Recombinomics reports on an emerging scenario for human-to-human transmission of avian influenza today.
The gist: influenzas with a particular single nucleotide polymorphism (S227N) have an affinity for human receptors. That polymorphism doesn't exist in H5N1. But it's present in H9N2.
H9N2 has become endemic in Israel and millions of migratory birds will be passing through the area in the upcoming days. Thus, the potential for dual infections by H9N2 and H5N1 is high. The 10 nucleotides of identity offers an opportunity for homologous recombination that would create the S227N polymorphism and increase the efficiency of H5N1 human transmission.
More geeky genomics jargon on the other side:
In the upcoming Virology paper "Evolution of the receptor binding phenotype of influenza A (H5) viruses, Gambaryan et al screen H5 isolates for affinity for human Sia2-6Gal (human-like) receptors. Two isolates, A/Hong Kong/212/2003 and A/Hong Kong/213/2003 were identified. Both isolates had the S227N polymorphism.
These two isolates were from bird flu patients who had returned from a visit to Fujian province, The daughter had died in China with bird flu symptoms. The two isolates were from her brother and father. The father subsequently also died. Thus, the change in receptor binding was associated with a familial cluster that probably involved human to human transmission.
Although the S227N change requires a simple G/A transition, it is rare. This is similar to another polymorphism E627K in PB2. The change is also encoded by a A/G transition, but is strictly conserved. E was exclusively in avian H5N1 isolates and K was found in all human isolates. The conservation could be explained by selection which may have been further restricted by opportunities for recombination. The conservation was broken at Qinghai Lake, where all 16 avian isolates contained the human E627K polymorphism.
Now, all of this does not mean "OMG! Human to human transmission is coming tomorrow!" It's just a scientifically valid mechanism by which a mutation that influenza needs to transmit from human to human COULD be picked up.
Efforts to limit the exposure of H9N2 infected birds to H5N1 infected wild birds should be aggressively pursued.