Preprint: H5N5 virus transmission is driven by repeated cross-Atlantic introductions and transmission among high latitude seabirds

 

#19,399

While most of the world's avian flu attention is focused on HPAI H5N1, in northern Europe, Iceland, and parts of Canada, a different variant - HPAI H5N5 - has been making substantial inroads in recent years, killing birds and spilling over into cats, foxes, seals, and racoons.  

In 2024's Cell Reports: Multiple Transatlantic Incursions of HPAI clade 2.3.4.4b A(H5N5) Virus into North America and Spillover to Mammals, we saw evidence of multiple introductions of H5N5 into North America, most likely by pelagic seabirds.

While only rarely reported in U.S. birds (just 58 out of 20,539 USDA samples), last November H5N5 infected and killed a resident of Washington State following contact with backyard birds.

Last March, in Viral Creep: H5N5 Update, we looked at its evolution as it has spread across the United States; shifting from almost exclusively EA H5N5 prior to last fall, to largely EA/AM H5N5 since November 2025.

 

Based on USDA Data - Graph created with Gemini

The USDA defines these two strains as:

EA = Eurasian; AM = North American; the EA H5 (2.3.4.4) viruses are highly pathogenic to poultry.

EA/AM: reassortant of H5 goose/Guangdong and North American wild bird lineage

Over the past summer, once again we saw several reports (see here,  here and here) of H5N5 in land mammals (polar bears, foxes, walruses) above the arctic circle. 

Although its current threat to humans appears low (see Preprint: A lethal human H5N5 influenza virus isolate exhibits low pandemic risk traits), there's still a lot we don't know about this emerging subtype.

All of which brings us to a new preprint which builds upon earlier reports, and finds evidence that H5N5 is more mobile than previously thought. They also report that it is more commonly detected in shore or seabirds, than in ducks and geese, as is common with H5N1.

The authors suggest that its carriage by seabirds and shorebirds necessitates `alternative surveillance strategies', and warns we may be missing cryptic transmission of the virus.

I've only posted some brief excerpts. Follow the link to read the preprint in its entirety. I'll have a postscript after the break

H5N5 virus transmission is driven by repeated cross-Atlantic introductions and transmission among high latitude seabirds

Clara R Malekshahi, Nicole St. Clair Knobloch, Alvin Crespo Bellido, Alexa Simulynas,  Stephanie Goya, Leigh Papalambros, William J. Fitzsimmons,  Alexander L. Greninger, Adam S. Lauring, Jonathan Runstadler, Martha I. Nelson, Wendy Puryear, Louise H. Moncla

doi: https://doi.org/10.64898/2026.10.29.762228
This article is a preprint and has not been certified by peer review 

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Abstract

In November 2025, the first global H5N5 infection was reported in a person in Washington State. Previously, these viruses had only been detected in seabirds and shorebirds near the Atlantic Ocean, suggesting potentially distinct ecology from dominant H5N1 strains.

Using 354 global H5N5 genomes, we show that H5N5 viruses have been carried by high latitude seabirds and shorebirds across the Atlantic at least 9 times, with introductions more frequent in the fall and spring. We reconstruct a plausible path of cross-continental movement via Arctic or sub-Arctic regions, followed by a year of cryptic circulation prior to human spillover.

These data show that H5N5 viruses have undergone far more intercontinental movements than previously recognized, and exhibit host and geographic patterns that are fundamentally distinct from dominant H5N1 lineages. The distinct host ecology, geography, and movement, paired with the capacity to infect a person, suggest that enhanced surveillance may be warranted.

       (SNIP)

Since its introduction into North America, surveillance and monitoring efforts for HPAI viruses are focused on H5N1 viruses in Anseriformes species. However, our data show that H5N5 viruses have been repeatedly introduced and have persistently co-circulated alongside predominant H5N1 strains, but with wholly distinct ecologies. 

These H5N5 viruses exhibit host distributions and geographic patterns of spread that are markedly different from those of H5N1, necessitating alternative surveillance strategies that are formulated to these lineages.

The close link between H5N5 transmission and seabird and shorebird ecology suggests that these viruses may continue to move across continents and circulate cryptically in these species, while the human infection shows that these viruses could pose spillover risk.

Increased surveillance in wild birds and mammals, as well as monitoring of key locations of repeated viral introductions, would be crucial for the prevention of further spread amongst wild birds and for the prevention of potential spillover into mammals.

        (Continue . . . )

Although this study is limited by small sample sizes, it does suggest that H5N5 may be better established in the environment than previously thought.  Given its persistence in high latitude birds, and repeated spillovers into mammals, it is definitely worthy of our continued attention.