The world's major migratory bird flyways - as depicted in the above graphic - serve as a global highway for avian influenza viruses. While primarily north-south conduits, there is enough overlap to allow for considerable east-west movement as well.
Every fall, migratory birds which have spent their summers in cooler high latitude nesting regions - and are the primary reservoir host for avian influenza viruses - begin their yearly journey to warmer climes for the winter.
A 2016 study (see Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1), suggests that waterfowl can pick up new HPAI viruses in the spring (likely from poultry or terrestrial birds) on their way to their summer breeding spots - where they spread and potentially evolve - and then redistribute them on their southbound journey the following fall.
We've also seen evidence that avian influenza viruses are very hardy (see EID Journal: Persistence Of H5N1 In Soil), and can remain infectious even after being frozen for months (see 2017's Appl Environ Microbiol: Survival of HPAI H5N1 In Infected Poultry Tissues).
The tissues were stored at +4°C or +20°C and viral isolation was performed at different times for 360 days. The maximum period for viral survival was observed in samples stored at +4°C in all tissue types, i.e., 240 days in feather tissues, 160 days in muscle, and 20 days in liver.
A recent study (see Evidence for interannual persistence of infectious influenza A viruses in Alaska wetlands) also demonstrated that avian influenza viruses could `overwinter' in Alaskan lakes and ponds, and remain infectious for up to a year (or longer).
All of which means there are ample opportunities for avian influenza viruses to spread widely among migratory birds, which share ponds, and feeding areas on their long journey south each winter.
While avian influenza tends to recede during the summer months in more temperate zones, it doesn't necessarily go away completely. We've been watching an unusual number of human infections in Mainland China over the summer with avian H5N6, and while subdued, Europe has continued to report scattered detections of HPAI H5 in wild birds and poultry over the past few months.
While most of the avian flu viruses carried by wild birds pose little or no public health threat, last year - and for the very first time - Russia reported confirmed human infections with avian H5N8 (see Russian Media Reports 7 Human Infections With Avian H5N8), and in May the CDC Added Zoonotic Avian A/H5N8 To Their IRAT List.
Nearly a month ago, in DEFRA: Epidemiological Report On 2020-21 Avian Flu & A Look Ahead To This Fall we looked at a detailed analysis of last year's avian epizootic, and a risk assessment for the UK that found:
The risk of HPAI H5 incursion in wild birds remains at LOW (event is very rare but cannot be excluded). Given all factors with ongoing detections in wild bird populations not only in central Asia and southern Russia, but also nearer to the UK mainland in the remote northern Scottish Isles and the Netherlands, the risk level may well increase through the autumn.
Today Germany's Friedrich-Loeffler-Institut, Federal Research Institute for Animal Health ( FLI ), released a less optimistic analysis of this fall's avian flu risk, calling the risk High. First a short announcement, followed by the link (and an excerpt) from the full report (in German).
14.09.2021 Short messages
Current risk assessment of the FLI
From the end of October 2020 to April 2021, Germany and Europe experienced the largest avian influenza event to date. While the last sporadic outbreaks in poultry were found in Germany on June 25th, 2021 and in Europe on September 6th, 2021, evidence of wild birds was constant over the summer, especially in the Nordic countries of Europe. This shows that, in contrast to earlier entries, the action did not come to a complete standstill.
The Friedrich Loeffler Institute ( FLI ) therefore rates the risk of a recurrence of HPAIV H5 in Europe and Germany in the course of the autumn months as high.
It is urgently recommended that monitoring measures with regard to dead or sick wild birds be further intensified and that biosecurity measures in the poultry holdings be re-examined and, if necessary, improved.
The full risk assessment (Risikoeinschätzung zum Auftreten von HPAIV H5 in Deutschland) is 10 pages in length, and can be accessed at:
Risk assessment of the occurrence of HPAIV H5 in Germany, as of September 13, 2021 ( PDF , accessible)
A (translated) excerpt follows:
The autumnal waterfowl migration has started, and in the coming weeks the bird densities will increase in rest areas. Even if the arrival of arctic geese and swans is not to be expected in Germany until October, the already migrating waterfowl could come out Scandinavia and the Baltic states favor the spread of the circulating viruses.
This increases the likelihood that the viruses already circulating in Western Europe will also spread to Water bird populations in Germany continue to spread. There are also cooler temperatures and Weaker UV radiation, which encourages HPAI viruses to persist in the environment.In addition, the current HPAIV H5N1 outbreaks in the western part of Russia and cases Wild birds near the border with northern Kazakhstan suspect that other viruses may be related with the beginning of the autumn migration of waterfowl in analogy to past epidemics Spread Europe.
Hence will1.) the risk of endemic virus circulation flaring up and2.) the risk of re-entryand the subsequent spread of HPAI H5 virus in waterfowl populations over the course of the year autumn bird migration and the increase in waterfowl densities at gathering points within Germany classified as high.The risk of HPAIV H5 contamination in German poultry holdings and bird populations in zoologicals establishments through direct and indirect contact with wild birds are classified as moderate.
Sporadic outbreaks in Poland in poultry from small-scale farming and currently in Belgium in kept birds are already interpreted as warnings. In areas where HPAI H5 infected wild birds are found or in their vicinity, the risk is higher than in regions in which no HPAI H5-Evidence has been provided in wild birds and no water bird populations are observed.Since the beginning of April, the outbreaks in poultry and kept birds have been falling across Europe, the last one Outbreak in poultry in Germany was on June 25th, 2021 and across Europe in kept birds Luxembourg registered on September 6th, 2021. The latter was related to the outbreak in Belgium and appears to be due to the live poultry trade.
With avian flu, you never know what you are going to get from year-to-year. 2020-21 was the first big uptick in HPAI in Europe since the record setting epizootic of 2016-17. This year could be a bust, or it could become a significant event.
And for more on how avian flu strains from Asia and Europe might make their way back into North America - either now or in the future - you may wish to revisit:
EID Journal: Introduction of Eurasian-Origin Influenza A(H8N4) Virus into North America by Migratory Birds