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Friday, February 10, 2006

The Skinny on the Latest PNAS Bird Flu Article

OK, I was finally able to get a copy of the PNAS bird flu article that has been reported on here and in the media this week. It isn't available online yet, but apparently only as a proof distributed to the media. Here's the basics:

Establishment of multiple sublineages of H5N1 influenza virus in Asia: Implications for pandemic control

H. Chen, G. J. D. Smith, K. S. Li, J. Wang, X. F. Fan, J. M. Rayner, D. Vijaykrishna, J. X. Zhang, L. J. Zhang, C. T. Guo, C. L. Cheung, K. M. Xu, L. Duan, K. Huang, K. Qin, Y. H. C. Leung, W. L. Wu, H. R. Lu, Y. Chen, N. S. Xia, T. S. P. Naipospos, K. Y. Yuen, S. S. Hassan, S. Bahri, T. D. Nguyen, R. G. Webster, J. S. M. Peiris,and Y. Guan

Abstract: Preparedness for a possible influenza pandemic caused by highly pathogenic avian influenza A subtype H5N1 has become a global priority. The spread of the virus to Europe and continued human infection in Southeast Asia have heightened pandemic concern. It remains unknown from where the pandemic strain may emerge; current attention is directed at Vietnam, Thailand, and, more recently, Indonesia and China. Here, we report that genetically and antigenically distinct sublineages of H5N1 virus have become established in poultry in different geographical regions of Southeast Asia, indicating the long-term endemicity of the virus and the isolation of H5N1 virus from apparently healthy migratory birds in southern China. Our data show that H5N1 influenza virus has continued to spread from its established source in southern China to other regions through transport of poultry and bird migration. The identification of regionally distinct sublineages contributes to the understanding of the mechanism for the perpetuation and spread of H5N1, providing information that is directly relevant to control of the source of infection in poultry. It points to the necessity of surveillance that is geographically broader than previously supposed and that includes H5N1 viruses of greater genetic and antigenic diversity.

OK so far...but here's where it gets tricky. This article is getting cited as evidence that wild birds are spreading H5N1 avian influenza from southern China, to Qinghai Lake in western China, Russia, and then this fall to Turkey and eastern Europe. Here's the new evidence given here:

1) Six wild ducks, apparently healthy Mallards, Spot-billed Ducks, and Falcated Teal, were found to have H5N1 in the wild in southern China in January and March 2005--indicating that some wild ducks may have the virus and appear healthy.

2) The exact strain of H5N1 that these ducks had is very similar to the strain found at Qinghai Lake and in Turkey.

That's it. Some wild birds have it (actually, we already knew this from reports in Russia this summer), but only very few individuals (six of 4,674 samples in this study). And the kind they have is similar to that found later in the year in Qinhai and eastern Europe.

It is slightly troubling that this article would seek to implicate wild birds in the recent outbreaks, even though it states that "the likely source of [the Qinghai] infection was thought to be from poultry in southern China" and that "the source of infection for these new outbreaks, including the initial Qinghai outbreak, has not been fully determined. Whether or not migratory birds could survive infection and carry H5N1 over long distances also remains unanswered."

As the only support for the wild bird transmission hypothesis, the paper cites a World Animal Health Organization report of Russian H5N1 viruses similar to those at Qinghai in several domestic ducks and geese and one wild duck last summer, and a Food and Agriculture Organization of the United Nations bulletin that only suggests in passing that wild birds may be transmitting the virus. None of this constitutes real evidence of wild birds spreading H5N1 during migration, and it seems irresponsible for the authors of this paper to base their claim of wild bird transmission of H5N1 on these weak sources.

So, while it remains possible that wild ducks may be able to spread H5N1 during migration, the case is far from proven and should remain a hypothesis for further testing. Meanwhile, there is lots of negative evidence showing that H5N1 must be rare in wild birds--even in this paper's study sites in southern China where the disease is thought to have originated and remained in poultry for the past 10 years.

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