It is well-established that influenza evolves primarily in Asia. This is thought to be due to the very dense population of humans living in close proximity to birds, which are the animal reservoir for flu viruses. Every year, newly evolved flu strains leave Asia on planes in their human carriers on their way to North America and Europe. Thus, it was surprising when H1N1 appeared first in Mexico, and then in the United States and Canada. Further, although initially termed “swine flu”, the sequences of the new H1N1 do not closely match any flu virus from swine. Indeed, these sequences are not good matches for any other known viral strain. Despite intense detective work, the “smoking pig”, an animal with a virus that would be a good match for the pandemic virus, has not been found. Although some imaginative theoretical reconstructions of the possible pedigree of pandemic H1N1 have been offered, none of these are particularly convincing without the “missing link” sequences.
So, where did the new H1N1 come from?
Dr. Frederick Leung Chi-ching, of Hong Kong University has an idea. From The Standard, October 13, 2009 [hat-tip, Pixie]:
“The initial patients of H1N1 had no recent exposure to swine and the current surveillance data does not support the circulation of human H1N1 in North American pigs,” he said.
He said it is possible the new strain had been transmitted to North America through “a human carrier” from Asia.
He said Asia and certain parts of the world could be “a missing link” in the evolution of the pandemic.
Naturally, the WHO is doubtful.
Me, I think Dr. Leung may be on to something.
Rambaut et al. (2008) The genomic and epidemiological dynamics of human influenza A virus. Nature. 453:615-619.
Russell et al. (2008) The Global Circulation of Seasonal Influenza A (H3N2) Viruses. Science. 320: 340 – 346