Viral Creep: H5N5 Update

 

Cell Reports: Multiple Transatlantic Incursions of HPAI 

clade 2.3.4.4b A(H5N5) Virus into North America and

 Spillover to Mammals (2024)

#19,098

While H5N1 remains the overwhelmingly dominant HPAI H5 subtype reported around the globe, it wasn't so very long ago (2014-2020) that HPAI H5N8 was the preeminent strain, and we continue to see other subtypes bubbling up around the globe. 

• Last month, in South Korea: H5N9 Rising, we looked at concerns over the arrival this winter of a triad of HPAI H5 viruses in Korea; H5N1, H5N6, and H5N9.

• In Mexico, HPAI H5N2 has emerged, infecting both humans and poultry (Preprint: Emergence of a Novel Reassorted HPAI A(H5N2) Virus Associated with Severe Pneumonia in a Young Adult).

• And in both Europe and in Eastern Canada, we've seen sporadic reports of HPAI H5N5 (see Cell Reports: Multiple Transatlantic Incursions of HPAI clade 2.3.4.4b A(H5N5) Virus into North America and Spillover to Mammals) since 2023.

Prior to 2025, H5N5 had only rarely been reported in the United States; mostly in seabirds along the Atlantic coast. Of 18 reports to the USDA, 16 had come from Massachusetts, 1 from Maine, and 1 from New York.  

Based on USDA Data - Graph created with Gemini

While the number of H5N5 detections in the United States remains small, we've witnessed several notable changes in the behavior and ecology of the virus over the past 12 months, not the least of which was the first (fatal) human infection reported last November in Washington state. 

While details remain scant, the local authorities reported:

The person had a backyard flock of mixed domestic birds. DOH testing identified avian influenza virus in the environment of the flock, making exposure to the domestic poultry, their environment, or wild birds the most likely source of exposure for this patient

This human infection was all the more surprising since the virus has never been reported this far west in wild or migratory birds.  Even in Canada, the virus has only been reported (once) as far west as Saskatchewan.

Most of our information comes from the USDA's Detections of HPAI in Wild Birds, which added 26 new detections in 2025, and 2 so far in 2026. 

Based on USDA Data - Graph created with Gemini

The most notable change - starting last summer -  is abrupt shift of detections from the Atlantic seaboard to the Central and Mississippi Flyways (Wisconsin, Iowa, Missouri), with outliers in Florida, Pennsylvania, Mississippi, and Virgina.

While the virus had been primarily reported in coastal and seabirds in 2023-2024, we have also seen a decided shift towards migratory waterfowl (Canadian & Snow Geese) in the latter half of 2025 and in early 2026. 

Based on USDA Data - Graph created with Gemini

At the same time we've seen a shift in the USDA's strain characterization - going from exclusively EA H5N5 prior to last fall, to EA/AM H5N5 since November. 

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

It is admittedly difficult to come to any conclusions about the trajectory of H5N5 based on the limited data we have. By all accounts, it remains a minor - albeit persistent - player in the avian flu world. 

But it has demonstrated its zoonotic potential, infecting both humans and other mammalian species, and it continues evolve - reassorting with North American LPAI viruses - as it expands both its geographic and avian host ranges. 

All of which makes HPAI H5N5 well worth our continued attention.