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December 20, 2007
Researchers have identified a new strain of swine influenza--H2N3--which belongs to the group of H2 influenza viruses that last infected humans during the 1957 pandemic. This new strain has a molecular twist: It is composed of avian and swine influenza genes.
The research team studied an unknown pathogen that in 2006 infected two groups of pigs at separate production facilities. Both groups of pigs used water obtained from ponds frequented by migrating waterfowl.
Molecular studies indicated the unknown pathogen was an H2N3 influenza virus that is closely related to an H2N3 strain found in mallard ducks
The above comments describe a PNAS ahead of the press publication on H2N3 in two swine isolates from Missouri. The 1957 pandemic was caused by H2N2, which was replaced by H3N2 in the 1968 pandemic. Therefore, people born after 1968 have not been exposed to H2 and its re-emergence in the human population is cause for concern.
Avian H2 has amino acids at position 226 and 228 which are distinct from the later human H2N2 isolates. Earlier human H2N2 only differed at position 226, but the swine isolates above have the sequence that matches the earlier human H2N2 at both positions (Q226L and S228G). Moreover, the swine H2N3 is pathogenic mice and can be efficiently transferred from ferret to ferret, although the infected ferrets were asymptomatic. However, the efficient transmission increases concerns for an emerging H2, which could continue to evolve.
Recent swine isolates in the US also frequently have a human H1 and swine H1N1 has recently been isolated from humans in the US, so there is exchange of genetic information. Similarly, recent seasonal flu isolates in the US have acquired swine polymorphisms, providing more evidence for co-infections.
Recombinomics Paper at Nature Precedings