Major Global Migratory Flyways – Credit FAO
Four months ago, in Bird Flu Spread: The Flyway Or The Highway?, we looked at the often contentious debate over the role that migratory birds play in the global spread of avian influenza viruses. A week later, in H5N8: A Case Of Deja Flu?, we compared the recent spread of HPAI H5N8 to the 2005-2006 geographic expansion of H5N1.
While poultry producers are often quick to blame wild or migratory birds, not everyone concurs, with a recent statement by the UN CMS/FAO Co-Convened Scientific Task Force on Avian Influenza and Wild Birds stating that typically the spread of HPAI virus is via contaminated poultry, poultry products and inanimate objects although wild birds may also play a role.
The way that HPAI H5N8 virus spread out of East Asia to Europe, Japan, Taiwan, and North America in less than 12 months certainly suggests that migratory birds played a role in its dissemination, although direct proof is lacking. We’ve seen that some birds can carry avian influenza viruses without ill effect, and when they encounter other birds, can `share’ their viral cargo along their migratory flyway.
And 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.
The USDA’s APHIS (Animal Plant Health Inspection Service) assessment on the arrival of HPAI H5 viruses to the Pacific Northwest this winter unambiguously focused on migratory birds. In Update on Avian Influenza Findings in the Pacific Flyway dated Feb 4, 2015, they state:
The H5N8 virus originated in Asia and spread rapidly along wild bird migratory pathways during 2014, including the Pacific flyway. In the Pacific flyway, the H5N8 virus has mixed with North American avian influenza viruses, creating new mixed-origin viruses. This is not unexpected. These mixed-origin viruses contain the Asian-origin H5 part of the virus, which is highly pathogenic to poultry. The N parts of these viruses came from North American low pathogenic avian influenza viruses.
But the abrupt shift of the H5N2 virus from the Pacific Northwest to the heartland of America – a 1000 mile leap to the east in the dead of winter – has some questioning whether migratory birds are behind every outbreak. Once introduced into a region’s poultry, we’ve seen avian flu viruses spread to other farms through lapses in biosecurity and the movement of poultry products, transport, or personnel.
With 900 poultry farms in Taiwan battling a multi-subtype outbreak of HPAI H5 this winter – even if the virus was initially introduced to the island by migratory birds – it stretches credulity that infected wild birds paid personal visits to each and every of them.
Mar 18, 2015
LHG Creative Photography / Flickr cc Pintail duck.
The notion that wild birds played a key role in bringing highly pathogenic avian influenza (HPAI) viruses from Asia to western North America and more recently to the Midwest has been implicit in government statements about recent outbreaks. But some wildlife disease experts are warning against jumping to easy conclusions.
The story goes back to last November and December, when an HPAI H5N2 virus struck several poultry farms in southern British Columbia. Those outbreaks triggered increased surveillance for avian flu in the United States, and a matching virus showed up in December in a wild northern pintail duck in northwestern Washington state. At the same time, a Eurasian strain of H5N8 virus was found in a captive gyrfalcon in the same area.
For more on this debate, you may wish to revisit: