Global Flyways – Credit FAO
It is pretty clear from the way the HPAI H5N1 virus spread out of South East Asia to Europe and the Middle East in the middle of the last decade, that migratory birds can play a major role in its dissemination. These birds can often carry avian influenza viruses without ill effect, and when they encounter other birds, can `share’ their viral cargo along their migratory flyway.
Where flyways overlap, there is a greater chance of spreading a virus from one region to another. And as you can see by the map above, they overlap a lot.
Japan and Korea – both overwintering sites for migratory birds that summer in Asia and Siberia – have seen the seasonal arrival of H5N1 in years past (see What Goes Around, Comes Around), and this winter Korea has found itself battling a viral foe: H5N8. Again, thought to have been introduced by migratory birds from China (see Korea: Migratory Birds Likely Source Of H5N8 Outbreak).
While the HPAI viruses originating in Asia have yet to make it to North America via these migratory birds, the concern remains that it could happen. Accordingly, the Pacific Flyways have been viewed with the most interest, as this would be the most direct route for H5N1, H7N9, or H5N8 to jump to the Americas.
But European birds can carry HPAI viruses as well, and new reassortant viruses can emerge anywhere, not just in Asia (see EID Journal: Predicting Hotspots for Influenza Virus Reassortment).
Reassortment of two Avian Viruses Producing a Hybrid (Reassortant) Virus
So while less obvious a threat, the North Atlantic Flyways are considered a potential air-bridge for the introduction of new (or components for a reassorted) virus to North America. Yesterday, the open access journal PLoS One published a study funded by the USGS and NIAID that confirms this potential.
First the press release from the USGS, then a link to the study and some excerpts. I’ll have a bit more after.
Released: 3/19/2014 5:10:00 PM
The North Atlantic region is a newly discovered important pathway for avian influenza to move between Europe and North America, according to a U.S. Geological Survey report published today.
USGS scientists and Icelandic partners found avian flu viruses from North America and Europe in migratory birds in Iceland, demonstrating that the North Atlantic is as significant as the North Pacific in being a melting pot for birds and avian flu. A great number of wild birds from Europe and North America congregate and mix in Iceland's wetlands during migration, where infected birds could transmit avian flu viruses to healthy birds from either location.
By crossing the Atlantic Ocean this way, avian flu viruses from Europe could eventually be transported to the United States. This commingling could also lead to the evolution of new influenza viruses. These findings are critical for proper surveillance and monitoring of flu viruses, including the H5N1 avian influenza that can infect humans.
"None of the avian flu viruses found in our study are considered harmful to humans," said Robert Dusek, USGS scientist and lead author of the study. "However, the results suggest that Iceland is an important location for the study of avian flu and is worthy of special attention and monitoring."
The study also highlighted the new finding that gulls play an important role in moving avian flu viruses across the North Atlantic.
During the spring and autumn of 2010 and autumn of 2011, the USGS researchers and Icelandic partners collected avian influenza viruses from gulls and waterfowl in southwest and west Iceland (see map). By studying the virus’ genomes — an organism’s hereditary information — the researchers found that some viruses came from Eurasia and some originated in North America. They also found viruses with mixed American-Eurasian lineages.
"For the first time, avian influenza viruses from both Eurasia and North America were documented at the same location and time," said Jeffrey Hall, USGS co-author and principal investigator on this study. "Viruses are continually evolving, and this mixing of viral strains sets the stage for new types of avian flu to develop."
Excerpts from PLoS One:
Robert J. Dusek mail, Gunnar T. Hallgrimsson, Hon S. Ip, Jón E. Jónsson, Srinand Sreevatsan, Sean W. Nashold, Joshua L. TeSlaa, Shinichiro Enomoto, Rebecca A. Halpin, Xudong Lin, Nadia Fedorova, Timothy B. Stockwell, Vivien G. Dugan, [ ... ], Jeffrey S. Hall
Avian influenza virus (AIV) in wild birds has been of increasing interest over the last decade due to the emergence of AIVs that cause significant disease and mortality in both poultry and humans. While research clearly demonstrates that AIVs can move across the Pacific or Atlantic Ocean, there has been no data to support the mechanism of how this occurs. In spring and autumn of 2010 and autumn of 2011 we obtained cloacal swab samples from 1078 waterfowl, gulls, and shorebirds of various species in southwest and west Iceland and tested them for AIV. From these, we isolated and fully sequenced the genomes of 29 AIVs from wild caught gulls (Charadriiformes) and waterfowl (Anseriformes) in Iceland. We detected viruses that were entirely (8 of 8 genomic segments) of American lineage, viruses that were entirely of Eurasian lineage, and viruses with mixed American-Eurasian lineage. Prior to this work only 2 AIVs had been reported from wild birds in Iceland and only the sequence from one segment was available in GenBank. This is the first report of finding AIVs of entirely American lineage and Eurasian lineage, as well as reassortant viruses, together in the same geographic location. Our study demonstrates the importance of the North Atlantic as a corridor for the movement of AIVs between Europe and North America.
In this study we isolated 11 unique AIVs in gulls and waterfowl from Iceland that contained unexpectedly high viral genetic diversity. Most significantly, we obtained viruses that were completely (all 8 segments) of Eurasian lineage or American lineage as well as reassortant American and Eurasian lineage viruses. Previous to this study there have been no reports of complete American or Eurasian lineage viruses in the same geographic location , .
When Eurasian AIV segments have been detected in the Americas, or American AIV segments in Eurasia, it has generally been only 1 or 2 segments per virus , , , . However, 2 recent studies in North America have found viruses with near complete (7 of 8 segments) Eurasian lineage genomes , . Both of these studies were conducted in regions where predominately Eurasian flyways overlap into North America (the East Asian Flyway and the East Atlantic Flyway). Iceland is within the East Atlantic Flyway (Figure 2); this flyway extends into North America (Greenland and eastern Canada) and tens of thousands of migratory birds move from North America into Europe along this route on their way to and from breeding and non-breeding grounds , .
Our data demonstrate that the North Atlantic serves as a route for intercontinental movement of AIV and it will be important to track the further dissemination of these viruses, in whole, or in part, into the Icelandic avian community and, more significantly, into the avian communities of Europe or North America.
While we’ve seen a lot of evidence to support the idea that migratory birds play an important role in the spread of avian viruses, not everyone agrees.
Last month in Korea: H5N8 Spreads, Debate Over Source Intensifies, the Scientific Task Force on Avian Influenza and Wild Birds argued against migratory birds being the source of that virus in Korea. Similarly, in years past we’ve seen other experts decry the blaming of migratory birds in the spread of H5N1 (see India: The Role Of Migratory Birds In Spreading Bird Flu & Another Migratory Bird Study).
Despite these somewhat partisan assertions, there have been plenty of other studies that strongly associate migratory birds with the spread of avian flu viruses. A few include:
Whether this North Atlantic flyway will provide a route for new HPAIs to show up in the Americas is unknown, but it does illustrate the folly in becoming too focused on one area of the world, when others are perfectly capable of serving up a viral surprise.