Although it doesn't begin or end at precisely the same time all over the world - twice each year billions of migratory birds begin their long migratory journey - either towards temperate mid-latitude or equatorial regions to overwinter, or towards their high latitude roosting areas for the summer.
In the Northern Hemisphere, particularly in the lower latitudes, many species have already begun moving north.Among those multitudes, some percentage will carry one or more LPAI and/or HPAI viruses with them as they return to their roosting areas in Siberia, Mongolia, Northern Canada and the Arctic for the summer.
Until recently, HPAI H5 didn't appear to persist long term in wild and migratory birds (see PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl). It was believed that natural immunity in aquatic birds - relatively quickly - defeated the virus.
It was assumed it would take periodic fresh introductions of new strains (presumably from poultry) to revive the threat from migratory birds.But starting in 2016 - with clade 188.8.131.52 of HPAI H5 - we've seen an unusual persistence of the virus in the wild - along with a greatly expanded host range - with sporadic outbreaks continuing throughout the summer months in Europe, and HPAI H5N8 crossing the equator in Africa (for the first time) and setting up shop the Southern Hemisphere.
Although the Southward Autumn bird migration has been linked to the highest number of poultry outbreaks - along with recent introductions of new HPAI viruses to Europe and North America (see Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1) - increasingly the return trek in the spring appears to play a major role in the evolution and spread of avian flu viruses.
Researchers have suggested that immunologically naive juveniles - born in the high latitudes over the summer - are the most likely to carry and spread HPAI viruses during the fall migration.The new reassortant HPAI H5N6 viruses that appeared in East Asia and Europe last November appear to be descendant from a similar virus that was first reported in Greece in late March of 2016, at the tail end of Europe's record H5N8 epizootic.
At the time, it appeared to be a one-off, fairly inconsequential finding.Six months later, after spending the summer (probably) in Siberia, it turned up almost simultaneously in East Asia and Northern Europe, instead of the H5N8 virus, and has persisted for months.
No one saw that coming.Also in March of last year, the United States saw a brief multi-state outbreak of a North American H7N9 virus (not related to the Asian H7N9), which appeared to be spread northward by migratory birds.
Meanwhile this winter, wild bird detections of H5N6 in Europe continue, and over the past few days Japan's Ministry of Environment has reported a surge in finding dead wild birds, with 23 birds either AI positive - or with results pending - since March 1st.
That's nearly 3 times more birds than were reported by the MOE between November and February. A reminder that the virus remains in the environment, and that late season poultry outbreaks are still possible.
While the spring migration is often less dramatic than the fall, and poultry outbreaks are far less common, this northern leg of their round trip nevertheless plays a huge part in the spread, and ongoing evolution, of avian flu.Much of it happens outside our view, and in ways we can't predict. But what is happening with the spring migration right now will very likely help set the stage for what we can expect next fall.