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

 Cats Infected With H7N2 - NYC 2016

#18,993

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?