Virus Research: Genetic Diversity of Clade 2.3.4.4b H5Nx High Pathogenicity Avian Influenza Viruses Detected in Korea During the 2025–2026 Winter Season and Pathogenicity of H5N1 and H5N9 Viruses

 

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Last December - in addition to reporting numerous biosecurity breaches on farms - South Korea's MAFRA Reported Increased Infectivity & Pathogenicity of This Year's Avian Flu Strains, which - for the first time - included 3 HPAI strains (H5N1, H5N6, and H5N9). 

Of particular concern, their Animal and Plant Quarantine Agency conducted an evaluation of the infectivity and pathogenicity of one highly pathogenic avian influenza virus (serotype H5N1) and found that the infectivity was more than 10 times higher than in previous years.

Details at the time were understandably scant, but today we've a research article which sheds new light on the panoply of viruses behind last year's difficult avian flu season in South Korea. 

While this is a fairly technical report, and the use of regional nomenclature for H5N1 genotypes further complicates matters, the gist is pretty simple. 

Last year South Korea was visited by a diverse panoply of H5Nx genotypes (n=16), with at least two -  H5N9 N9-G2 and H5N1 25G2 - displaying unusually high infectivity in chickens.

H5N1 25G2 was also deadly to ducks, whereas H5N9 N9-G2 spread efficiently in ducks, albeit without producing serious illness. 

H5N1 25G2 was the source of nearly half (n=30) of last year's 62 South Korean poultry outbreaks, but it was only detected in environmental samples once, making its prevalence in wild birds unknown. 

Whether 25G2 (or H5N9 N9-G2) will return to South Korea this fall - or turn up in other regions - remains to be seen.   

Due to its technical nature, I've only provided the link, abstract, and a few excerpts.  Follow the link to read the report in its entirety. 

Genetic Diversity of Clade 2.3.4.4b H5Nx High Pathogenicity Avian Influenza Viruses Detected in Korea During the 2025–2026 Winter Season and Pathogenicity of H5N1 and H5N9 Viruses

Yunyueng Jang, Ra Mi Cha, Min-Ji Park, Jong-Min Kim, Eui Hyeon Lim, Gyeong-Beom Heo, Se-Hee An, Bina Lee, Hyun-ji Seo, Kwang-Nyeong Lee, Youn-Jeong Lee, Eun-Kyoung Lee

10.1016/j.virusres.2026.199807

PDF

Highlights

• During 2025-2026, clade 2.3.4.4b H5N1, H5N6, and H5N9 HPAIVs were detected in Korea, causing 62 H5Nx HPAI outbreaks in poultry, and 63 cases in wild birds.
• Ten genotypes (24G1, 25G2–25G7, and N9-G1–3) identified in poultry, among which 25G2 was the dominant genotype.
• Most genotypes were reassortants containing gene segments derived from East Asian clade 2.3.4.4b H5N1 HPAIVs since 2022 and LPAIVs from wild birds.
• The 25G2 and N9-G2 genotype showed high infectivity in chickens with 103.3 and 103.2 EID50 of LD50 values, respectively.
• The 25G2 caused rapid duck mortality, whereas H5N9 caused asymptomatic infection with efficient transmission.

ABSTRACT

Clade 2.3.4.4b H5N1, H5N6, and H5N9 high pathogenicity avian influenza viruses (HPAIVs) were detected in poultry and wild birds during the 2025–2026 winter season in Korea. During this period, 62 H5Nx HPAI outbreaks occurred in poultry, and 63 cases were reported in wild birds.

Genome constellation analysis revealed substantial genotype diversity. Most H5N1 and H5N9 genotypes were reassortants containing gene segments derived from East Asian clade 2.3.4.4b H5N1 HPAIVs since 2022 and low-pathogenicity avian influenza viruses from wild birds. 

Notably, the European H5N1 genotype EA-2024-DI.2.1, which spread rapidly across Europe in 2025, and its reassortants were identified in Korea. The novel poultry genotypes were also detected in wild birds, supporting wild bird to poultry spillover, whereas one genotype detected in previous season was identified in small-scale poultry farms.

H5N1 (C588) of major 25G2 genotype in poultry and H5N9 (C722) of N9-G2 genotype showed high infectivity in chickens with 103.3 and 103.2 EID50 of median lethal dose (LD50) values, respectively. The predominance of the 25G2 genotype in poultry may have been influenced by efficient infection and virus introduction into high-density poultry production areas, potentially facilitating horizontal transmission under field conditions.

In young ducks, H5N1 caused rapid mortality, whereas H5N9 induced limited clinical signs without mortality. Both viruses were transmitted to all contact ducks, increasing the risk of transmission to poultry. Despite intensified surveillance and early detection efforts to limit virus spread, H5Nx HPAIV outbreaks occurred during the 2025–2026 winter season. These findings highlight the need for continued surveillance, together with rapid genomic and pathogenic characterization of newly introduced viruses.

(SNIP)

Genotype 25G2 was the predominant genotype detected in poultry during the 2025–2026 season, accounting for 30 outbreaks. However, it was identified in only one environmental sample collected during an epidemiological investigation of migratory bird habitats. This outbreak pattern differed from those observed during previous seasons in Korea, in which the predominant genotypes were generally similar between wild birds and poultry (Cha et al., 2023; Cha et al., 2025; Cha et al., 2026). 

The predominance of genotype 25G2 in poultry may have been influenced by higher infectivity under field conditions, particularly in layer farms located in high-density poultry farming areas where HPAI outbreaks were concentrated. However, introduction through environmental contamination associated with wild birds cannot be excluded, and the potential influence of sampling bias resulting from spatial and temporal variations in bird migration and surveillance should also be considered (Hayes et al., 2025; Llanos-Soto, Yaffy, Pavlak and Ivanek, 2025).

This study revealed multiple introductions of genetically diverse H5 viruses during the 2025–2026 winter season and demonstrated their relatively high infectivity in chickens. Epidemiological investigations of concurrent outbreaks, together with virus detection and genetic analyses, helped identify potential sources of virus introduction in the affected regions and provided evidence of viral contamination in the surrounding environment. Despite enhanced surveillance and early detection efforts to limit virus spread, H5 HPAI outbreaks still occurred in poultry farms. These findings highlight the importance of continued surveillance in wild birds and poultry, coupled with the rapid genomic and pathogenic characterization of newly emerging viruses, to support the implementation of effective avian influenza control disease.

        (Continue . . . )

ECDC/EFSA Quarterly Avian Influenza Overview June-August 2026

 
2025-2026: HPAI's Biggest Surge in Wild Birds to Date (Europe)

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As the above EDC/EFSA chart illustrates, after a couple of lackluster years (2023-2025) avian flu returned to Europe with a vengeance last year. We saw similar upticks in places like South Korea, and North America, but reporting from large swaths of the world - particularly in Asia and Africa - is often lacking (see FAO graphic below).

Every 3 months the ECDC/EFSA publishes a highly detailed avian influenza surveillance report, and while they tend to be EU centric, in its 67 pages you'll find ample coverage of outbreaks and infections from around the world on a wide variety of avian subtypes.

While today's report covers the slowest part (June-August)  of the year, these highly detailed quarterly reports make excellent reference material, and are well worth perusing.

This edition does contain one milestone, however; the long-dreaded arrival of HPAI to Australia/New Zealand, which was first reported in June.  I've posted the Abstract, and a few excerpts below. 

I'll have a brief postscript after the break.

Abstract 

Between 5 June and 28 August 2026, 110 highly pathogenic avian influenza (HPAI) A(H5N1) virus detections were reported in domestic (7) and wild (103) birds in 12 countries in Europe.The number of detections remained at a seasonal low throughout the summer, continuing the decline observed since spring. In contrast to previous years, fewer colony-breeding seabirds were affected, and the geographical range of detections was more limited. Further sporadic detections of HPAI A(H5) virus were reported in wild terrestrial carnivores and pinnipeds.

Outside Europe, the epidemic continued in the Americas, where also the HPAI A(H7N3) subtype was detected in Mexico. Following its first introduction to mainland Australia, HPAI A(H5N1) virus spread within local wild bird populations and spilled over to terrestrial carnivores and marine mammals. In the US, the number of dairy cattle farms reportedly affected by HPAI A(H5N1) increased, while detections in captive American minks were reported for the first time.

Between 5 June and 31 August 2026, 15 cases of avian influenza virus infection were publicly reported in humans (no fatal cases) in three countries and territories: Bangladesh (one A(H5N1) case), Cambodia (one A(H5N1) case) and China (13 A(H9N2) cases). All human cases reported exposure to poultry or a poultry environment prior to detection or onset of illness. Human infections with avian influenza viruses remain rare and no sustained human-to-human transmission has been documented.

The risk posed by avian influenza A(H5N1) clade 2.3.4.4b viruses currently circulating in Europe remains low for the general public in the European  Union/European Economic Area (EU/EEA) and low-to-moderate for those occupationally or otherwise exposed to infected animals or contaminated environments.

©2026 European Food Safety Authority, European Centre for Disease Prevention and Control,European Union Reference Laboratory for Avian Influenza. EFSA Journal published by Wiley-VCH GmbH on behalf of European Food Safety Authority

        (Continued . . . )

As the chart at the top of this blog indicates, each avian flu season (which runs Oct 1st -Sept 30th) is different, sometimes punctuated by the arrival of new subtypes or genotypes, and often varying in intensity. 

As we go into a new fall season, migratory birds in the Northern Hemisphere are expected to bring with them a new wave of HPAI from their high-latitude breeding grounds (see H5Nx: Reassort & Repeat).

While we can't know what kind of avian flu season lies ahead, now would be a good time to review the things you can do to lower your (currently, low) risk of exposure. 

WHO Interim Guidance to Reduce the Risk of Infection in People Exposed to Avian Influenza Viruses

Backyard Flock Owners: Protect Yourself from Bird Flu (CDC Guidance)

UF/IFAS Extension: What Backyard Flock Owners Need to Know about Bird Flu (Influenza H5N1)

EM&I: Limited added benefit of seasonal influenza vaccination before A(H5) vaccination in mice and ferrets challenged with A(H5N1)

 

Credit ACIP/CDC

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One of the biggest challenges during any future H5Nx pandemic would be the rapid production and timely deployment of a two-dose (given 30 days apart) H5 vaccine to billions of people (see Referral: SCI AM - A Bird Flu Vaccine Might Come Too Late to Save Us from H5N1).  

While it's not a new idea (see 2008's Seasonal Flu Vaccine May Offer Some Protection To H5N1), the notion of using the existing seasonal flu vaccine as either a `stop-gap measure' - or as a `prime-boost' for the first pandemic jab, during the opening months of an H5 pandemic - has persisted. 

An early, encouraging 2010 study in the journal Vaccines (Seasonal influenza vaccine elicits heterosubtypic immunity against H5N1 that can be further boosted by H5N1 vaccination) reported:

Recent findings indicate that seasonal influenza vaccination or infection of healthy humans may contribute to heterosubtypic immunity against new influenza A subtypes, such as H5N1. Here, we investigated whether seasonal influenza vaccination in a mouse model could induce any immunity against the H5N1 subtype.

It could be demonstrated that, largely due to the H1N1 component strain A/NewCaledonia/20/99, parenteral immunization of mice with a trivalent seasonal influenza vaccine elicited heterosubtype H5-reactive antibodies able to confer partial protection against H5N1 influenza virus infection.

Furthermore, the trivalent seasonal influenza vaccine was found to be compatible with a whole virus H5N1 vaccine in a heterologous prime-boost immunization regimen, achieving superior efficacy compared to a single immunization with an equivalent low-dose of the H5N1 vaccine.

More recent evidence remains both mixed and modest - and has often relied on ferret or mouse models (which may not fully apply to humans) -  including:

Nature Comms: Adjuvanted Influenza Vaccination Increases Pre-existing H5N1 Cross-reactive Antibodies

mBio: Low levels of influenza H5N1 HA and NA antibodies in the human population are boosted by seasonal H1N1 infection but not by H3N2 infection or influenza vaccination

EID Journal: Effect of Seasonal Influenza Vaccines on Avian Influenza A(H5N1) Clade 2.3.4.4b Virus Infection in Ferrets

EID Journal: Investigation of Influenza A(H5N1) Virus Neutralization by Quadrivalent Seasonal Vaccines, United Kingdom, 2021–2024

Note: Regardless of whether the seasonal flu vaccine directly protects against H5N1 infection, there are still good reasons to get the jab, prior to - and even during - an H5Nx pandemic.

1. It still may provide some small boost to your immune response.  Neutralizing antibodies aren't the only immune defense against infection. 
2. It could help prevent a co-infection with H5N1 and seasonal flu, which has the potential for being more severe.
3. Co-infections could also help generate a reassorted `hybrid' virus (see Preprint: Intelligent Prediction & Biological Validation of the High Reassortment Potential of Avian H5N1 and Human H3N2 Influenza Viruses).

While it may not be the last word on the topic, today we've a study in EM&I from researchers at the CDC's NCIRD Influenza Division, which frankly, offers little encouragement on the effectiveness of using seasonal flu vaccines as a `prime-boost' for H5N1 vaccines. 

I've only reproduced the abstract and a few excerpts, so follow the link to read the report in its entirety.  I'll have a brief postscript after the break.

Limited added benefit of seasonal influenza vaccination before A(H5) vaccination in mice and ferrets challenged with A(H5N1)

Masato Hatta , Ying Huang , Jeremy A. Duke , Joseph R. Rouse , Chenchen Feng ,
Xudong Lin , show all
Article: 2731495 | Received 29 Jun 2026, Accepted 05 Sep 2026, Published online: 21 Sep 2026
 https://doi.org/10.1080/22221751.2026.2731495

Limited A(H5)-specific vaccine supply is expected early in a potential A(H5N1) pandemic, raising the question of whether licensed seasonal influenza vaccines could enhance protection when administered before A(H5) vaccination. We evaluated this strategy in mouse and ferret models using clade 2.3.4.4b A(H5N1) viruses.

Seasonal influenza vaccination induced antibodies to seasonal haemagglutinins but did not induce detectable antibodies against A(H5) and did not consistently enhance A(H5)-directed antibody responses after A(H5) vaccination.

In lethal challenge studies, seasonal vaccine priming before A(H5) vaccination was associated with improved outcomes compared with A(H5) vaccination alone in one of three mouse experiments, but this effect was not observed in the other two mouse experiments or in ferrets. These findings suggest that seasonal influenza vaccine priming provides limited added benefit to A(H5) vaccine-mediated protection against A(H5N1) under the conditions tested.

(SNIP)

Accordingly, clinical studies are needed to further evaluate the strategy of administering seasonal influenza vaccine prior to A(H5) vaccination. However, such studies would likely rely on immunological endpoints rather than direct assessment of protection. Evidence of cross-reactivity alone should therefore be interpreted cautiously when informing decision-making, as it may not reliably predict protection against severe or lethal A(H5N1) infection.

In the absence of a clear added benefit, prioritizing seasonal influenza vaccination as a priming strategy to enhance protection against A(H5N1) during the early phase of a pandemic may offer limited value, particularly when resources may need to be directed toward A(H5)-specific vaccination and other countermeasures. Continued efforts to improve the development, availability, and deployment of antigenically similar A(H5) vaccines should remain a priority for pandemic preparedness [13,14].

       (Continue . . . )

Although the prospect of using seasonal vaccine as a bridge - or as a primer for a first H5 dose - remains biologically plausible, the evidence of its effectiveness has been both inconsistent and weak. 

Today's CDC study reports that prior seasonal vaccination offered little reproducible improvement over H5 vaccination alone (at least, in mice and ferrets). While disappointing, it is hardly unexpected. 

Which suggests that during the opening months of any novel flu pandemic we will (once again) have to rely on NPIs (Non-pharmaceutical Interventions) like social distancing and face masks - and our limited supply of antivirals -  while a strain-specific vaccine is produced and deployed.

For some specific preparedness advice, you may wish to revisit #Natlprep 2026: Pandemic Planning At Both Ends of the Spectrum. 

California Declares State of Emergency to Bolster Statewide El Niño Preparedness

 

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Earlier this month, in #NPM26: Preparing For An El Niño Winter,  we looked at the potential for weather anomalies across the country due to a strengthening - and possibly historic - El Niño expected this winter. 

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

While that may mean more more snow or ice for the southern tier of states, heavy rains and possible severe weather across the Gulf states, while warmer and drier in the north - for California this often means repeated atmospheric rivers - bringing bringing flooding rains and a heavy Sierra snowpack. 

How bad any of these scenarios will play out is anyone's guess. Weather is complicated - and while this winter's El Niño could be unprecedented - there are no hard and fast rules about its effects.

Yesterday, the Governor of California declared a State of Emergency to aid in statewide preparation for El Niño's arrival later this year.  He also proclaimed a state of emergency in several states due to recent storm activity. 

I've only reproduced a brief excerpt from the announcement. Follow the link to read the statement in its entirety.  

        Sep 21, 2026

Governor Newsom proclaims state of emergency to bolster statewide El Nino preparedness, protect California

Proclaims states of emergency in several counties to support recovery from earlier storm activity and other events

What you need to know: Governor Newsom is taking early action to prepare California for what is shaping up to be a historic El Niño season, which is forecast to be very strong, even the strongest on record. The state of emergency helps California move faster to protect communities, stage critical resources, and strengthen readiness before severe weather intensifies.

SACRAMENTO – As a historic El Niño storm season develops, Governor Newsom is proclaiming a state of emergency today, mobilizing state resources ahead of potential winter storms and other impacts. The emerging El Niño event is already breaking records, and forecasters expect it to continue to strengthen, potentially becoming the strongest in recorded history. Today’s emergency proclamation streamlines measures to help California prepare for this historic event.

The Governor’s action puts California in a stronger position before winter weather intensifies. It directs state agencies to begin protecting vulnerable communities, prepare roads and critical infrastructure, stage emergency supplies, and help local partners reduce flood, landslide, and coastal risks.

Read the full proclamation here.

        (Continue . . . )
 

Those interested in diving into one of the more extreme (albeit, historic) 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. 

While California may be first in line to feel El Niño's effects, ill effects, it won't be alone. So, if the power goes out, or flood waters rise, or the ground begins to shake . . . 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.

While being prepared can't guarantee you a good outcome, it can certainly increase your chances.

Preprint: Serological Evidence of Widespread Exposure to H5 Avian Influenza Virus in Arctic Foxes in Svalbard, Norway

Location of Svalbard in the Arctic Ocean

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The Svalbard Archipelago, located well above the Arctic Circle - roughly midway between the northern coast of Norway and the North Pole - is about the size of West Virginia and is home to fewer than 3,000 people.

Since 2022, Svalbard has also become a frequent center of HPAI H5 activity, reporting a mixture of  H5N1 and H5N5 (see Norwegian Veterinary Institute : HPAI Detected In Arctic (Svalbard) For the First Time).

This becomes all the more important because Svalbard - like many high-latitude avian nesting regions - is the summer home for millions of  migratory seabirds, waterfowl, waders, and other birds. As we saw in 2016's Sci Repts.: Southward Autumn Migration Of Waterfowl Facilitates Transmission Of HPAI H5N1, it is thought that these breeding areas may contribute to viral evolution and global spread of new reassortants. 

As we discussed in 2025's H5Nx: Reassort & Repeat, each fall we watch for the introduction of new genotypes and/or subtypes with the autumn arrival of migratory birds. 

We revisited the Svalbard story again in 2024's HPAI H5N5: A Variation On A Theme, and over the past few months, we've also seen extensive reporting on the detection of  H5N5 in polar bears in the region. 

Norwegian Veterinary Institute Reports Another HPAI H5N5 Infected Polar Bear on Svalbard Island

Preprint: HPAI H5N5 in a Polar Bear and Atlantic Walrus, Svalbard, 2026, with Widespread Seroconversion in Polar Bears

Norway Veterinary Institute Reports HPAI H5N5 In Polar Bears on Svalbard Island

 

While most of the world continues to deal primarily with clade 2.3.4.4b HPAI H5N1, since 2023 we've been following a relatively small - but persistent - presence of H5N5 in Northern Europe, Iceland, Eastern Canada, and even the Pacific Northwest. 

Credit: Multiple transatlantic incursions of HPAI 

clade 2.3.4.4b A(H5N5) virus into North America and spillover to mammals

Like H5N1, H5N5 appears to have an affinity for spilling over into mammalian species, including seals, walruses, polar bears, raccoons, mink, deer, domestic cats, and humans (n=1). 

While these infections often prove fatal, we've also seen growing serological evidence of non-lethal infection. 

In August of 2025, the Norwegian Veterinary Institute reported the first detection of H5N5 in Arctic Foxes on Svalbard Island.  Their press release stated: 

Arctic foxes can become infected with avian influenza through direct contact with sick or dead animals. Foxes are scavengers that are exposed to high infection pressure when they eat infected birds. Studies of red foxes on the mainland indicate that foxes do not have the ability to infect each other. Whole-genome sequencing of the viruses from arctic fox pups will be carried out to investigate whether there are signs of mammalian adaptation in the viruses.

This week researchers from the Norwegian Veterinary Institute have published a follow-up, in the form of a 26-page preprint, documenting serological evidence of widespread H5 exposure among Svalbard's population of  Arctic foxes. 

Due to its length I've only posted some excerpts (reformatted for readability), so you'll want to follow the link to read it in its entirety. I'll have a bit more after you return.

Serological Evidence of Widespread Exposure to H5 Avian Influenza Virus in Arctic Foxes in Svalbard, Norway

Johanna Hol H Fosse, Eva Fuglei, Francesco Bonfante, Line Olsen, Luca Bordes, Rebecca K. Davidson, Torill Mork, Ida Kristin Myhrvold, Lone Thiel Engerdahl, Sandra Venema, Kjersti Sturod, Johan Akerstedt, Olav Hungnes, Monika Z. Ballmann, Lineke Begeman, Ingebjorg Helena Nymo, Ragnhild Tonnessen, Bjornar Ytrehus
doi: https://doi.org/10.64898/2026.09.17.752278
This article is a preprint and has not been certified by peer review [what does this mean?].
 
Preview PDF

Abstract

The continued circulation of H5 clade 2.3.4.4b highly pathogenic avian influenza virus (HPAIV) has caused extensive mortality in wild bird populations worldwide with increasing spillover to mammals. In 2022, H5 HPAIV emerged in Svalbard, Norway, with subsequent detections in wild birds, walruses, polar bears, and arctic foxes. 

To understand the population-level exposure among Svalbard arctic foxes, we analysed body fluids from carcasses trapped in 2006-2015 (n = 56), 2023-2024 (n = 112), and 2024-2025 (n = 94) for antibodies to H5 avian influenza (anti-H5), influenza A nucleoprotein (anti-NP), and neuraminidase subtypes using ELISAs, haemagglutination inhibition (HI), and a multiplex assay.

• Only three samples from 2006-2015 tested positive for anti-H5 and were interpreted as false positives.  
• In 2023-2024, seropositivity for anti-H5 was high (95%), supported by a lower anti-NP seropositivity (85%) and antibody profiles consistent with mixed H5N1 (42%) and H5N5 (52%) exposure. 
• In 2024-2025, anti-H5 and anti-NP seroprevalences remained high (83% and 57%), with H5N5 (87%) exposure predominating over H5N1 (3%). 

A subset of anti-H5-positive samples tested positive by HI (2023-2024: 30%; 2024-2025: 14%). Juveniles with exposure limited to the previous season had higher odds of anti-H5 seropositivity in 2023-2024 than in 2024-2025 (OR 5.5). Analysis of paired lung extracts from a subset of individuals (n = 63) yielded results concordant with body fluids, using an indirect anti-H5 ELISA adapted for carnivores. Our findings demonstrate widespread H5 virus exposure. 

Together with occasional reports of progression to fatal HPAI, this highlights the need for continued population monitoring to evaluate ecological consequences.

       (SNIP)

Discussion

Our study reveals widespread exposure to H5 avian influenza in Svalbard arctic foxes trapped during the winters 2023-2024 and 2024-2025, with no evidence of H5 exposure during the preemergence period (2006-2015), as the few anti-H5-positive results detected in that period were interpreted as likely false positives.

Antibody profiles changed over time, suggesting a transition from mixed H5N1 and H5N5 exposure in 2023-2024 to predominantly H5N5 exposure in 2024-2025.

Our findings demonstrate that many arctic foxes survive exposure to H5 avian influenza viruses and develop a detectable humoral response. However, recent detections of fatal HPAI in arctic fox pups suggest that the virus may have profound effects in some individuals, although its potential effects on population dynamics remain unknown. The temporal changes in antibody profiles should be viewed in the context of HPAIV detections in Svalbard wildlife. Both H5N1 and H5N5 viruses were detected in birds and marine mammals in 2023, whereas no detections were reported in 2024. However, H5N5 was subsequently detected in arctic foxes in 2025 and in a polar bear and an Atlantic walrus in 2026 [13,18].

        (SNIP)

In conclusion, our findings demonstrate widespread exposure of Svalbard arctic foxes to H5 avian influenza virus following the emergence of clade 2.3.4.4b viruses in the High Arctic, with unknown consequences for the population. The exceptionally high seroprevalence, combined with temporal changes in antibody profiles, indicates substantial and evolving exposure within the Svalbard ecosystem. Arctic foxes may represent a useful sentinel species for monitoring HPAIV circulation in remote regions where conventional surveillance is challenging.

Continued integrated surveillance of wildlife hosts, predators, and scavengers will be important for understanding the long-term ecological consequences of HPAIV circulation and for detecting future changes in virus distribution, subtype composition, and transmission dynamics [18].

        (Continue . . . )

Interestingly, while H5N1 remains the dominant HPAI across the rest of the world, in Svalbard (and potentially other poorly-monitored high-latitude regions); H5N5 appears to have - at least temporarily - gained the upper hand. 

It’s not yet clear whether H5N5’s apparent success in Svalbard reflects some inherent viral advantage, favorable local environments or hosts, or simply chance.

But it's an outlier, which makes it well worth our continued attention.  

Emerg. Microbes & Inf.: Novel emerging reassortant H6 avian influenza viruses with internal genes from G57 genotype of H9N2 pose potential zoonotic risk

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A little over a year ago, in JOI: Prevalence and Transmission of Influenza A (H6) Viruses Pose a Potential Threat to Public Health, we revisited the H6 family of influenza A viruses, which are endemic in Asian poultry and are believed to have some degree of zoonotic potential. 

Long time readers will recall that H6N1 briefly made headlines in 2013 and 2014 after a college student in Taiwan was hospitalized with pneumonia (see Taiwan CDC: Epidemiological Analysis Of Human H6N1 Infection) and several dogs were found infected (see EID Journal: Influenza A(H6N1) In Dogs, Taiwan).

But as an LPAI virus (and neither H5 or H7), it isn't considered a `reportable' disease in poultry or wild birds by WOAH, and therefore our knowledge of its spread and evolution is limited.  

That said, over the past decade we've seen growing interest by Chinese scientists in the spread and evolution of H6, particularly since it reassorts readily with another zoonotic risk: LPAI H9N2.  A sampling of recent blogs include:

J. Inf.: Zoonotic Threat of Novel H6N2 Avian Influenza Virus with Internal Genes Exclusively Derived from H9N2, China, 2025

Preprint: Progressive Adaptation of H6N1 Avian Influenza Virus in Taiwan Enhances Mammalian Infectivity, Pathogenicity and Transmissibility

Study: Influenza A (H6N6) Viruses Isolated from Chickens Replicate in Mice and Human lungs Without Prior Adaptation

H9N2 is a ubiquitous, and highly promiscuous avian influenza virus, which reassorts easily with other influenza A strains and has a record of lending its internal genes to more dangerous HA subtypes (see PNAS: Evolution Of H9N2 And It’s Effect On The Genesis Of H7N9).

Today's study describes 6 H6 reassortants - all carrying the internal genes from H9N2 - and 5 with its NA from H5N1, and 1 from H9N2.  Of note, the H6N2 isolate showed dual receptor binding to both avian and human-like receptor cells. 

The three representative isolates tested (NJ2501, AH25107, and CZ56) produced high virulence in mice, while all 6 produced respiratory symptoms in chickens, and transmitted efficiently among birds.

Due to its length and technical nature, I've only posted some excerpts. Follow the link to read the report in its entirety.  I'll have a postscript after you return.

Novel emerging reassortant H6 avian influenza viruses with internal genes from G57 genotype of H9N2 pose potential zoonotic risk

Zhimin Wana,b,c,d*, Wenjie Jianga,b,c,d*, Xudong Caoa,b,c,d, Xingyao Guoa,b,c,d, Xinyan Hea,b,c,d, Jianjun Zhange,Xinran Chua,b,c,d, Xinyi Jia,b,c,d, Yu Liua,b,c,d, Xuefeng Yina,b,c,d, Zhehong Zhaoa,b,c,d, Jixiang Wanga,b,c,d,Wei Gaoa,b,c,d, Quan Xiea,b,c,d, Tuofan Lia,b,c,d, Hongxia Shaoa,b,c, Aijian Qina,b,c, Yuhai Bif,g,h andJianqiang Yea,b,ca 

In recent decades, multiple novel reassortant avian influenza viruses (AIVs) carrying internal genes derived from H9N2 viruses have emerged in poultry in China and have repeatedly caused human infections, highlighting the pivotal role of the H9N2 internal genes cassette in facilitating cross-species transmission. 

In this study, five H6N1 and one H6N2 AIVs were isolated from chickens and Houdan chickens exhibiting respiratory symptoms. Genetic analyses revealed that the hemagglutinin (HA) genes of these H6 AIVs belonged to the ST339-like lineage and originated from H6 AIVs circulating in domestic geese in China, whereas the neuraminidase (NA) genes were derived from H9N2 or H5N1 AIVs, respectively. 

Notably, all six internal genes of these H6 isolates were derived from the G57 genotype H9N2 virus. In vitro studies demonstrated that these H6 AIVs replicated effectively in both avian and mammalian cells.

The H6N2 isolate displayed dual receptor binding affinity for both avian-like and human-like receptors, whereas the five H6N1 viruses showed no detectable receptor binding affinity under the assay conditions in vitro. Reverse-genetics reassortant assay further revealed that the H6N1 HA itself possesses intrinsic dual receptor binding affinity.

Furthermore, these H6 isolates exhibited high virulence in mice, and induced obvious respiratory symptoms in chickens, with efficient transmission among birds. 

Collectively, our findings demonstrate that H6 AIVs carrying the G57 H9N2 internal gene constellation are actively circulating in chicken populations and pose a substantial threat to poultry health, underscoring their potential risk to public health and the need for continued surveillance at the poultry-human interface.

(SNIP)

Discussion 

The  continuous emergence of the novel reassortant AIVs poses a great threat to both poultry industry and public health. In this study, we identified and characterized six novel reassortant H6N1 and H6N2 AIVs circulating in chickens with respiratory symptoms, all of which harbour internal genes from the G57 genotype H9N2 virus. 

Our findings provide further evidence that H9N2 viruses serve as a critical genetic backbone facilitating the generation of novel reassortant AIVs with enhanced fitness in poultry and increase zoonotic potential.

(SNIP)

In addition to their mammalian pathogenicity, these H6 viruses demonstrated efficient infection and transmission in chickens. Viral shedding was detected in both infected and contact chickens (Figure 8), indicating effective direct-contact transmission of these novel H6 viruses. The ability to spread efficiently within poultry is a critical factor for viral maintenance and amplification, which increases the likelihood of further reassortment and spill-over events. 

Compared with previous reports in which H6 viruses showed limited transmission in poultry [26], the strains described here appear to have acquired enhanced transmissibility, possibly due to their adapted internal genes of H9N2 virus origin.

        (SNIP)

In conclusion, our study highlights the evolution of H6 AIVs through reassortment with G57 genotype H9N2 viruses and demonstrates that such reassortment can result in AIVs with enhanced pathogenicity and cross-species transmission potential. 

These findings underscore the importance of continuous surveillance of AIVs in poultry flocks in China, particularly those carrying H9N2 internal genes, as they may serve as precursors to zoonotic or even pandemic AIVs.

       (Continue . . . )

While most people think first of `H5N1' when they hear about novel flu strains, the CDC's IRAT lists 27 avian, canine, or swine subtypes/variants with pandemic  potential.  

Among the top 10 zoonotic influenza A viruses (ranked by likelihood of emergence) the CDC has placed H5N1 as 7th on the list, while H9N2 comes in at #5.

And despite having caused documented human infections, a number of novel flu subtypes (e.g. H3N8, H5N5, H6N1, and H7N4) have yet to be added to this list. 

The reality is, most of these viruses will never pose a serious public health threat.  But none of them can be ruled out as having pandemic potential.

 Which is why we follow their progress with considerable interest.