#18,760
Over the summer of 2023 we saw two high-profile H5N1 outbreaks in domestic cats; one in Poland and the other in South Korea. Unlike the Polish outbreak - where details were slower to emerge - the South Korean outbreak was quickly linked to contaminated cat food.
Both outbreaks occurred nearly a full year before the USDA added domestic cats to their Mammals With H5N1 list, which now contains 139 domestic cats, and scores of other wild and captive felines.
While felines had long been considered poorly susceptible to influenza A viruses (IAVs), in 2004 we saw the deaths of dozens of captive tigers who'd been fed H5N1 contaminated chicken. Since then we've learned that cats are susceptible to a wide range of IAVs (see A Brief History Of Avian Influenza In Cats).
In the Polish outbreak both indoor - and indoor/outdoor cats - across a wide swath of the country were affected (see map above). The authors of today's report summarized that outbreak:
The epizootic in Poland in 2023 was unprecedented in scale, with HPAI H5N1 virus infection confirmed in 29 domestic cats, one captive caracal, a few pet ferrets, and a dog [3,4,14,15]. However, the number of affected animals was likely higher, with cases reported nationwide.
The authors noted: All sequences carried mammalian-adaptive mutations in the polymerase gene, specifically PB2-E627K and PB2-K526R, associated with enhanced replication at lower temperatures [2,3,4,14,15].
We've previously looked at these cases, including in:
Eurosurveillance: Two Papers On HPAI H5N1 clade 2.3.4.4b virus in cats, Poland, June to July 2023
Viruses: Natural Infection With HPAI H5N1 In Pet Ferrets - Poland, 2023
Microorganisms: Case Report On Symptomatic H5N1 Infection In A Dog - Poland, 2023
While some experts suspected H5N1 contaminated poultry had entered the food chain, Poland's MOA vigorously denied the claim (see Poland : Ministry of Agriculture Statement On H5N1 & Food Safety). In the end no single route of infection was ever identified.
Instead, a combination of 3 different routes was suggested:
- Contaminated poultry meat
- Direct contact with infected birds
- Environmental contamination
Out of those 835 cat serum samples - 68 cats (8.1%) tested positive for influenza A virus antibodies in the primary screening (with 3 more suspected). Of those 68 IAV positive samples, 23 were positive for H5-specific antibodies.
Intriguingly, 2/3rds of the IAV positive samples were of a non-H5 origin, but further identification was beyond the scope of this study, and so we don't know if they were human or avian IAVs.
While most documented feline H5N1 infections have proved fatal, a seroprevalence of 3% of H5 antibodies is significant, because it suggests these felines are survivors of a past H5 encounter.
Which means that some mild, perhaps even subclinical, feline infections with the H5 virus are likely going unreported. Due to its length, I've just posted the link, abstract, and a few excerpts.
Follow the link to read the study in its entirety. I'll have a postscript after the break.
Anna Golke 1,,Tomasz Dzieciątkowski 2,Olga Szaluś-Jordanow 3,Michał Czopowicz 4,Lucjan Witkowski 4,Monika Żychska 4,Ewa Domańska 5,Dawid Jańczak 6,Tomasz Nalbert 4,Stephanie Lesceu 7,Marzena Paszkowska 8,Justyna Giergielewicz 8 and Tadeusz Frymus 3 D
Viruses 2025, 17(6), 855; https://doi.org/10.3390/v17060855
Published: 16 June 2025
Abstract
Recently, cats have emerged as potential incidental hosts for avian and human influenza A viruses (IAVs), including the highly pathogenic avian influenza (HPAI) H5N1 virus. Following an unprecedented outbreak of H5N1 HPAI in cats in Poland in June 2023, we conducted a cross-sectional epidemiological study to assess the seroprevalence of IAV, especially H5Nx, infections in domestic cats.
Eight hundred thirty-five serum samples collected in June 2023 were tested using a competitive ELISA for antibodies to IAV nucleoprotein. Positive or doubtful samples were further screened for H5-specific antibodies. The overall seropositivity for IAV was 8.5% (CI 95%: 6.8%, 10.6%; 71/835 cats), and 23/68 IAV-seropositive cats (33.8%) were also seropositive for H5 antigen. Multivariable analysis identified young age (≤8 years) and male sex as significant risk factors for H5 seropositivity, while non-H5-IAV seropositivity was more common in cats aged ≥12 years.
These findings suggest different exposure pathways and host risk profiles for H5 and non-H5 IAVs and underscore the importance of enhanced surveillance in cats, particularly in regions affected by HPAI outbreaks. Given the susceptibility of cats to both avian and human IAVs, including subclinical infections, there is a theoretical risk for viral reassortment. Preventive measures, including vaccinating humans and restricting outdoor access for cats, should be considered in endemic areas.
(SNIP)
The observed anti-H5 seroprevalence in Polish domestic cats raises essential questions about the likely sources of exposure, which we cannot definitively answer. It is possible that outdoor cats encounter infected birds or other wild animals. Since well-fed domestic cats often leave their prey uneaten, owners may remain unaware of such interactions [29].
Our findings suggest a need to reconsider the role of companion animals as potential intermediate hosts in the ecology and evolution of IAVs. The U.S. CDC’s Influenza Risk Assessment Tool (IRAT) and the WHO’s Tool for Influenza Pandemic Risk Assessment (TIPRA) estimate a low pandemic risk for H5N1 clade 2.3.4.4b [39]. However, these tools assess only the current risk and do not predict the virus’s future evolutionary trajectory or reassortment potential. Considering that on March 24, the UK Department for Environment, Food & Rural Affairs and the Animal and Plant Health Agency issued a statement confirming that the Chief Veterinary Officer had identified a case of avian-origin influenza (H5N1) in a sheep in Yorkshire, it is likely that we will soon have to contend with new routes of virus transmission in Europe [40].
We now have more information about the global distribution, host range, and genetic diversity of the H5N1 virus than for most other zoonotic pathogens. So far, most surveillance systems remain biased toward symptomatic or dead animals. In contrast, the COVID-19 pandemic has underscored that subclinical infections play a substantial role in pathogen transmission and contribute to maintaining epidemics at the population level [41].
In light of the above, future research should prioritize the following: (i) including feral and free-roaming cat populations in serological studies; (ii) confirming ELISA results with other methods to determine subtype-specific exposure; and (iii) investigating behavioral risk factors and their relation to infection dynamics through case–control or cohort designs. These steps will help address current methodological limitations and enhance our understanding of IAV ecology in companion animals.
While the CDC continues to rank the risk to general public from avian flu as low, they do provide very specific guidance to pet owners on how to limit their risk of infection from the virus.
Bird Flu in Pets and Other Animals
Key points
- Avian influenza viruses, which can cause bird flu illness, mainly infect and spread among wild birds and domestic poultry. However, some avian influenza viruses can infect and spread to other animals, including pets.
- While it is unlikely that you would get sick with bird flu from direct contact with your infected pet, it is possible.
- If your pets (including pet birds, cats or dogs) go outside and eat or are exposed to sick or dead birds, dairy cows, or other animals infected with avian influenza viruses, they could become infected.
- Prevent pets from interacting with wild birds, backyard poultry, cows, or other outdoor animals.
- Keep pets away from clothes, surfaces or environments that could potentially be contaminated with avian influenza viruses.
- Do not feed pets raw pet food or unpasteurized (raw) milk.
Prevention measures for people
- As a general precaution, people should avoid direct contact with wild birds and observe wild birds only from a distance.
- Pet owners should prevent their pets (including pet birds, dogs, and cats) from interacting with potentially infected dairy cows, backyard flocks, and wild animals.
- Pet owners should not let their pets consume raw pet food or raw (unpasteurized) milk.
- Pet owners should prevent their pets from touching clothes or other surfaces or environments that could potentially be contaminated with avian influenza viruses.
- Do not touch sick or dead birds, their feces, litter, or any surface or water source (ponds, waterers, buckets, pans, troughs) that might be contaminated with their saliva, feces, or any other bodily fluids without wearing PPE.
