Virulence: Cats Are More Susceptible to the Prevalent H3 Subtype Influenza Viruses Than Dogs

 Cats Infected With H7N2 - NYC 2016

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Nine years ago this week the New York City Health Department Issued a Statement On An Avian H7N2 Outbreak In Cats, that quickly spread across multiple animal shelters in NYC. 

Initially (on Dec 14th, 2016) it was announced that that 45 cats at a  Manhattan shelter were infected with an LPAI H7N2 virus, but that number would increase nearly 10-fold in the days to come. 

A week later, the plot thickened, when the NYC Health Department released a Statement On the Human H7N2 Infection of a veterinarian working at one of the shelters. 

While not at the top of our pandemic concerns, in 2007 four people were presumed to have been infected by H7N2 in the UK following local outbreaks in poultry (see Eurosurveillance report). 

Two years later, in J Infect Dis: Serological Evidence Of H7N2 Infection Among Animal Shelter Workers, NYC 2016, we would see evidence that at least at least one other animal worker showed serological evidence of prior infection, and 5 others exhibited low positive titers to the virus, indicating possible infection.

While the exact source of this outbreak remains unknown, it likely started with a stray cat's chance encounter with an infected bird.

Although we'd seen sporadic reports of cats infected with H5N1 - particularly in Asian zoos which fed raw chicken to captive tigers - this was a wake-up call for the potential for cats (and potentially other companion animals) to serve as intermediary hosts for other types of zoonotic influenza. 

Cats As Potential Vectors/Mixing Vessels for Novel Flu

Since then, we've seen hundreds of cats infected with HPAI H5 (see One Health: Outbreak of HPAI a(H5N1) Among House Cats: A Case Series Involving Oseltamivir Treatment) in both the United States and around the world, along with sporadic reports of other avian flu spillovers, including H3N8 in China. 

A few (of many) past blogs includes:

J. Virulence: HPAI H5N1 Virus Infection In Companion Animals

Viruses: The Seroprevalence of Influenza A Virus Infections in Polish Cats During a Feline H5N1 Influenza Outbreak in 2023

CDC MMWR: HPAI H5N1 Virus Infection of Indoor Domestic Cats Within Dairy Industry Worker Households — Michigan, May 2024

Emerg. Microbes & Inf.: Marked Neurotropism and Potential Adaptation of H5N1 Clade 2.3.4.4.b Virus in Naturally Infected Domestic Cats

As we've previously noted (see below), China has recently become increasingly concerned over the evolution and spread of several novel H3 viruses. 

• Preprint: Fatal Human H3N8 Influenza Virus has a Moderate Pandemic Risk.

• Emerg Microbes Infect: A fatal Case of Acute Encephalitis Associated with a Novel influenza H3N2 Recombinant Virus Possessing Human-origin H7N9 Internal Genes 

• Virology: Assessment of the Public Health Risk of Novel Reassortant H3N3 Avian Influenza Viruses That Emerged in Chickens

• Preprint: Fatal infection of a novel canine/human reassortant H3N2 influenza A virus in the zoo-housed golden monkeys

All of which brings us to new study, published this week in the journal Virulence, which finds that cats are far more susceptible to influenza H3 infection than dogs, making them a plausible intermediate host. 

First, the link and some excerpts from the study (but follow the link to read it in its entirety), after which I'll have a brief postscript. 

 

Cats are more susceptible to the prevalent H3 subtype influenza viruses than dogs

Jie Deng , Chunhui Ma , Junting Yu , Bo Chen , Shoujun Li  & Pei Zhou 

Article: 2605799 | Received 06 May 2025, Accepted 13 Dec 2025, Accepted author version posted online: 17 Dec 2025

Cite this article https://doi.org/10.1080/21505594.2025.2605799  

 

ABSTRACT

Recent reports have highlighted the increasing frequency of influenza A virus (IAV) spillover events from other species to dogs and cats. IAV, particularly the H3 subtype, exhibits a broad host range and a propensity for interspecies transmission, as exemplified by the sustained circulation of H3N2 and H3N8 canine influenza viruses in dog populations. This raises concerns about the potential role of companion animals as intermediate hosts in influenza virus transmission. 

To evaluate the susceptibility of dogs and cats to the prevalent H3 subtype influenza viruses, we experimentally inoculated groups of both species with three prevalent influenza viruses: H3N2 avian influenza virus (AIV), H3N8 avian influenza virus, and H3N2 swine influenza virus (SIV). Results showed that while all inoculated dogs exhibited seroconversion to all three viruses at 7, 14, and 21 days post-inoculation (dpi), they displayed no clinical signs, viral shedding, or evidence of viral replication in their organ tissues. 

In contrast, despite the cats did not exhibit apparent clinical signs, all inoculated cats exhibited seroconversion to all viruses at 7, 14 and 21 dpi, sustained nasal viral shedding for approximately one week, and demonstrated viral replication in their lungs, trachea, and nasal turbinate.

Our findings underscore the higher susceptibility of cats compared to dogs to H3 subtype influenza viruses. These results emphasize the critical need for enhanced surveillance of cats within the influenza virus transmission network.

        (SNIP)

Through experimental infection, our study significantly advances our understanding of the infectivity and pathogenicity of various H3 subtype influenza viruses in dogs and cats. Based on our previous studies and the current findings, we can conclude that dogs are resistant to H3N8 EIV2, H3N2 AIV, H3N8 AIV, and H3N2 SIV, while cats are susceptible to H3N8 EIV2, H3N2 AIV, H3N8 AIV, H3N2 SIV, and H3N2 CIV (Table 1).

This pronounced susceptibility of cats, however, contrasts with the epidemiological observation that stable influenza virus circulation has become established in dog populations (e.g., equine-origin H3N8 CIV and avian-origin H3N2 CIV) but not in cats.

A critical factor explaining this paradox is the demographic behavior of cats. Specifically, efficient cat-to-cat transmission is highly dependent on close contact, a condition typically limited in dispersed household settings but readily met in high-density confined environments such as animal shelters.

This demographic constraint is supported by historical evidence of H7N2 and H3N2 influenza outbreaks in feline shelters (Figure1), underscoring the imperative for targeted surveillance in populations where close proximity may promote viral adaptation and sustained transmission.

       (Continue . . . )

While we are understandably focused on the potential for HPAI H5 to spark a pandemic, history suggests that H1, H2, or H3 (avian/swine/canine) novel flu viruses are far more likely to do so (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?). 

Trying to predict what pathogen will win the genetic lottery and spark the next global public health crisis is a mug's game. There are just too many viral contenders, and far too little surveillance and reporting. 

But another pandemic - whether from influenza A, another coronavirus, or something completely out of left field - is all but inevitable. 

The only real question is, will we be ready?

WHO EURO Statement: More than half of WHO European Region experiencing intense, early influenza season driven by new strain

Credit WHO Global Respiratory Rept #556

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For the past 6 weeks we've been following reports of unusual seasonal flu activity - both in the Southern and Northern Hemispheres - which has been attributed to the recent emergence of a `drifted' H3N2 virus (subclade K), which dominated the Southern Hemisphere's flu season.

A month ago, in UKHSA Preprint: Early Influenza Virus Characterisation and Vaccine Effectiveness in England in Autumn 2025, A Period Dominated by Influenza A(H3N2) Subclade K, we saw early evidence that the 2025-2026 Northern Hemisphere vaccine may not be as effective against this emerging strain, although it is still expected to provide some protection against severe disease. 

Unusually -  Australia's flu season - which would normally have ended in August - continues (albeit well off the highs of July) well into their summer (see Australian CDC chart below)

Over the past six weeks we've also seen reports of uncharacteristically early, and robust, flu outbreaks in places like Japan, South Korea, China, and much of Europe. Some countries are now urging the public to wear face masks in public, and many school closures have been reported.

According to the most recent CDC FluView (Week 49), influenza is slowly rising in the United States, but it hasn't really taken off yet (see map below).   

While Europe's flu woes aren't guaranteed to be duplicated here, the odds are pretty high that the above picture will change radically in the the weeks ahead.  Which means, if you haven't gotten a flu shot, there is still time. 

More than half of WHO European Region experiencing intense, early influenza season driven by new strain
17 December 2025
Media release

17 December 2025 | Copenhagen

Influenza is sweeping across the European Region earlier than usual, with a newly dominant virus strain placing health systems under significant pressure in some countries. The public is recommended to take simple protective action to keep themselves and others safe this winter.

The influenza season has begun roughly 4 weeks earlier than in previous seasons. At least 27 of the 38 countries reporting data in the WHO European Region are now seeing high or very high influenza activity. In 6 countries – Ireland, Kyrgyzstan, Montenegro, Serbia, Slovenia and the United Kingdom – more than half of patients tested for influenza-like illness were positive for influenza.

“The flu comes around every winter, but this year is a little different,” said Dr Hans Henri P. Kluge, WHO Regional Director for Europe. “A new strain – A(H3N2) subclade K – is driving infections, though there is no evidence that it causes more severe disease. This new variant of seasonal flu now accounts for up to 90% of all confirmed influenza cases in the European Region. It shows how even a small genetic variation in the flu virus can place enormous pressure on our health systems because people do not have built-up immunity against it.”

While it may not prevent infection, early data from the United Kingdom confirm that the current seasonal influenza vaccine lowers the risk of severe health outcomes from A(H3N2) influenza virus. Vaccination remains the single most important preventive step to avoid severe health outcomes. This is especially important for those at higher risk, including older people, those with underlying conditions, pregnant women and children. In addition, health workers are also a priority group for vaccination to protect their own health and the health of their patients.

As in other seasons, school-aged children are the primary drivers of community spread. However, adults aged 65 and older constitute the majority of severe cases requiring hospitalization, highlighting their critical priority for vaccination.

How to stay safe this winter

Cases will continue to rise until the influenza season peaks, likely in late December or early January. Most people will recover from influenza on their own. People with severe symptoms or other medical conditions should seek medical care.

WHO reiterates proven measures to curb transmission and save lives.

• Get vaccinated. This is the best defence, especially for high-risk priority groups and health workers, who should also follow infection prevention measures and wear a mask when necessary.

• Stop the spread. Stay home if unwell. If you have respiratory symptoms, wear a mask in public to prevent passing the virus to others. When sneezing or coughing, cover your mouth and nose. Clean hands regularly. Open windows and doors frequently to improve air flow indoors.

“The current flu season, though serious, does not represent the level of global emergency we faced during the COVID-19 pandemic. Our health systems have decades of experience managing influenza, we have safe vaccines that are updated annually and we have a clear playbook of protective measures that work. If we use the proven tools that we already have – vaccination, health-conscious behaviour and strong public health systems to protect the most vulnerable – then we will weather this predictable, seasonal storm.”

“It is also vital in the current climate of mis- and disinformation to seek credible information from trusted sources like national health agencies and WHO,” concluded Dr Kluge. “In a challenging flu season, trustworthy evidence-based information can be life-saving.”

CIDRAP Unveils New Public Health Alerts

 

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In this age of diminished public health reporting - particularly by many government agencies - I'm happy to report that CIDRAP (Center for Infectious Disease Reporting & Policy) at the the University of Minnesota has unveiled a new feature; Public Health Alerts. 

The first two reports went live yesterday.  Follow the links to read them in their entirety. 

 

Public Health Alerts

RSS feed

Published by NEJM Evidence in collaboration with CIDRAP, Public Health Alerts are brief reports that inform public health officials, clinicians, policymakers, and the public of important and emerging health issues. Freely available on both the NEJM Evidence site and here, these reports are published on a rapid review track in support of efforts to make timely decisions for the medical community and public health.

For more information, read the news release and NEJM Evidence editorial on the launch.

To receive notice of new reports on the day they are published, sign up for our Daily News Headlines newsletter here.

Detection of community transmission of clade Ib mpox virus in the United States

In October 2025, clade Ib mpox virus infection was confirmed in 3 unvaccinated hospitalized men in California who had not traveled internationally.
(Dec 17, 2025)

 

 

Influenza virus characteristics in the Department of Defense populations, 2024-2025 

Report describes the influenza viruses circulating during the 2024-25 flu season in the US.
(Dec 17, 2025)

Declining Mammalian Spillover Submissions To USDA Over Past 8 Months

 

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The USDA maintains a dashboard of Detections of Highly Pathogenic Avian Influenza in Mammals - which is highly dependent upon State Animal Health Officials submitting samples for testing - has recorded 674 confirmed infections in mammalian wildlife (excluding livestock) since early 2022.

As you can see by the map above, some northern and western states have reported literally scores of infected wildlife, while others (particularly in the south and east) have reported few or none.  

Admittedly, it may come down to differences in climate and terrain (swamps vs. forests vs. deserts) and the fact that infected wildlife often die in remote places where their carcasses are quickly scavenged by other animals, making them unlikely to be discovered or tested.

But it also possible that some states are more proactive in investigating, and reporting suspected cases, than others. 

Curiously, despite increased reports of infected birds in 2025, we've seen a huge drop off in reports of infected wildlife confirmed by the USDA since last spring. This comes after a significant surge in submissions in late 2024 and into early 2025 (see graph below). 

The above graph is based on collection dates, not confirmation dates (which may come months later). During the 1st 3 months of 2025, the USDA averaged 43 positive submission per month.

In the 8 months since the, that number has dropped 90%, averaging just 4 per month (note: given delays in testing/reporting those numbers could change). Exactly what accounts for this precipitous plunge isn't clear. 

The USDA officially added domestic cats to their list on May 30th, 2024 (they subsequently backfilled a number of  previous reports), and - as the following graph illustrates - infected house cats were a big story for the rest of that year and into early 2025.

But once again, we see the same dramatic drop off in positive reports since last spring. 

It is possible that these declining numbers truly represent a drop in mammalian spillovers (which would be a positive sign) since we are also seeing a similar trend in infected U.S. livestock and mammalian wildlife reports from around the world. 

But they may simply mirror our growing weariness from - or indifference to - the HPAI threat. The public remains largely apathetic (see Two Surveys (UK & U.S.) Illustrating The Public's Lack of Concern Over Avian Flu), and many governments are more than happy to downplay the threat for political or economic reasons.

We continue to see pleas from public health agencies and researchers for better surveillance and reporting (see here, here, here, here, here, and here); yet many countries continue to hold outbreak information close to their vest (see From Here to Impunity)

The problem with all of this is; viruses don't read and are not influenced by sanitized reports. They simply do whatever they can do.

And while ignorance on our part may be bliss, it can also be tragically short-lived.

The Lancet: The Threat of Another Coronavirus Pandemic

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Although influenza A has historically been the cause of most major respiratory pandemics, three times between 2002 and 2020 we watched as (3 different) novel  coronaviruses emerged from the wild, and threatened  a global health crisis. 

• First, in 2002, SARS-CoV (see SARS & Remembrance) spread hidden in China for months before it burst on the international scene.  Luckily, its inability to spread asymptomatically - and strict quarantines - limited its impact. 

• Ten years later (2012) another coronavirus - MERS-CoV - began spilling over from camels on the Arabian Peninsula, and then spreading (primarily in hospitals) from human-to-human (see Ziad Memish: Two MERS-CoV Hospital Super Spreading Studies).

• While deadlier than SARS-CoV, MERS-CoV never quite achieved the same level of transmission, and so outbreaks have remained limited. Still, thousands have been confirmed infected, and roughly 35% have proven fatal. 

• And of course in 2020, a novel coronavirus (SARS-CoV-2) sparked the worst human pandemic in over 100 years.  While estimates vary, many experts put the number of deaths in the tens of millions. 

MER-CoV and SARS-CoV are still very much with us - both continue to cause illness and death - and most importantly, both continue to evolve (see Nature: CoV Recombination Potential & The Need For the Development of Pan-CoV Vaccines).

All of which suggests a new, more virulent COVID variant, or a more transmissible MERS-CoV mutation, could still be in our future (see Health Sci Rpts: Pathogenicity and Potential Role of MERS-CoV in the Emergence of “Disease X”).

But at the same time we've seen numerous reports of newly discovered coronaviruses circulating in the wild which also appear to have zoonotic potential. Just a few, of many, include:

Preprint: A Divergent Betacoronavirus with a Functional Furin Cleavage Site in South American Bats

J. of Infection: Novel Coronaviruses Identified in Livestock

Viruses: Novel Rodent Coronavirus-like Virus Detected Among Beef Cattle with Respiratory Disease in Mexico

Nature: Study on Sentinel Hosts for Surveillance of Future COVID-19-like Outbreaks

Preprint: Human Cell adaptation of the Swine Acute Diarrhea Syndrome Coronavirus Spike Protein

While it may seem overly sanguine to ask how we are preparing to deal with another coronavirus pandemic when we seem incapable of dealing with far more immediate threats that are already on our plate (i.e. COVID & HPAI) - given the above examples - is still a question worth asking. 

Today we've a short, but somewhat technical commentary on recently identified ACE2-using merbecoviruses - like HKU5-CoV-2 - which may represent potential future coronavirus pandemic threats. 

Interestingly, this article suggests that prior COVID-19 infection is unlikely provide meaningful antibody protection against these HKU5-like merbecoviruses, reporting that sera from people who had SARS‑CoV‑2 did not neutralize HKU5 in vitro. 

While the serum from patients with a previous SARS-CoV-2 infection contained antibodies that failed to neutralise HKU5-CoV, three of 28 patients with a past MERS-CoV infection harboured antibodies that neutralised HKU5-CoV.7  This finding highlights the vast antigenic differences between merbecoviruses and sarbecoviruses, which likely stem from structural changes in the spike protein.

Due to its technically dense content, I've elected to post the link and a very brief excerpt. Follow the link to read it in its entirety.  I'll have a bit more after the break.

The threat of another coronavirus pandemic: how are we preparing for it?

Marcus G Maha,c ∙ Neha Dikshita,c ∙ Ramona Alikiiteaga Gutierreza,b ∙ David Chien Lyea,b,c,d,e,f ∙ Lin-Fa Wanga,c,g linfa.wang@duke-nus.edu.sg

Published December 12, 2025

DOI: 10.1016/j.lanmic.2025.101308 External Link

Copyright: © 2025 The Author(s). Published by Elsevier Ltd.

User License: Creative Commons Attribution (CC BY 4.0)

 

Merbecovirus is a subgenus within the genus Betacoronavirus that is genetically distinct from Sarbecovirus (eg, SARS-CoV-2). Recently, a few merbecoviruses isolated from the bat genus Pipistrellus were characterised and shown to have the ability to bind to angiotensin-converting enzyme 2 (ACE2) as an entry receptor.

These merbecoviruses include HKU5-CoV isolated from Hong Kong,1 HKU5-CoV-2 isolated from China,2 PnNL2018b isolated from the Netherlands,3 and MOW15-22 isolated from Russia.3 

This unexpected ability to bind to ACE2 was first observed in the African bat merbecoviruses NeoCoV and PDF-2180.4 Furthermore, these bat merbecoviruses bind only to ACE2, whereas the prototype merbecovirus MERS-CoV binds only to dipeptidyl peptidase-4 (DPP4).

(SNIP)

Coronaviruses will continue to remain a threat to public health. In this view, the scientific community can better prepare the world through the formation of the CORC-CoV, which fosters increased global cooperation, in line with the newly minted pandemic agreement. The comprehensive road map being developed by the CORC-CoV will also serve to engage funders towards supporting global efforts to overcome the identified primary research challenges and address key needs essential for advancements in the field of coronavirus research. Most importantly, the road map will help to drive priority research activities in preparation for the next coronavirus pandemic that could occur at any time.

(Continue . . . )

While we've heard a lot of voices urging stepped up surveillance, reporting, and pandemic preparedness in the wake of the last crisis (see WPRO Table-Top Exercise Crystal: A `Bovine' Novel Flu Outbreak Scenario), it is far less clear whether anyone in power is truly listening. 

Special interests, economic concerns, and the desire for short-term political gains all appear to cancel out - or marginalize - those voices. 

While I've no special insight into what will spark the next global health crisis - when that will come, or how severe it might be - history tells us another pandemic is inevitable. 

Assuming, of course, that we're actually listening. 

 

USDA: Wisconsin Becomes 18th State to Detect HPAI H5N1 in Dairy Cattle

 

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While the above graphic from the USDA's National Milk Testing Strategy (NMTS) website hasn't been updated to show it yet, late yesterday the USDA announced that Wisconsin - the second-largest milk producer in the U.S. - has at least one herd that has tested positive for HPAI H5N1.

First, the USDA's announcement, after which I'll have a bit more.

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

Contact: aphispress@usda.gov
Milk Supply Safe; Considered Low Risk to Human Health and Safety
WASHINGTON, DC- December 14, 2025 – 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) and ELISA (Enzyme-Linked Immunosorbent Assay) assay a detection of highly pathogenic avian influenza (HPAI) H5 clade 2.3.4.4b in a dairy cattle herd in Wisconsin. APHIS will complete genetic sequencing and announce final results as soon as they are available. This confirmation was a result of routine National Milk Testing Strategy testing, not pre-movement surveillance.

This marks the first known case of HPAI in cattle in Wisconsin. While dairy cattle in a total of 18 states have been infected since the start of the outbreak in March 2024, APHIS has seen cases in only a small number of states this year. APHIS is working closely with the Wisconsin Department of Agriculture, Trade, and Consumer Protection 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.

While reports of infected dairy herds have dwindled markedly during the second half of 2025, the USDA does not publish data on reinfected herds, as noted below by the California Department of Food & Agriculture website, and it isn't entirely clear what percentage of U.S. herds are currently tested on a regular basis.  

National Detections

This nationwide outbreak of H5N1 Bird Flu began in poultry in 2022 and was first detected in US dairy cattle in March 2024. All national detections in cattle can be found on the USDA website at: HPAI Confirmed Cases in Livestock | Animal and Plant Health Inspection Service (usda.gov). All national detections in poultry can be found on the USDA website at: Confirmations of Highly Pathogenic Avian Influenza in Commercial and Backyard Flocks | Animal and Plant Health Inspection Service (usda.gov)

Note: The USDA website reports new (first time detections) in dairies; it does not capture when a herd is cleared, ongoing cases, or reinfections on premises that had previous detections.

It will be of considerable interest to learn which genotype of HPAI H5N1 has been detected.  During all of 2024, only genotypes B3.13 was detected in cattle, but in early 2025 genotype D1.1 was reported in at least 2 states. 

For more information on the USDA's National Milk Testing Strategy you may wish to visit: 

Frequently Asked Questions: National Milk Testing Strategy