Although migratory birds have been labeled a `convenient scapegoat’ by conservationists who instead point their finger at the poultry industry for the spread of avian flu (see this recent statement by the UN CMS/FAO Scientific Task Force on Avian Influenza and Wild Birds), a fair review of the data makes the wild bird connection difficult to ignore.
Not that migratory birds are the only factor, since evidence strongly suggests poultry operations (including legal and illegal trade & transportation) have contributed mightily to the evolution and spread of avian viruses as well.
Yesterday, in EID Journal: A Proposed Strategy For Wild Bird Avian Influenza Surveillance, we looked at recommendations for a more consolidated and cost effective program to monitor avian flu viruses in birds, while previously (see The North Atlantic Flyway Revisited & Satellite Images Show Where The Wild Goose Goes) we’ve looked at some of the individual migratory bird studies.
With the recently emerged HPAI H5N8 avian virus showing up not only in Europe, but in North America this fall, and an upstart H5N6 making inroads in across China and Vietnam, defining the role of migratory birds in the spread of these viruses gains even more importance.
Today we’ve a report from PNAS that looks back over much of the data gathered since the emergence of H5N1 in Southeast Asia, and presents the case that migratory birds have play a significant role in its propagation. Among other things, they cite that along migratory flyways, H5N1 outbreaks closely match the seasonal arrival of migratory flocks.
They are quick to clarify, however, that migratory birds are likely just one of several underlying transmission networks, and that in some cases where the virus was detected in wild birds, the virus may have transmitted from poultry to birds instead of the other way around.
As with all studies, the authors list a number of limitations, and so you’ll probably want to read this report in its entirety. I’ve only excepted a couple of highlights, so follow the link to read:
Huaiyu Tiana,1, Sen Zhoub,1, Lu Dongc,1, Thomas P. Van Boeckeld,1, Yujun Cuie,1, Yarong Wue, Bernard Cazellesf,g, Shanqian Huanga, Ruifu Yange, Bryan T. Grenfelld,h,2, and Bing Xua,b,i,2
The spatial spread of the highly pathogenic avian influenza virus H5N1 and its long-term persistence in Asia have resulted in avian influenza panzootics and enormous economic losses in the poultry sector. However, an understanding of the regional long-distance transmission and seasonal patterns of the virus is still lacking. In this study, we present a phylogeographic approach to reconstruct the viral migration network. We show that within each wild fowl migratory flyway, the timing of H5N1 outbreaks and viral migrations are closely associated, but little viral transmission was observed between the flyways. The bird migration network is shown to better reflect the observed viral gene sequence data than other networks and contributes to seasonal H5N1 epidemics in local regions and its large-scale transmission along flyways. These findings have potentially far-reaching consequences, improving our understanding of how bird migration drives the periodic reemergence of H5N1 in Asia.
Highly pathogenic avian influenza virus H5N1 first emerged in Asia and subsequently unfolded into the first avian influenza panzootic, causing major economic losses in the poultry sector. However, we still do not understand the regional long-distance transmission and seasonal patterns of H5N1. In this study, we addressed this issue by combining H5N1 outbreak records, whole-genome sequences of viral samples, and satellite tracking data for four species of migratory birds in Asia. We show that timing of H5N1 outbreaks and viral migration are closely associated with known bird migration routes. The flyway is the major viral transmission barrier to the intracontinental spread of H5N1 by migratory birds in Asia, whereas geographic distances within the flyways have little effect on H5N1 transmission.
For more on the recent emergence and spread of the H5N8 virus, and how it compares to the sudden expansion of H5N1 in the middle of the last decade, you may wish to revisit: