Major Global Migratory Flyways – Credit FAO
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In November of 2014 - just one month prior to the first detection of HPAI H5N8 in North America - the debate over the role of migratory birds in the spread of HPAI H5 was still going strong (see Bird Flu Spread: The Flyway Or The Highway?).Although it had been apparent since the mid-2000s that migratory birds played some role in the spread of HPAI, in 2010 a paper in the British Ecological Society's Journal of Applied Ecology claimed that the global spread of the H5N1 virus through migratory birds was possible . . . but unlikely.
“There is currently no evidence that wild birds are the source of this virus and they should be considered victims not vectors ”
and that “. . . focusing attention on wild birds can misdirect critical resources away from effective disease control and result in negative conservation outcomes and loss of biodiversity.”
To be fair, prior to 2016 evidence of long-term carriage of HPAI viruses by migratory birds was fairly limited. But the first diaspora of HPAI H5 from Southeast Asia to Europe and the Middle East in 2006 provided a pretty good indication that migratory birds played at least some role.
In late 2014 - after H5N8 had been discovered in both Europe and North America - the UN Scientific Task Force on Avian Influenza and Wild Birds released a revised statement which allowed:
Typically, spread of HPAI virus is via contaminated poultry, poultry products and inanimate objects although wild birds may also play a role.By the summer of 2015, H5N8 had vanished outside of Asia (see PNAS: The Enigma Of Disappearing HPAI H5 In North American Migratory Waterfowl), and to everyone's surprise it did not return the following winter to Europe or North America.
The persistence of HPAI H5 infection in migratory birds was still limited.
But over the following summer, H5N8 underwent a significant reassortment which appears to have increased its ability to be carried by migratory birds. By fall we were seeing reports of H5N8 turning up in India, then Kazakhstan, and by late October H5N8 had returned to Europe (see FAO Notification Of H5N8 In Hungary).
Since June 2016, countries in both Europe and Asia have detected infections in wild birds and/or domestic poultry with A(H5N8) including Austria, Croatia, Denmark, Germany, Hungary, India, Israel, Netherlands, Poland, Russian Federation and Switzerland. Many of these recent detections were associated with mortality in wild birds.A chance reassortment (see EID Journal: Reassorted HPAI H5N8 Clade 2.3.4.4. - Germany 2016) had radically changed the virus's behavior; increasing its virulence in some wild birds, while at the same time expanding its avian host range, and giving it unusual environmental persistence through the summer.
Armed with these (and many subsequent) evolutionary changes, today's H5Nx virus is a far cry from the H5N8 virus we were able to contain in 2015 with increased biosecurity and culling.
But we continue to treat H5N1 as primarily a `poultry problem', when in reality, it now permeates our shared ecosystem; which includes wild birds, poultry, cattle, marine mammals, and even peridomestic mammals.
While farm-to-farm spread is still a concern, migratory birds now appear to play the biggest role in the spread of HPAI viruses in North America, which suggests that culling and/or poultry vaccines aren't going to be sufficient to contain the problem.All of which brings us to a lengthy (47-page) preprint written by researchers from the Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania on the dramatic shift in HPAI's ecology in North America.
Complicating matters, HPAI has become widespread in cattle, and may be quietly simmering in other mammalian species (e.g. foxes, skunks, cats, mice, etc.).
`Prevention of agricultural outbreaks may now require novel strategies that reduce transmission at the wild bird/agriculture interface.'
Intensive transmission in wild, migratory birds drove rapid geographic dissemination and repeated spillovers of H5N1 into agriculture in North AmericaLambodhar Damodaran, Anna Jaeger, Louise H Moncla
doi: https://doi.org/10.1101/2024.12.16.628739
This article is a preprint and has not been certified by peer review
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Abstract
Since late 2021, a panzootic of highly pathogenic H5N1 avian influenza virus has driven significant morbidity and mortality in wild birds, domestic poultry, and mammals. In North America, infections in novel avian and mammalian species suggest the potential for changing ecology and establishment of new animal reservoirs.
Outbreaks among domestic birds have persisted despite aggressive culling, necessitating a re-examination of how these outbreaks were sparked and maintained. To recover how these viruses were introduced and disseminated in North America, we analyzed 1,818 Hemagglutinin (HA) gene sequences sampled from North American wild birds, domestic birds and mammals from November 2021-September 2023 using Bayesian phylodynamic approaches.
Using HA, we infer that the North American panzootic was driven by ~8 independent introductions into North America via the Atlantic and Pacific Flyways, followed by rapid dissemination westward via wild, migratory birds.
Transmission was primarily driven by Anseriformes, shorebirds, and Galliformes, while species such as songbirds, raptors, and owls mostly acted as dead-end hosts. Unlike the epizootic of 2015, outbreaks in domestic birds were driven by ~46-113 independent introductions from wild birds, with some onward transmission. Backyard birds were infected ~10 days earlier on average than birds in commercial poultry production settings, suggesting that they could act as early warning signals for transmission upticks in a given area.
Our findings support wild birds as an emerging reservoir for HPAI transmission in North America and suggest continuous surveillance of wild Anseriformes and shorebirds as crucial for outbreak inference. Future prevention of agricultural outbreaks may require investment in strategies that reduce transmission at the wild bird/agriculture interface, and investigation of backyard birds as putative early warning signs.
(SNIP)
Taken together, our findings implicate a critical shift in highly pathogenic avian influenza ecology in North America, with wild birds playing the central role in transmission and dispersal in the 2021-2023 epizootic. Persistent and intensive transmission in wild birds provides an explanation for the rapid cross-continental spread, and continued agricultural outbreaks despite aggressive culling.
Our results highlight the utility of wild bird surveillance for accurately distinguishing hypotheses of epizootic spread, and suggest continuous surveillance as critical for preventing and dissecting future outbreaks.
A little over a century ago French Prime Minister Clemenceau is quoted as complaining that ` . . . generals are always preparing to fight the last war', particularly if they had won that war.Our data underscore that continued establishment of H5N1 in North American wildlife may necessitate a shift in risk management and mitigation, with interventions focused on reducing risk within the context of enzootic circulation in wild birds.
At the time of writing, outbreaks in dairy cattle highlight the critical importance of modeling the ecological interactions within and between wild birds and domestic production. Future work to effectively model viral evolution and spread hinges critically on effective surveillance across wild and domestic species to capture key transmission pathways across large geographic scales. Ultimately, these data are essential for informing biosecurity, outbreak response, and vaccine strain selection.
The tactics of culling and increased biosecurity that successfully ended the 2015 HPAI epizootic after just 7 months have proved inadequate to halt this new and improved avian virus after more than 43 months.
While this strongly suggests we need a new playbook, the only thing that appears to be changing is the virus.
