How Bird Flu Could Come to America

If avian flu makes its way to the U.S., it will most likely hitch a ride on migratory birds flying in from Latin America, having first entered the hemisphere through trade in infected poultry. That's the conclusion of a new study, which finds that lax quarantine systems in Mexico and Brazil could give the virus a foothold in North America.

H5N1 has decimated poultry flocks in Asia, where it originated before spreading to Africa and Europe (ScienceNOW, 9 February). It has caused 154 human fatalities, and many scientists worry that if it acquires the ability to pass easily from human to human, it could touch off a devastating human pandemic and claim millions of lives.

To understand how the virus has spread geographically, disease ecologist Marm Kilpatrick and colleagues at the Consortium for Conservation Medicine in New York City, and other institutions in the U.S. and the U.K., analyzed its entry into 52 countries. To do so, they integrated data on migratory bird movements, trade in poultry and wild birds, and the relationship between various H5N1 viruses. The researchers concluded that the primary route of transmission in Asia was poultry movements, although migratory birds were most responsible for introductions into European countries. Both factors were responsible for spreading the virus into Africa. More importantly, "There is overwhelming evidence that wild birds and poultry are both involved, and that the two synergistically enable a very rapid spread of the virus," Kilpatrick says.

The interaction between poultry and wild birds is particularly significant for the United States. The researchers used the patterns of infection that emerged from their studies to predict future spread. They concluded that there was a relatively low risk of H5N1 introduction into the United States from countries where it is currently circulating. But they also warn that the chance of H5N1 spreading to Mexico and Brazil is significant because, unlike in the U.S., there is no testing or quarantine system for imported poultry. If the virus gets established in the Western Hemisphere, migratory birds could then bring it to the U.S., the team reports online this week in Proceedings of the National Academy of Sciences.

"This paper is significant in pointing out a possible transmission pathway to the U.S. that has previously been given little consideration," says Yi Guan, a virologist at University of Hong Kong. "The results concur with what had been thought of as the probable transmission pathways of H5N1, particularly throughout Eurasia." But Guan cautions that the conclusions rest on a large number of estimates and assumptions, such as the number of wild birds that might have been exposed to the virus in Europe and how close the birds passed to an outbreak site. In addition, he notes that better poultry and wild bird surveillance data will be needed to accurately predict the future spread of the virus.

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