EM&I: Emergence of a Distinct Evolutionary Cluster of Clade 2.3.4.4b H5 Viruses in China during 2024-2025 with Rapid Evolution and Antigenic Divergence

 

Antigenic Distance Map

#19,403

As we've discussed often over the years, there are scores of genotypes of HPAI H5 clade 2.3.4.4b viruses circulating around the world, and many genetic variations may exist within each genotype. 

Beyond that, there are other subclades of the HPAI H5 virus circulating in places like Cambodia, Bangladesh, and India (see 2025's WHO: Influenza at the Human-Animal Interface Report - Identifies New H5 Clade 2.3.2.1e Infections in Cambodia & Vietnam). 

Simply put, the H5Nx virus currently infecting cattle and humans in Washington state is closely related to - but genetically distinct from - numerous H5 viruses circulating in Europe, Australia, or South Korea. 

Since 2022 more than 100 different genotypes have been identified in the United States alone.  Some are more pathogenic in birds or mammals - while others are more benign - but more are expected to emerge over time. 

Most of these variants are destined to become evolutionary failures; ultimately unable to compete with more biologically `fit' variants already in circulation. 

Over the past 20+ years we've seen hundreds of them appear - sometimes only briefly - before fading away.  Occasionally, however, a new variant will bring something new to the party.

In early 2014 an emerging H5N8 virus in South Korea improved its ability to be carried long distances by migratory birds. In less than a year it had crossed the Pacific and sparked the first major North American epizootic with HPAI H5, and the following year (2016) conquered Europe. 

In 2021 a new and improved H5N1 emerged in Europe and quickly supplanted H5N8, winging its way first to North America, then South America, and then the Antarctic. In 2026, it reached Australia and New Zealand. 

This new H5N1 has shown an increased affinity for infecting mammals, and a much wider avian host range (see DEFRA: The Unprecedented `Order Shift' In Wild Bird H5N1 Positives In Europe & The UK).

Since then, we've seen HPAI H5 increase its ability to infect dairy cattle, sheep, goats, and other ruminants, even as it continues to occasionally spill over into humans. 

Although H5's evolutionary trajectory is both erratic and unpredictable, as long as they circulate, these avian influenza viruses will undoubtedly continue to reinvent themselves. 

All of which bring us to a new research article, published yesterday (Nov 5th) in Emerging Microbes & Infections, which describes the detection of an antigenically distinct cluster of HPAI H5 viruses which appeared in China in 2024-2025 (see distance map at top of blog).

While the authors didn't report any immediately alarming red-flags (e.g. increased virulence or transmissibility, mammalian markers, etc.), this is a reminder that HPAI H5 viruses continues to evolve, and what we say about their threat today may not hold true tomorrow. 

Much of this report is highly technical, and of interest primarily to virologists. Those desiring a deeper dive will want to follow the link to read it in its entirety.  

Research Article

Emergence of a Distinct Evolutionary Cluster of Clade 2.3.4.4b H5 Viruses in China during 2024-2025 with Rapid Evolution and Antigenic Divergence

Xinkui Zhang ,Yujia Yang , Xirong Huang, Shunyin Fang ,Xinyu Han ,Dandan Wei , show all
Article: 2760125 | Accepted author version posted online: 05 Nov 2026
Cite this article https://doi.org/10.1080/22221751.2026.2760125

Abstract

Clade 2.3.4.4b H5 viruses have undergone continuous evolution since their emergence in China, but the extent of genetic and antigenic diversification among recently circulating viruses remains unclear. 

Here, we characterised 18 H5 viruses isolated through routine surveillance in China during 2024-2025, together with publicly available clade 2.3.4.4b H5 sequences collected since 2020. 

Phylogenetic analysis revealed that these isolates formed a distinct evolutionary cluster within clade 2.3.4.4b, designated the G2-derived emerging cluster (G2-EC), which emerged around mid-2024 and exhibited clear genetic divergence from previously circulating H5N1 viruses. 

G2-EC viruses accumulated a characteristic set of hemagglutinin amino acid substitutions, most of which were located in antigenic regions. Across the clade 2.3.4.4b H5 virus population, the estimated mean HA evolutionary rate was higher during 2024-2025 than during preceding periods. 

These genetic changes were accompanied by marked antigenic divergence from the Re14 vaccine strain, with 4- to 32-fold reductions in HI titres. Genomic analysis revealed acquisition of PB1 segment from a distinct genetic background in G2-EC H5N1 viruses, followed by the emergence of H5N6 viruses sharing a highly similar genetic backbone but carrying a distinct NA segment, indicating sequential reassortment. 

Animal experiments showed that representative G2-EC viruses retained high pathogenicity and efficient direct-contact transmission in SPF chickens, while exhibiting relatively limited lethality in mice. Collectively, these findings document a recent evolutionary transition of clade 2.3.4.4b H5 viruses in China, characterised by genetic and antigenic divergence accompanied by sequential reassortment, underscoring the need for continued genomic and antigenic surveillance.

        (SNIP)

Evolutionary rate analysis further suggested that the estimated mean evolutionary rate of the HA gene of clade 2.3.4.4b H5 viruses was higher during 2024–2025 than during the preceding two periods. Together with the emergence of the new evolutionary cluster and the accumulation of multiple characteristic substitutions in its HA protein, this observation suggests that 2024–2025 may represent an important phase of enhanced genetic divergence within the clade 2.3.4.4b H5 virus population in China.

However, the number of HA sequences available for evolutionary rate estimation during this period was relatively limited, which may have increased uncertainty in the posterior estimates and resulted in partial overlap of the 95% HPD intervals among different periods. Therefore, although the estimated mean evolutionary rate was higher during this period, this trend still requires further validation using larger-scale surveillance datasets.

       (Continue . . . )
 

Whether we are seeing the start of a new (and potentially important) branch in HPAI H5Nx's evolution - or simply a flash in the pan - is hard to say. This study is based on just 18 sequenced viruses (15 H5N1, 3 H5N6) acquired from routine surveillance in China. 

The authors wrote:

To date, viruses belonging to this cluster have been detected predominantly in China. Apart from a small number of highly similar sequences identified in environmental samples from Japan in November 2025, no evidence of widespread detection of G2-EC-related viruses has been observed among publicly available H5N1 virus sequences from other countries or regions.

Limited surveillance, along with delays in depositing sequences (see Nature: Lengthy Delays in H5N1 Genome Submissions to GISAID), mean that we are only catching glimpses of H5's evolution.

Which is why we should be prepared for surprises going forward. 

EID Journal: Serologic Evidence of Avian Influenza Virus Exposure in Livestock, Egypt, 2024–2026

#19,402

In the spring of 2024 a mysterious illness spreading among Texas, Kansas, and New Mexico dairy cattle was identified as avian H5N1, an outbreak that has now spread to at least 21 states, and has affected > 1200 herds.  

At that time cattle were believed poorly susceptible to influenza A (see A Brief History Of Influenza A In Cattle/Ruminants), and so testing for H5N1 was delayed.   

The immediate global reaction was that this was an `American problem', caused by a new (and presumably unique to North America) strain (B3.13) of H5N1 (see DEFRA Risk Assessment Of HPAI H5N1 Occurring in Cattle In the UK), and unlikely to occur in Europe. 

This strain has not been detected in the UK, and none of the viruses detected in the UK over the 2020 to 2024 HPAI outbreak period have contained genetic material originating from North or South American viruses. 

 The risk to GB of H5N1 in livestock is assessed at very low. 

That assumption was challenged in June of 2024 (see Germany: FLI Statement On Experimental Infection Of Dairy Cows With European H5N1 Virus) when researchers demonstrated that a German virus isolate could replicate in a cow's udder after experimental infection.

Over the summer of 2024 the USDA reassuringly suggested that B3.13 entered dairy cattle via a single spillover from birds - and was subsequently spread by the interstate transport of infected dairy cows. That assumption held until early 2025 when another genotype (D1.1) turned up in dairy cattle in two states (Nevada & Arizona).

In March of 2025, the UK's Defra reported H5N1 Detected In Domestic Sheep with Mastitis, and over the previous year we'd seen reports of other non-B3.13 H5N1 in goats and sheep in Pakistan, horses in Mongolia, and pigs in Oregon.

Also in the spring of 2025, Chinese scientists warned that Clade 2.3.4.4b HPAI H5N1 influenza Viruses from Birds in China Replicated Effectively in Bovine cells.

Last April, in Preprint: Infection of the Bovine Mammary Gland by Avian H5N1 Subclade 2.3.4.4b Influenza viruses. we saw even more evidence that diverse HPAI H5 strains could infect dairy cattle. The authors wrote:

Our findings show that mammary tissue from common cattle breeds is permissive to clade 2.3.4.4b H5N1 infection. These results support the view that bovine mammary epithelium represents a susceptible tissue environment and suggest that IAV infection in cattle may be more widespread than previously assumed.

Four months ago, in Eurosurveillance: Detection of antibodies against avian influenza in European dairy cattle, the Netherlands, January 2026), we saw additional evidence of HPAI spillover into dairy cattle outside of the United States. 

A little over a a year ago, WOAH embraced an 11-page OFFLU technical document (see OFFLU Guidelines for High Pathogenicity Avian Influenza Virus Risk Mitigation in Cattle) which - among other things - finally made HPAI in cattle a `reportable disease'.

But surveillance remains passive, and testing largely up to the discretion of the livestock owner. Something that many farmers find unappealing, fearing the quarantining of their herds and the stigma of infection.

Today we've an EID Journal research letter which reports compelling serological evidence not only of HPAI H5 infection in Egyptian cattle, but LPAI H9N2 as well.  The authors write:

Our findings indicate that spillover of avian influenza viruses to cattle and other livestock is not geographically restricted or caused by certain genotypes of H5N1 virus.

Testing included both hemagglutination inhibition (HI) and microneutralization (MN) assays, and positive results weren't confined to cattle.  Sheep and goats tested positive for LPAI H9N2 as well.  

I've only posted some excerpts from this letter. Follow the link to read it in its entirety. I'll have a postscript after the break.

Volume 32, Number 12—December 2026
Research Letter
Serologic Evidence of Avian Influenza Virus Exposure in Livestock, Egypt, 2024–2026

Mokhtar Gomaa, Rabeh El-Shesheny, Ahmed El Taweel, Omnia Kutkat, Hager Emam, Basant Helal, Ayat Swelam, Mohamed GabAllah, Ahmed Kandeil, Pamela P. McKenzie, Richard J. Webby, Mohamed Ahmed Ali, and Ghazi Kayali

Abstract

Spillover of influenza A(H5N1) virus to cattle in the United States sparked global concern. We investigated whether endemic avian influenza viruses are potentially spilling over into domestic livestock in Egypt. We detected influenza H9N2 antibodies in 2.5% and H5N1 antibodies in 2.9%–4.5% of tested animals. Additional surveillance will elucidate host infection dynamics.

       (SNIP) 

Clade 2.3.4.4b H5N1 virus and low-pathogenicity avian influenza A(H9N2) virus are widely and endemically circulating in poultry in Egypt on farms and backyards and at live bird markets (7). Subtype H5N1 was detected in feral rats in poultry production areas (8). In addition, reassortment events between the H5N1 and H9N2 subtypes led to the emergence of novel clade 2.3.4.4b influenza A(H5N2) viruses (9). To determine whether those viruses are spilling over to livestock in Egypt, particularly in mixed-species settings, we conducted a serosurveillance study in northern Egypt.

During October 2024–January 2026, we collected 444 serum samples from 33 conveniently selected small farms (≈10 cattle, no mixed species) and backyards housing cattle (1–3 cattle with mixed species) in 3 governorates (n = 155, n = 59, and n = 230) in northern Egypt, representing common practices in Egypt. Sheep and goats were also sampled if housed in the same backyards as cattle. All sampled animals were apparently healthy adults.

We tested serum samples for antibodies against a 2025 Egyptian clade 2.3.4.4b H5N1 (A/duck/Egypt/BA20361C/2022) virus using hemagglutination inhibition (HI) and microneutralization (MN) assays and against a G1-like Egypt influenza A(H9N2) (A/chicken/Egypt/RA20191OP/2022) virus by HI in accordance with World Health Organization protocols (10). We considered an antibody titer >1:40 as positive for both assays. Data on the number of cows, presence of other mammals (e.g., donkeys and horses), and presence of avian species (poultry and/or wild birds) were available for 271 samples. We performed statistical comparisons with χ2 or Fisher exact tests; we considered p<0.05 statistically significant.

Overall, 11 (2.5%) serum specimens tested positive for influenza A(H9N2) antibodies, 20 (4.5%) were positive for influenza A(H5N1) antibodies by HI, and 13 (2.9%) were positive for influenza A(H5N1) antibodies by MN (Figure, panel A). Positive titers were 40–160 (Figure, panel B). None of the samples tested positive for antibodies against both viruses. Of the 20 H5N1 virus specimens positive by HI, 12 tested positive by MN; 1 specimen was negative by HI and positive by MN. All influenza H9N2-positive specimens were from 3 locations in the same governorate (p = 0.005). Influenza A(H5N1) antibodies were detected in 2 governorates; the differences in detection rates by governorate were statistically significant (0%, 1.3%, and 4.8%; p = 0.049). All H5N1 virus–seropositive specimens were from cattle (Table).

We detected H9N2 antibodies in 6/374 (1.6%) of cattle specimens, 1/31 (3.2%) of goat specimens, and 4/39 (10.3%) of sheep specimens (p = 0.004). Presence of other animals did not predict seropositivity. Small farms had a significantly higher proportion of H5N1 virus–positive animals tested by MN (p = 0.039).

Our findings indicate that spillover of avian influenza viruses to cattle and other livestock is not geographically restricted or caused by certain genotypes of H5N1 virus. Clade 2.3.4.4b H5N1 virus of the EA-2021-AB genotype and G1-like H9N2 virus appear to be spilling over into livestock in Egypt; however, because our evidence is serologic, we could not ascertain whether true infection is occurring and whether the same signs seen in the United States are common in cattle in Egypt. 

Even so, the evidence is a strong indicator that spillover is occurring, similar to that reported in Europe (4,6). Furthermore, spillover is not restricted to cattle; some sheep and goats tested in our study were seropositive for influenza A(H9N2) virus. Surveillance for avian influenza viruses in Egypt and other countries, especially those in which avian influenza is endemic, will verify infection and provide data on frequency, routes of transmission, and factors affecting infection of and transmission to livestock.

       (Continue . . . )
 

Despite the accrued evidence to date -  for a myriad of political, societal, economic, and logistical reasons - governments around the world appear slow to respond to HPAI (and now LPAI) spillover into mammalian livestock.   

We see this same reluctance in investigating HPAI spillover into other species, including farmed mink (see Virological (Referral): Preliminary report on H5N1 highly pathogenic avian influenza in Utah mink, August 2026).

Although this laissez faire approach may work out in the short run, it provides these viruses with ample opportunities to spread, to evolve, and to adapt. 

Admittedly, most of these viruses will likely never pose a serious public health threat. 

But it only takes one overachiever - aided and abetted by inaction on our part - to plunge the world back into another pandemic crisis. 

NOAA Climate Prediction: West Coast Atmospheric River Event (Nov 10th-16th)

 

NOAA Forecast Issued Nov 3rd

#19,401

As we discussed 2 months ago in #NPM26: Preparing For An El Niño Winter, the west coast and the southern tier of states have been warned to expect an unusually wet and stormy winter due to the record Pacific El Niño. 

Predicting exactly where unusual or severe weather will occur this winter   is impossible, but generally, the jet stream across the United States shifts storm tracks to the southern tier of states during an El Niño winter.

Parts of Florida have already seen record flooding in the past week, but it appears that next week Northern California and the Pacific Northwest will be faced with a substantial atmospheric river event; one which could bring heavy rain and snow to much of the west coast. 

 

NOAA's Climate Prediction Center published the following warning yesterday:

Key Messages from the NOAA Climate Prediction Center
Issued November 2, 2026 and Updated November 3, 2026

Atmospheric River Expected to Affect the West Coast States

Atmospheric river activity is expected to pull significant amounts of moisture into much of the West Coast during the second week of November and weekend, with a potential secondary moisture surge later in the period. Strong surface low pressure just north of the region will also likely drive high winds along the immediate coastline.

(a) A moderate risk (40 to 60%) of heavy precipitation and possible flooding is indicated for Nov 11 to 15 across parts of the Pacific Northwest and northern/central California, while a slight risk (20 to 40%) extends to Nov 17 across the West Coast into the Desert Southwest, Great Basin, and Southern Rockies. High winds are possible across much of the West, including a moderate risk along the immediate West Coast Nov 11 to 15 and a broader slight risk extending to Nov 17.

(b) A moderate risk of heavy snow is indicated across the higher elevations of the Klamath and Sierra Nevada Nov 11 to 15. Slight risks of heavy snow extend to Nov 17 for high elevations of the Cascades, Klamath, and Sierra Nevada, as well as the Northern Intermountain, Great Basin, and Central and Northern Rockies.

Monitor local forecasts for safety alerts, avoid driving through flooded roadways, and use caution if traveling through high elevations. Be prepared for power outages due to high winds.

This is likely just the first of many such storms expected to impact the west coast this winter, and inland freshwater flooding is a major concern.  Six weeks ago, in anticipation of these events, California Declared a State of Emergency to Bolster Statewide El Niño Preparedness. 

The LA County website is urging both community and individual preparedness for what is expected to be a difficult winter, and advises:

LA County is preparing its flood control and emergency response systems now. Crews are inspecting and maintaining dams, debris basins, channels, storm drains, and pump stations; monitoring burn areas, hillsides, and other areas at risk of flooding and debris flows; and preparing for evacuations, emergency shelter, and other storm response. County teams are also preparing to assist people experiencing homelessness, older adults, and others who may need additional help during severe weather.

Here’s what residents can do now:

• Sign up for emergency alerts: Go to alert.lacounty.gov to receive emergency notifications, including evacuation warnings and orders.
• Know the risks where you live: If you are near a hillside, canyon, recent burn area, or area that can flood, learn your flood and mudflow risk before a storm.
• Make an evacuation plan: Know where you will go and how you will get there if you need to leave quickly.
• Have supplies ready: Keep enough food, water, medications, and other essentials for up to 10 days.
• Prepare your home: Clear gutters, downspouts, and storm drains, fix roof leaks, and get sandbags before heavy rain is in the forecast.
• Consider flood insurance: Standard homeowners insurance does not cover flood damage. Flood insurance is separate and can take at least 30 days to go into effect.
• Need help with mud or debris flow risk? Call LA County Public Works at 1-800-933-0930 to speak with an engineer.
• Know someone experiencing homelessness who needs help? Visit homeless.lacounty.gov/get-help/

While this first round is expected to impact further north, now is the time for everyone along the west coast - and much of the southern United States - to prepare for severe storms and elevated flood risks. 

Those interested in diving into one of the more extreme (albeit, based on  historic events) scenarios of Atmospheric river flooding may wish to revisit 2022's California's ARkStorm Scenario Revisited & National Preparedness Month, but even a far less severe event could be devastating. 

Whether it's coastal storms,  inland flooding, or ice storms or tornado outbreaks in the southland, El Niño has the potential to adversely affect millions of Americans this winter (and potentially billions worldwide). 

So, if the power goes out today for a week, or if flood waters begin to rise . . . do you already have?

• A battery operated NWS Emergency Radio to find out what was going on, and to get vital instructions from emergency officials
• A decent first-aid kit, so that you can treat injuries
• Enough non-perishable food and water on hand to feed and hydrate your family (including pets) for the duration
• A way to provide light when the grid is down.
• A way to cook safely without electricity
• A way to purify or filter water
• A way to handle basic sanitation and waste disposal.
• A way to stay cool (fans) or warm when the power is out.
• A small supply of cash to use in case credit/debit machines are not working
• An emergency plan, including meeting places, emergency out-of-state contact numbers, a disaster buddy, and in case you must evacuate, a bug-out bag
• Spare supply of essential prescription medicines that you or your family may need
• A way to entertain yourself, or your kids, during a prolonged blackout

If not, you've got some important work to do.

A good place to get started is by visiting Ready.gov, or by hitting this link which will serve up a series of my preparedness blogs.

When Flu Viruses Collide

 

Antigenic `Shift' or Reassortment 

#19,400 

Influenza co-infections in humans - or in any other susceptible host (birds, pigs, cattle, sheep, etc.) - are a public health concern because they can allow two (or more) influenza viruses to swap genetic material (reassort) and produce a hybrid virus.

It's more than just a theoretical concern, as twice in my lifetime (1957 & 1968) a reassortment between seasonal flu and an avian flu virus - possibly in a human host - produced a pandemic virus. 

• The first (1957) was pandemic H2N2, which According to the CDC  emerged after three different genes from an H2N2 avian influenza A virus combined with 5 genes from the existing H1N1 virus.

• In 1968 a novel H3N2 virus emerged (a reassortment  consisting of 2 genes from a low path avian influenza H3 virus, and 6 genes from seasonal H2N2) which continues to spark yearly epidemics more than 58 years later.

While we'll never know exactly where or how these novel viruses came about, the genetic fingerprints of reassortment are unmistakable. This is the reason why yearly flu vaccination has long been recommended for people who raise pigs or work with poultry and the CDC now urges vaccination in dairy workers.

Coinfection is rare, but not unheard of, and as any virologist will tell you; Shift happens.

With three recent dairy-linked H5N1 cases (1 fatal) reported in Washington state - and previous evidence suggesting mild and/or asymptomatic cases may go unreported (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers), the arrival of the fall flu season raises concerns over the possibility of seeing a rare coinfection.  

In addition to vaccination, the CDC also promotes (see Reducing Exposure for Workers to Avian Influenza A Viruses) the use of PPEs, but it isn't clear how many are following this advice.

Since 2009, with increased surveillance and better tools, we've seen a growing number of reports of human co-infection with influenza A:

EID Journal: Nosocomial Co-Transmission Of H7N9 & H1N1pdm09

EID Journal: Human Co-Infection with Avian and Seasonal Influenza Viruses, China

Lancet: Coinfection With H7N9 & H3N2

But evidence of the generation of `biologically fit' reassortant viruses from a human co-infection remain rare.

• In 2025's Eurosurveillance: Novel influenza A(H1N2) Seasonal Reassortant Virus Identified in a Patient, Sweden, April 2025 we saw a rare reassortment between seasonal H1N1 and seasonal H3N2.

• In April of 2020, during the opening of the COVID pandemic, we saw an MMWR Report on a Seasonal H3N2 & H1N1pdm09 Reassortant Infection - Idaho, 2019), involving a 13 year-old boy.

• In 2015's J Clin Virol: Influenza Co-Infection Leading To A Reassortant Virus, we looked at co-infection in a Cambodian patient with both seasonal H3N2 and H1N1pdm09 which led to a new, reassortant H3N2 virus.
• And in 2011 we saw a similar co-infection in a 16 month old boy in Canada that led to the creation of a unique hybrid reassorted virus (see Webinar: pH1N1 – H3N2 A Novel Influenza Reassortment).  

In 2024, Chinese scientists looked at the reassortment potential of H5N1 and H3N2 (see Preprint: Intelligent Prediction & Biological Validation of the High Reassortment Potential of Avian H5N1 and Human H3N2 Influenza Viruses) and predicted the risks to be relatively high.

Another, older (2010) study (see PNAS: H3N2 And H5N1 Reassortment) found that Reassortment between avian H5N1 and human H3N2 influenza viruses creates hybrid viruses with substantial virulence.

While it is difficult to judge the probability of H5N1 successfully swapping genes with H1N1, I've not seen any studies that have shown that these viruses cannot reassort.

Of course, reassortment with seasonal flu doesn't have to happen in a human.  Swine, cattle, and other species can be susceptible to both human and avian flu. 

Just a few of many scenarios

While no one can predict whether H5Nx and seasonal flu will ever hook up and produce a viable threat, there is historical precedence. Which makes anything we can reasonably do to reduce that risk worth doing. 

Simple things; like getting the seasonal flu vaccine, wearing PPEs when in high-risk contact with dairy cattle and poultry, and not consuming raw milk could go a long way in reducing those risks. 

Of course, whether we can be bothered to take these types of proactive steps, remains to be seen. 

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 

Preview PDF

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.

Washington Adds 6 More H5N1 Infected Herds (n=40) - Indiana Reports 1st Infected Herd

 

#19,398

As the USDA dashboard indicates (above), 6 more dairy herds have been reported as being infected with H5N1 in Washington state, bringing to 40 the number identified since October 8th.  

All but one have been reported from Yakima County.

Although it is not yet reflected in the USDA dashboard, the USDA (h/t Maggie Fox on Twitter/X) has posted the following statement on Indiana's first detection of HPAI H5N1 B3.13 in a dairy herd. 

USDA Confirms Highly Pathogenic Avian Influenza in a Dairy Herd in Indiana

Contact: aphispress@usda.gov

Milk Supply Safe; Considered Low Risk to Human Health and Safety

WASHINGTON, D.C., November 2, 2026—Today, the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) National Veterinary Services Laboratories (NVSL) confirmed via PCR (polymerase chain reaction) detection of highly pathogenic avian influenza (HPAI) H5N1 genotype B3.13 in a dairy cattle herd in Indiana. This detection was a result of routine National Milk Testing Strategy testing, not pre-movement surveillance.

This marks the first reported case of HPAI in cattle in Indiana. While dairy cattle in a total of 21 states have been infected since the start of the outbreak in March 2024, APHIS has seen cases in previously unaffected states this year. APHIS is working closely with the Indiana State Board of Animal Health to conduct additional on-farm investigations, diagnostic testing, and epidemiological data collection to better understand the detection and prevent further disease spread.

The detection does not change USDA’s HPAI eradication strategy. Biosecurity is still key to mitigating the risk of disease introduction or spread between premises. APHIS recommends enhanced biosecurity measures for all dairy farms. Producers should immediately report any livestock with clinical signs, or any unusual sick or dead wildlife, to their state veterinarian.

This detection does not pose a risk to consumer health or affect the safety of the commercial milk supply. The U.S. Food and Drug Administration is confident that pasteurization is effective at inactivating HPAI virus, and that the commercial, pasteurized milk supply is safe. Dairies are required to send only milk from healthy animals into processing for human consumption; milk from impacted animals is being diverted from the commercial milk tank or destroyed so that it does not enter the human food supply.

According to the U.S. Centers for Disease Control and Prevention (CDC), HPAI viruses circulating in birds and U.S. dairy cattle pose a low risk to the general public in the United States. However, people who have job-related or recreational exposures to infected birds or mammals are at higher risk of infection and should take appropriate precautions outlined in CDC guidance

To recap:

• In addition to 40 infected herds in Washington  
• There have been 3 confirmed human infections in Washington state (1 fatal) 
• The first H5N1 infected herd reported by Colorado in nearly 2 years
• and now, this first spillover in Indiana.

All in the space of 3 weeks.