine flu specialist at the Agriculture Department’s laboratory in Ames, Iowa, referring, of course, to people.

The 1st person to carry the flu to North America from Asia, assuming that is what happened, has never been found and never will be, because people stop carrying the virus when they get better. Moreover, the officials said, the chances of proving their theory are diminishing as the virus infects more people globally. It has now reached more than 90 countries, according to the World Health Organization. Since some of those people will inevitably spread it to pigs, its history will become impossible to trace. “To tell whether a pig is newly infected by a human or had the virus before the human epidemic began really can’t be done,” said Dr. Kelly M. Lager, another Agriculture Department swine disease expert.

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