A view of the transmission pathway

There has been previously a classic perception regarding the genetic segregation of influenza viruses all over the world: the virus has been divided into west hemisphere clade and east hemisphere clade, or to say, North American lineage and Eurasian lineage, as a result of long-term geographical and ecological separation of migrating wild birds. We all know that the global transmission of avian influenza is due to its natural reservoir, multiple kinds of wild birds. The viruses alway cause asymptomatic infection in wild birds, who carry them to travel the world. However, because of the climatic change and geographical distance, the birds cannot "make it" through the gigantic pacific ocean. The gene pool of influenza viruses, therefore, is categorized into lineages. Just take a look at M segments of H8N4, it obviously indicates that the gene is actually different between the viruses that identified in North America or Eurasian continent. 

It is rare to find virus circulation across the hemisphere, but this separation is not alway the case. There exists some well-established flyways of wild duck that are in the overlapping region across hemisphere. For example, the east Atlantic flyway seems to be a promising way to lead to virus transmission across the bound. Some researchers even proved the evasion of Eurasian clade towards North American clade by phylogenetic analysis. HPAI H5N1 is originated from Asia, without any evidence founded in North America. Either to evade or to take over right away by seasonal migrating pathway, all that HPAI needs is just time.

(well I would say it is quite fascinating to see the migrating viruses)

References

Global Patterns of Influenza A Virus in Wild Birds

•   Björn Olsen, Vincent J. Munster, Anders Wallensten, Jonas Waldenström, Albert D. M. E. Osterhaus,and Ron A. M. Fouchier

Science 21 April 2006: 312 (5772), 384-388. [DOI:10.1126/science.1122438]