Abrupt Shift From H5N8 to H5N1 in 2020
#18,247
As recently as a decade ago, the ability of HPAI H5 viruses to be carried long distances by migratory birds was still being hotly debated, with many experts claiming birds were being falsely accused (see Bird Flu Spread: The Flyway Or The Highway?).
While some outbreaks were clearly driven by migratory birds, until recently most birds either succumbed to the virus - or cleared the infection quickly - limiting their ability to carry the virus over long distances (see PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl).
By 2016, the H5N8 virus had overtaken H5N1 as the dominant subtype, and while it was more adept at hitching rides on migratory birds, it showed few signs of becoming a zoonotic threat. H5N1 waned, as H5N8 continued to spark major epizootics in North America (2015), in Europe (2016-17), and crossed the African equator in 2017.
But as the graphic at the top of this blog illustrates, in 2020 H5N8 gave way to a new, and improved, H5N1 virus. One that retained (and improved upon) H5N8's ability to spread by wild birds, but was suddenly more adept at infecting mammals (including humans).
In 2021 HPAI H5N1 went from being a seasonal epizootic threat primarily in Europe and Asia to being a frequent (Pacific & Atlantic) ocean crossing traveler to North America. And unlike in the past - while the frequency of outbreaks dropped during the summer - the virus never really went away.
In 2022 we also saw a report from Defra on The Unprecedented `Order Shift' In Wild Bird H5N1 Positives In Europe & The UK, with many species of birds that were previously unaffected, suddenly dying from - or carrying - they H5N1 virus.
A year later, the virus spread into South America, and has been reported in both Antarctica and above the Arctic circle (see map sequence below).
Numbers are impossible to quantify, but hundreds of millions of wild birds, hundreds of millions of captive birds, and hundreds of thousands - perhaps millions - of wild mammals have succumbed to the HPAI H5 virus over the past 5 years.
A few past blogs on this carnage include:
Travel Med. & Inf. Dis.: Pacific and Atlantic Sea Lion Mortality Caused by HPAI A(H5N1) in South America
EID Journal: Recent Changes in Patterns of Mammal Infection with Highly Pathogenic Avian Influenza A(H5N1) Virus Worldwide
EID Journal: Mass Mortality of Sea Lions Caused by HPAI A(H5N1) Virus (Peru)
While surveillance and reporting is spotty at best, we are arguably witnessing the largest, most diverse, and widespread epizootic in human history.
Even if it turns out that H5N1 doesn't have what it takes to spark a human pandemic, the damage it has done (and continues to do) to our shared ecosystem is incalculable, and the knock-on effects of these losses may not be fully appreciated for years.
All of which brings us to an excellent 48-page (preprint) review examining the changing role of wild birds in the spreading of HPAI H5. Due to its length, I've only provided the link, abstract, and a brief excerpt from the discussion.
Follow the link to read it in its entirety. I'll have a postscript after the break.
The role of wild birds in the global highly pathogenic avian influenza H5 panzootic
M Couty, C Guinat, D Fornasiero, FX Briand, PY Henry, B Grasland, L Palumbo, GL Loc'harXiv preprint arXiv:2504.11910, 2025•arxiv.org
The highly pathogenic avian influenza (HPAI) H5 clade 2.3.4.4b has triggered an unprecedented global panzootic. As the frequency and scale of HPAI H5 outbreaks continue to rise, understanding how wild birds contribute to shape the global virus spread across regions, affecting poultry, domestic and wild mammals, is increasingly critical.
In this review, we examine ecological and evolutionary studies to map the global transmission routes of HPAI H5 viruses, identify key wild bird species involved in viral dissemination, and explore infection patterns, including mortality and survival.
We also highlight major remaining knowledge gaps that hinder a full understanding of wild birds role in viral dynamics, which must be addressed to enhance surveillance strategies and refine risk assessment models aimed at preventing future outbreaks in wildlife, domestic animals and safeguard public health.
(SNIP)
Importantly, this panzootic has affected an unprecedented diversity of wild bird species, including many not previously reported infected by HPAI H5 viruses27, raising serious conservation concerns.
Colonial breeding species, such as seabirds or terns, have suffered mass mortality events, resulting in sharp population declines and probable disruptions of colony social structures. Long-lived species with low reproduction rates, restricted geographic ranges or threatened status are particularly at risk, as outbreaks may have caused long-term effects on their populations.
The breadth of affected species also raises broader ecological concerns, suggesting an expansion of the virus’s host range and the emergence of novel wild bird reservoirs capable of sustaining viral circulation. This may help explain the unusual persistence of HPAI H5 viruses throughout the year in multiple regions—a major difference from patterns observed in previous epizootics—although the underlying mechanisms remain poorly understood158.
Increased viral circulation has also led to repeated spillover events into poultry1,28, and into both domestic and wild mammals3–6. Both terrestrial and marine mammals have been affected, including large-scale mortality events among marine mammals in South America6,23,171.
Reports of probable mammal-to-mammal transmission raise additional concerns about the virus’s adaptation to new hosts, including humans3,4,6,23. These developments underscore the urgent need for continued research and international collaboration to mitigate the impacts of HPAI H5 viruses on wildlife, poultry and public health, while enhancing preparedness for future epizootics.
Nearly two years ago, in Avian Flu's New Normal: When the Extraordinary Becomes Ordinary, I wrote about the numbing effect that comes with the constant barrage of HPAI H5 reports from around the world.
Events that were nearly unthinkable four years ago (e.g. Repeated trans-Atlantic introduction of avian flu from Europe, the spread of HPAI H5 across the length of South America, numerous spillovers of H5 into mammalian species, and > 77 human cases in the U.S. ) have somehow become routine.
While the future course and impact of HPAI H5 remains unknown, HPAI's recent trajectory represents an escalation of its threat level, and we'd do well to take that seriously.
