#16,964
Trans-Atlantic and cross-Pacific carriage of HPAI avian flu viruses to North America from Europe and Asia have long been considered possible (see here, here, and here), but until recently - with one notable exception (see below) - North America has been largely spared.
In late 2014, (see EID Journal: Novel Eurasian HPAI A H5 Viruses in Wild Birds – Washington, USA), a previous incarnation of our current clade 2.3.4.4b HPAI H5 virus made history, crossing the the Northern Pacific into Alaska, then making its way south through Canada to the Pacific Northwest and sparking the largest North American Epizootic to that date.
While involving longer distances, the same scenario has been considered possible for European migratory birds crossing the Atlantic (see 2014's PLoS One: North Atlantic Flyways Provide Opportunities For Spread Of Avian Influenza Viruses). This notion was reinforced in 2017 when the Icelandic government Warned On the arrival of Bird Flu.It did serve as a proof-of concept, however. Proving that under the right circumstances, HPAI could spread from Asia to North America by migratory birds.
Since 2015 HPAI H5Nx viruses (both in Asia and Europe) have undergone numerous evolutionary changes, morphing from H5N8, to H5N6, and more recently to H5N1 and H5N5 subtypes.
Over time these H5 viruses have become better suited for carriage by migratory birds, have increased their avian host range (see DEFRA: The Unprecedented `Order Shift' In Wild Bird H5N1 Positives In Europe & The UK), and have shown a greater affinity for infecting mammals (see CDC Adds Zoonotic Avian A/H5N1 Clade 2.3.4.4b To IRAT List).
And defying expectations - over the past 10 months they have demonstrated their ability to cross vast oceans - not once - but twice.
We now know (thanks to the following report) that at roughly the same time, migratory birds from Asia - carrying genetically distinct variants of HPAI H5 - were making their way across the Northern Pacific, arriving in Western Canada last winter.
First a link, and the abstract from the report, after which I'll return with a postscript.
A Threat from Both Sides: Multiple Introductions of Genetically Distinct H5 HPAI Viruses into Canada Via both East Asia-Australasia/Pacific and Atlantic Flyways
Tamiru N Alkie, Sara Lopes, Tamiko Hisanaga, Wanhong Xu, Matthew Suderman, Janice Koziuk, Mathew Fisher, Tony Redford, Oliver Lung, Tomy Joseph ... Show more
Virus Evolution, veac077, https://doi.org/10.1093/ve/veac077
Published: 25 August 2022
Abstract
From 2016-2020, high pathogenicity avian influenza (HPAI) H5 viruses circulated in Asia, Europe, and Africa, causing waves of infections, the deaths of millions of wild and domestic birds and presenting a zoonotic risk. In late 2021, H5N1 HPAI viruses were isolated from poultry in Canada and also retrospectively from a Great Black-backed Gull (Larus marinus), raising concerns that the spread of these viruses to North America was mediated by migratory wild bird populations.
In February and April 2022, H5N1 HPAI viruses were isolated from a Bald Eagle (Haliaeetus leucocephalus) and broiler chickens in British Columbia, Canada. Phylogenetic analysis showed that the virus from Bald Eagle was genetically related to H5N1 HPAI virus isolated in Hokkaido, Japan in January 2022. The virus identified from broiler chickens was a reassortant H5N1 HPAI virus with unique constellation genome segments containing PB2 and NP from North American lineage LPAI viruses and the remaining gene segments genetically related to the original Newfoundland-like H5N1 HPAI viruses detected in November and December 2021 in Canada.
This is the first report of H5 HPAI viruses introduction to North America from the Pacific and the North Atlantic-linked flyways and highlights the expanding risk of genetically distinct virus introductions from different geographical locations and the potential for local reassortment with both the American lineage LPAI viruses in wild birds and with both Asian-like and European-like H5 HPAI viruses.
We also report the presence of some amino acid substitutions across each segment that might contribute to the replicative efficiency of these viruses in mammalian host, evade adaptive immunity and posing a potential zoonotic risk.
Twenty years ago HPAI H5 was limited to Southern China and parts of Southeast Asia, where it smoldered in poultry and wild birds until it evolved into clade 2.2 at Qinghai Lake - and quickly spread to Europe, the Middle East, and Africa - in 2005-2007.
A new H5N8 clade 2.3.4.4 emerged in South Korea in 2014, breathing new life into the subtype, spreading with unexpected speed around the globe, and reassorting readily with other LPAI viruses. While not nearly as pathogenic in humans as the original H5N1, this new virus continues to evolve rapidly, and in unpredictable ways.For the next decade it set up housekeeping in places like Egypt, Cambodia, and Indonesia - forming geographically distinct clades and variants - but by 2012 H5 appeared to be declining globally.
Today's study confirms that North America is no longer shielded by vast oceans from Asian and European avian flu viruses. If simultaneous introductions can occur across two intercontinental flyways, then future incursions seem likely, and that could help increase the speed of H5's evolution.
Complicating matters, there are now concerns that the `new and improved' HPAI H5 viruses introduced over the winter have far greater staying power, and will become endemic in North American birds just as they have in Asia and in Europe.
Our next big test will probably come in October of November of this year, when we will see if a new wave of (possibly reassorted) HPAI H5 arrives on the wings of migratory birds this fall.
Stay tuned.
