Avian Flu: What Goes Around, Comes Around (Twice Each Year)

 

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A decade ago, some experts were still bitterly divided over the impact of  migratory birds on the spread of avian flu (see 2014's Bird Flu Spread: The Flyway Or The Highway?).  Many conservationists had long insisted  that `sick birds don't fly', and that it was illicit poultry trade that was driving the spread of the virus. 

A 2015 statement by UN CMS/FAO Co-Convened Scientific Task Force on Avian Influenza and Wild Birds maintained that `. . .  typically the spread of HPAI virus is via contaminated poultry, poultry products and inanimate objects although wild birds may also play a role.'

While the poultry industry undoubtedly contributed to the local spread of the virus, after the H5N8 virus crossed the Pacific in late 2014 and sparked a major North American Epizootic (see map below), the role of migratory birds became much harder to ignore.

But curiously, this epizootic caused relatively few wild bird deaths, and completely disappeared over the summer of 2015.  A year later, in PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl, Robert Webster et al. reported that migratory birds were not a good `long-term' reservoir host for HPAI viruses. 

While could be infected, and spread HPAI H5 across long distances, their pre-existing immunity prevented long-term carriage of the virus. 

But 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.  A 2016 reassortment event  in Russia led to Europe's record setting H5 epizootic of 2016-2017, which also featured unusual mortality in wild birds.  

And unlike in North America in 2015, while avian flu reports decreased sharply over the summer of 2017 in Europe, the virus never completely disappeared.  A trend which has only increased in the years since (see Ain't No Cure For the Summer Bird Flu).

Over time H5 viruses have become better suited for carriage by migratory birds, have greatly 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.

Over the past decade these changes have enabled the virus to spread globally, crossing both the Atlantic and Pacific Oceans repeatedly (see Multiple Introductions of H5 HPAI Viruses into Canada Via both East Asia-Australasia/Pacific & Atlantic Flyways).

With their newfound persistence, these viruses are carried north each spring to the high latitude roosting areas of migratory birds (mostly above the Arctic Circle) where they are shared by hundreds of species - and can potentially reassort - before heading south once again in the fall. 

We looked at this twice yearly migratory cycle in 2016's Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1.  In North America more than 200 bird species spend their summers in the Alaskan Arctic Refuge, and then funnel south each fall via 4 distinct North American Flyways (see map below).

Complicating matters, the roosting areas in Alaska are also overlapped by the East Asian flyway, which may allow birds from Siberia and Mongolia to intermix with native migratory bird population (see USGS: Alaska - A Hotspot For Eurasian Avian Flu Introductions).

Albeit on a smaller scale, a mirror-image of this occurs in the Southern Hemisphere as well.  A process that may eventually bring HPAI H5 to Australia/NZ by way of Antarctica (see Australia : Biodiversity Council Webinar on HPAI H5 Avian Flu Threat).

Over the next two months the spring northbound migration of wild birds will peak across the United States (see map at top of blog), during which time, we may see another increase in spillovers.  This week the Washington State Department of Agriculture has published a lengthy and informative blog on the topic (see below).

I've only posted the link, and a few excerpts.  Follow the link to read it in its entirety.  I'll have more after you return. 

Clearing the air — What to know about avian influenza and spring bird migration

As spring bird migration brings thousands of birds to Washington state, it also increases the risk of avian influenza (bird flu) spreading through the region. Migratory waterfowl like ducks, geese, and swans, can carry the virus without showing symptoms and can introduce it to local wildlife. This migration typically peaks from March to May, when birds traveling along the Pacific Flyway stop to rest and feed in Washington’s wetlands and coastal areas.

In this blog, we’ll explore how migrating waterfowl spreads avian influenza, what exactly it is, where it came from, symptoms to watch for, preventive measures you can take, and what to do if you suspect your animals have the virus.

          (Continue . . . )

While the focus is on the next couple of months - after which we will hopefully have a few months respite - the reality is the virus continues to expand both its geographic and host ranges, and with that comes increased genetic diversity.

In North America alone, more than 100 genotypes have been identified over the past 3 years, with new ones fully expected to emerge over time.  Over the past year, four new reassortments of note have emerged:

• B3.13 aka the `bovine' strain affecting dairy cattle in at least 16 states and infecting dozens of humans
• D1.1   a wild bird/poultry strain which has spilled over into > a dozen people in Washington State and a teenager in British Columbia
• D1.2  a wild bird/poultry strain which recently detected in poultry and 2 pigs in Oregon

• D1.3  a recently detected wild bird/poultry strain which has been infection poultry and at least 1 human

While we can't predict what kind of changes may occur in the HPAI H5 virus over the summer - or during the Southern Hemisphere's winter - history suggests that we can expect some surprises when the virus returns next fall.   

A perilous pattern that we can expect to be repeated twice yearly, ad nauseum, for the foreseeable future.