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.

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. 

 

South Korea: MAFRA Report on H5N1 In wild leopard cat in Seosan, South Chungcheong Province

Location of Seosan, South Korea

#19,397

While human infections get the bulk of our attention, the impact of HPAI H5 on avian and mammalian wildlife around the world is enormous and growing. 

• Devastating outbreaks in marine mammals (see here, here, and here) have killed tens of thousands of seals, sea lions, and dolphins. 

• Thousands of vultures, hawks, and falcons have succumbed to HPAI (see here, here, and here), including highly endangered California Condors. 

• Felines, both wild and captive, are highly susceptible to the virus, with the USDA's Detections of Highly Pathogenic Avian Influenza in Mammals dashboard lists more than 200 feline detections in the U.S. alone.

• Recent seroprevalence studies report a high incidence of infection among polar bears above the arctic circle (see Preprint: HPAI H5N5 in a Polar Bear and Atlantic Walrus, Svalbard, 2026, with Widespread Seroconversion in Polar Bears).

But most wildlife infections go unnoticed.  Marine mammals die at sea, or on remote beaches, and are never recorded.  When a massive die-off of birds occurs, no one counts the bodies.  When an infected fox dies in the woods, the evidence is quickly erased by scavengers. 

Australian citizens have reported over 70,000 suspected wildlife incidents over the past 5 months, yet only a tiny percentage have been investigated. 

As we discussed in early 2025 (see Nature Reviews: The Threat of Avian Influenza H5N1 Looms Over Global Biodiversity), no one really knows the full extent of  H5's carnage, or its likely impacts going forward. 

In the spring of 2025 South Korea reported their first mammalian wildlife infection with H5N1 in an endangered leopard cat. Today they report another wild leopard cat has succumbed to the virus. 

While this is only the second such detection, it is clear from this report they are taking it seriously, as is evidenced by their extensive guidance to the public. 

        (Translated . . .)

Highly pathogenic avian influenza (AI) detected in a wild leopard cat in Seosan, South Chungcheong Province… Surveillance and disease control measures strengthened

2026.11.02 15:30:08 Quarantine Policy Bureau Avian Influenza Prevention Division

The Ministry of Agriculture, Food and Rural Affairs ( Minister Song Mi-ryeong ) and the Ministry of Climate, Energy and Environment ( Minister Kim Sung-hwan ) announced on November 2 that the H5N1 highly pathogenic avian influenza (AI) virus was detected as a result of a detailed examination conducted by the National Wildlife Disease Control Center on the carcass of a wild mammal, a leopard cat * , found on October 23 near Ganwol Lake in Buseok-myeon, Seosan-si, Chungcheongnam-do, following a report by a local visitor .

* Class II Endangered Species, primarily feeds on rodents and birds ( home range approx . 10–20 km² )

This is the second time that highly pathogenic avian influenza (AI) virus has been detected in a wild mammal in Korea since it was first detected in a wild leopard cat in Hwasun, South Jeolla Province in March 2025 , and the first time this winter .

Meanwhile, the National Wildlife Disease Control Center, an agency under the Ministry of Climate, Energy and Environment, has been investigating whether wild mammals * secured through wildlife rescue centers , resident reports, and surveillance of major migratory bird habitats nationwide are infected with avian influenza (AI) .

Carnivorous and omnivorous mammals such as raccoons , weasels , badgers , wildcats , otters , martens , seals , and finless porpoises

※ Avian Influenza (AI) testing on 245 wild mammal samples from January to October 2026

The Ministry of Climate, Energy and Environment is implementing response measures in accordance with the Standard Operating Procedures (SOP) for Avian Influenza (AI) in wild mammals established following the case of infection in a wildcat in Hwasun-gun last year , and is strengthening cooperation with relevant agencies to reduce the risk of infection between wild animals , poultry , and humans .

As soon as highly pathogenic avian influenza (AI) was confirmed, the situation was shared with relevant agencies , including the Ministry of Agriculture, Food and Rural Affairs and the Korea Disease Control and Prevention Agency, and special surveillance was carried out in areas within a 10km radius of the detection site , including searching for dead wild mammals and conducting detailed on-site investigations , and emergency quarantine measures, such as on-site disinfection and access control, were requested from competent local governments, including Seosan City .

In addition , viral gene analysis is performed to identify the characteristics and origin of the infecting virus , and to investigate whether there is additional infection to surrounding wild animals and the possibility of transmission .

The Ministry of Agriculture, Food and Rural Affairs implemented emergency quarantine measures, such as surveillance and inspection , to prevent the spread to poultry farms within the quarantine zone ( 10km radius ) , and took measures to thoroughly clean and disinfect the interior and exterior of farms , facilities , and vehicles to preemptively block risk factors for the transmission of avian influenza .

Designation and operation of wild bird surveillance areas in accordance with the Avian Influenza Prevention and Control Guidelines

The Korea Disease Control and Prevention Agency ( Chungcheong Region Disease Response Center ) and local governments, including South Chungcheong Province, have stated that they are continuing to monitor the health status of field response personnel, including those who discovered the wildcat and rescue center staff, taking into account the virus's incubation period (10 days) to prevent human infection , and that there have been no suspected cases to date .

In addition , the Ministry of Climate, Energy and Environment provided the following information to the public to prevent the transmission and spread of highly pathogenic avian influenza (AI) .

First , you should refrain from visiting wildlife habitats, such as migratory bird sanctuaries, as much as possible . If you must visit, you should avoid contact with wild animals and be careful not to step on feces or bodily fluids . Furthermore , you should strictly observe personal hygiene rules, such as disinfecting shoes and vehicles and washing your hands after returning home, and avoid entering livestock farms .

In addition , even if you discover dead wild birds or mammals, or wild animals exhibiting abnormal behavior, do not touch or move them directly, but report them immediately to the competent local government so that the National Wildlife Disease Control Center can conduct avian influenza (AI) testing promptly .

※ Report to the relevant city , county , or district environmental department via the Government Civil Complaint Information Call Center (110 ) or the competent local government ( area code + 120 ) .

Livestock farm officials must strictly adhere to basic disease prevention guidelines, such as prohibiting entry to migratory bird habitats , controlling the entry of outside people and vehicles, and disinfecting the inside and outside of the farm daily. In addition , if suspected symptoms of highly pathogenic avian influenza (AI), such as the death of more poultry than usual, are confirmed , they must immediately report to the disease control authorities (1588-4060/9060) .

Lee Chae-eun, Director General of Nature Conservation at the Ministry of Climate, Energy and Environment, stated, “ As highly pathogenic avian influenza (AI) has been confirmed in wild mammals, specifically leopard catesses, again this year following last year, we will strengthen surveillance and inspections to closely investigate whether there are any additional infections in wild animals , and we will do our best to ensure the safety of field response personnel and the public . ” She added , “ We also ask the public to report any dead wild animals immediately without direct contact if they find them . ”

Lee Dong-sik , Director General of the Quarantine Policy Bureau at the Ministry of Agriculture, Food and Rural Affairs, stated, “ With highly pathogenic avian influenza (AI) confirmed in wild mammals ( leopard cats ) following wild birds, the risk of outbreaks has significantly increased as the related virus has entered poultry farms. ” He urged poultry farm officials to “ strictly adhere to basic quarantine rules , such as spraying rodenticides , ensuring the safe storage of carcasses, controlling the entry of outside people and vehicles, and disinfecting the inside and outside of the farm daily, to prevent wild animals from entering the farm . ”

A little over 3 years ago, in Avian Flu's New Normal: When the Extraordinary Becomes Ordinary, I wrote about the numbing effect that comes with the constant barrage of HPAI H5 reports from around the world.

Events that were nearly unthinkable four or five years ago (see Preprint: Mass mortality of southern elephant seals during multi-species outbreak of HPAI H5N1 on sub-Antarctic Heard Island) have now become `the new normal'.

While we may have become inured to these happenings, that doesn't make them any less impactful to our shared global ecosystem. What future challenges may arise from these continued viral insults to non-human species is difficult to predict.

But given our largely passive response to avian flu, we appear on track to find out.

Washington: Hospitalized Avian Flu Case Died - 3rd Case Reported

 

#19,396

While details are sparse, overnight the Yakima Health District reported that their 2nd avian flu case of 2026 - announced on Saturday - has died, and that a 3rd case (reportedly not severe) has been identified. 

While the patient apparently had (unspecified) `underlying conditions', it isn't clear whether that had anything to do with the course of this patient's illness. 

Pertinent details - like onset date, date of hospitalization and/or confirmed H5 diagnosis, gender, or age - are not provided.  The mode of exposure `at a dairy with infected cattle', is also vague. 

Today's announcement does state that contact tracing is being done on the newly reported third case, and contacts of the second case are being monitored for symptoms. 

The Yakima press release follows, after which I'll have a postscript.

For immediate release: November 1, 2026

Contact: Stephanie Ruiz, Communications Specialist (509) 941-7364

Yakima County Resident Dies from Complications of Avian Influenza

Yakima, WA- The Yakima County resident who was hospitalized with severe illness after contracting avian influenza (bird flu) following exposure at a dairy with infected cattle died today. 

The Yakima Health District offers its heartfelt condolences to the individual’s family, friends, and loved ones during this difficult time.

The individual had underlying health conditions. Out of respect for the family’s privacy, the Yakima Health District is not releasing their name, gender, or age.

The risk of avian influenza infection to the general public remains low. People at greater risk include dairy workers and others who have close or prolonged contact with infected animals or environments contaminated with the virus. Transmission of avian influenza between humans is extremely rare and has never been documented in the United States. Public health officials are continuing to monitor individuals who had close contact with the patient for symptoms.

Third Confirmed Human Case of Avian Influenza

The Yakima Health District is reporting a third confirmed human case of avian influenza in a Yakima County resident who was exposed to infected dairy cattle at a Yakima County dairy. The  individual has not experienced severe illness or been hospitalized.

The Yakima Health District is working with public health partners to identify and contact individuals who may have been in close contact with the person to monitor for symptoms.     

        (Continue . . . . )

This press release goes on to state: 

To keep the public informed, Yakima Health District plans to update its avian influenza webpage every Monday and Thursday at 2:00pm with the latest available information, including confirmed human case counts in Yakima County. Updates will be shared as new information becomes available.

For the latest information, visit www.YakimaHealthDistrict.org/BirdFlu.

The latest update (Nov 1st) shows:

Hopefully local officials will be more forthcoming about the modes of exposure  (and whether PPE was worn), onset dates, and other details.  The release of sequencing data is also anxiously awaited. 

Stay tuned.

Washington: Yakima County Reports 2nd Human H5N1 Case (Hospitalized with severe illness)

 

#19,395

Yakima County, Washington is now reporting their second human infection with H5N1 associated with their ongoing (and rapidly expanding) outbreak in dairy cattle.  

Worth noting: While `bovine' B3.13 H5N1 infection has generally been associated with mild illness in humans, this case is reportedly hospitalized with severe illness.  

This patient is described as having unspecified `underlying health conditions', and of course, we don't as yet have official confirmation of the genotype (presumably B3.13).

First the press release, after which I'll have a bit more.

For immediate release: October 31, 2026

Contact: Stephanie Ruiz, Communications Specialist (509) 941-7364

Second Yakima County Resident Tests Positive for Avian Influenza

Yakima, WA- The Yakima Health District (YHD), in coordination with the Washington State Department of Health (DOH), is reporting a second human case of avian influenza, also known as bird flu, in a Yakima County resident after exposure at a dairy with infected cattle. 

The individual is currently hospitalized with severe illness and has underlying health conditions. 

The risk of avian influenza infection to the general public remains low. People at greater risk include dairy workers and others who have close or prolonged contact with infected animals or environments contaminated with the virus. Transmission of avian influenza between humans is extremely rare and has never been documented in the United States.

The Yakima Health District is working with public health partners to identify and contact individuals who may have been in close contact with the patient to monitor for symptoms. YHD is also continuing to work with impacted dairies and public health partners to identify people who may have been exposed to infected cattle, monitor their health, and provide guidance and resources to help protect exposed individuals. This includes providing testing for exposed individuals who develop symptoms to support prompt diagnosis and appropriate medical care.

Symptoms and When to Seek Guidance

Symptoms of avian influenza in people can include eye redness or irritation, fever or feeling feverish, cough, sore throat, runny or stuffy nose, muscle or body aches, headache, and fatigue.Illness can range from mild to severe.If you work on a dairy farm and develop symptoms after potential exposure to infected cattle, contact your healthcare provider and let them know about your exposure or the Yakima Health District at (509) 480-3770 for guidance. 

Updated Information on Avian Influenza in Dairy Cattle

As of October 29, 33 dairy herds in Yakima County have tested positive for highly pathogenic avian influenza (HPAI) H5N1. The Washington State Department of Agriculture (WSDA)maintains updated information on confirmed dairy herd detections through its website. The Yakima Health District will also share public health updates and prevention guidance on its webpage: www.YakimaHealthDistrict.org/BirdFlu.

(Continue  . . . )

Those who work with poultry, wild birds, or dairy cattle are presumed to be at greatest risk of exposure to avian flu, but other exposures (dogs, cats, other peridomestic animals) are possible.  

There are also a handful of cases here in the United States where no known exposure was identified. 

 

For people at the highest risk, the CDC continues to recommend the use of PPEs (see below).  

In early October we looked at: 

AVMA: How can veterinary personnel recognize and protect themselves from domestic cats infected with HPAI A(H5N1) viruses?

And the CDC offers the following advice to pet owners on how they may keep their animals safe from avian flu, and what to do if they suspect infection.

While the risk to the general public is still presumed to be low, it is not zero.  

Since every human infection is another opportunity for the virus to adapt, the steps we take today to reduce human exposure could conceivably lower future risks from the virus. 

But for that to happen, people need to take the threat seriously.

Colorado Reports 1st H5N1 In Dairy Samples Since 2024 - Washington State adds 4 more Infected Herds

 

#19,394

Just 24 hours ago I reported that Washington State had added 3 More Dairy Herds Infected with H5N1 (n=30), but yesterday the WSDA added 4 more herds to their rapidly expanding list. 

Meanwhile, Colorado - which was one of the earliest affected states back in 2024 - is reporting its first detection in bulk milk in almost 2 years. The following press release was published yesterday by Colorado's Department of Agriculture.

Routine Milk Tank Testing Detects Colorado's First Dairy HPAI Case Since 2024

Friday, October 30, 2026 

The risk to the general public is low and pasteurized milk continues to be safe because pasteurization kills viruses

The U.S. Department of Agriculture’s National Veterinary Services Laboratory (NVSL) has confirmed a detection of Highly Pathogenic Avian Influenza (HPAI) in a dairy in Weld County. This is the first HPAI detection in a Colorado dairy since August 2024. Since the initial detection of HPAI in dairy cattle in April 2024, Colorado has implemented routine bulk milk tank surveillance testing to ensure Colorado’s milk supply is not impacted.

“This detection underscores the importance of our routine bulk milk tank testing to keep our herds healthy and our food supply secure,” said Colorado State Veterinarian Dr. Maggie Baldwin. “Colorado has taken a proactive approach to monitoring and responding to avian influenza, ensuring that early detection leads to swift mitigation measures to protect consumers and our food supply.”

The Colorado State Veterinarian’s Office was notified by the Colorado State University Veterinary Diagnostic Laboratory that a sample from a routine bulk milk tank test tested presumptive positive for HPAI. The National Veterinary Services Laboratory (NVSL) issued the official confirmation of the detection of HPAI (B3.13 strain) in a dairy in Weld county on Saturday, October 24. The dairy has been put under a quarantine order.

Under the quarantine order, several mandatory stipulations are in effect to prevent further transmission of the virus. The quarantine restricts the movement of lactating dairy cattle from the affected facility. In addition, the operation must immediately implement strict biosecurity protocols across all farm activities and daily routines. To monitor and verify disease status, weekly bulk milk tank testing is required until two consecutive negative test results are obtained at least one week apart.

CDA is working closely with the Colorado Department of Public Health and Environment and local partners to protect the health of farmworkers and facility staff. Public health officials routinely monitor individuals following potential exposure to infected animals or raw animal products as a standard precaution. People most at risk for avian flu are those who come directly into contact with infected birds or other animals. The risk to the general public remains low and pasteurized milk continues to be safe, because pasteurization kills viruses.

Colorado originally confirmed its first positive detection of HPAI in dairy cattle on April 25, 2024, during an outbreak that grew to affect 64 individual dairies across the state. In response to the widespread outbreak, Colorado became the first state in the nation to institute mandatory bulk milk tank testing for all commercial dairies in Colorado. That comprehensive monitoring effort successfully cleared all affected herds, with all dairies released from quarantine by December 2024.

Colorado was rightfully applauded back in 2024 for their aggressive response to their initial outbreak (n=64 herds) (see Colorado Orders Mandatory Statewide Weekly Bulk-Tank Testing for HPAI), which was copied by many states. 

It will be of considerable interest to learn exactly how this new case came about, and whether this is a one-off event. 

In Washington state, the big questions are what is driving the spread of the virus, and how long it may have been going on before it was detected.  

Until we have answers to those questions, it is impossible to know whether these events are unrelated occurrences or the start of a new trend. 

Stay tuned.