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H5N1 Expanded Host Range
August 20, 2004
The isolation of H5N1 in swine has drawn considerable media attention. The sequence of three of the genes has been available at GenBank for over a month. Analysis of these genes show that they a very similar to older H5N1 isolates found in Asia. The isolate is from 2003, so a close relationship with older isolates is not unexpected.
The virus in Asia has continued to evolve via recombination, and although the more recent isolates have not swapped entire genes with humans there is swapping happening via re-assortment (whole genes) and recombination (pieces of genes). Analysis of recent sequences from H9N2 isolates (the most widely detected avian virus in Asia) from Hong Kong in 2003 indicate that there has been some swapping of genes with H5N1 (PB2, PA, NP) and these genes are more recombined.
Most alarming however, is the presence of pieces of genes (polymorphisms) that have not been previously seen in H5N1 isolates. These novel sequences are normally found in human and swine isolates (H1N1, H1N2, H2N3))., indicating recombination between avian and mammalian isolates has already happened (especially in isolates from Thailand and Vietnam). Paralleling the recombination has been an expanded host range for more recent isolates (both H5N1 and H9N2). These more recent isolates grow more efficiently in mice and many are neurotropic (can be isolated from mouse brains).