The headlines are today loudly announcing the return of H5N1 to the United Kingdom (CNN, New York Times) at a Turkey farm near Lowestoft.  Though nobody can say for sure, the virus probably arrived via migrating birds.  It appears that the likelihood of transmission by migrating bird or smuggled poultry has a geopolitical dependence.

Last month, Kilpatrick, et al., published a paper in PNAS ("Predicting the global spread of H5N1 influenza") that looked at a variety of factors to classify historical outbreaks and predict new ones.  The abstract does a decent job of summarizing the paper, so here it is:

The spread of highly pathogenic H5N1 avian influenza into Asia, Europe, and Africa has resulted in enormous impacts on the poultry industry and presents an important threat to human health. The pathways by which the virus has and will spread between countries have been debated extensively, but have yet to be analyzed comprehensively and quantitatively. We integrated data on phylogenetic relationships of virus isolates, migratory bird movements, and trade in poultry and wild birds to determine the pathway for 52 individual introduction events into countries and predict future spread. We show that 9 of 21 of H5N1 introductions to countries in Asia were most likely through poultry, and 3 of 21 were most likely through migrating birds. In contrast, spread to most (20/23) countries in Europe was most likely through migratory birds. Spread in Africa was likely partly by poultry (2/8 introductions) and partly by migrating birds (3/8). Our analyses predict that H5N1 is more likely to be introduced into the Western Hemisphere through infected poultry and into the mainland United States by subsequent movement of migrating birds from neighboring countries, rather than from eastern Siberia. These results highlight the potential synergism between trade and wild animal movement in the emergence and pandemic spread of pathogens and demonstrate the value of predictive models for disease control.

Of course, the only way to know if the model really works is, alas, to wait for more outbreaks.  Anyway, it seems the U.S. is safe from poultry smuggling, which we have a chance of intercepting, but susceptible to migrating birds, a pathway that almost certainly resists any defensive measures.