The swine flu pandemic likely originated in pigs in Asia, not on factory farms in Mexico, and was brought to North America by an infected human, say U.S. agriculture officials.
They said there's no evidence that the new swine flu virus -- a combination of North American and Eurasian genes -- has ever circulated in North American pigs. However, there is proof that a closely related "sister virus" has circulated among pigs in Asia, The New York Times reported.
But the agriculture officials added there's no way to prove their theory, which has only sketchy data to support it. As the virus spreads and pigs are infected by humans, it becomes more difficult to determine the origin of the pandemic.
"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," Dr. Kelly M. Lager, a U.S. Agriculture Department swine disease expert, told the Times.
Since the swine flu pandemic began, the popular assumption is that it originated in Mexico. The pandemic has now reached more than 90 countries, according to the World Health Organization.