Influenza A Virus in Cats

Overview and Taxonomy of Influenza A Virus in Cats

Taxonomic Classification and Virological Basis

Influenza A viruses (IAVs) belong to the family Orthomyxoviridae, a group of enveloped, negative-sense, single-stranded RNA viruses characterized by a segmented genome comprising eight gene segments. The taxonomic distinction of IAVs is fundamentally determined by the antigenic properties of the two major surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). To date, 18 HA subtypes (H1–H18) and 11 NA subtypes (N1–N11) have been identified, with H17N10 and H18N11 being exclusive to bat species. The natural reservoir for the vast majority of IAV subtypes is wild aquatic birds, particularly those belonging to the orders Anseriformes (ducks, geese, swans) and Charadriiformes (gulls, terns). From this avian reservoir, IAVs have demonstrated a remarkable capacity for cross-species transmission, successfully establishing sustained lineages in a diverse array of mammalian hosts, including humans, swine, horses, dogs, and, as increasingly documented, domestic cats (Felis catus) [12, 19].

The classification of IAVs extends beyond HA and NA subtyping to include pathotype designation, specifically, the distinction between low pathogenicity avian influenza (LPAI) and highly pathogenic avian influenza (HPAI) viruses. This distinction is primarily based on the presence of a multibasic cleavage site (MBCS) in the HA0 precursor protein, which facilitates systemic replication in gallinaceous poultry. Critically, HPAI viruses, particularly those of the H5 and H7 subtypes, have been responsible for the most severe documented infections in cats, often resulting in systemic disease with high mortality [1, 5, 7, 17]. The emergence of the HPAI H5N1 clade 2.3.4.4b virus, and its subsequent reassortment into genotypes such as B3.13 and D1.1, has fundamentally altered the epizootiological landscape for feline influenza, marking a departure from previous paradigms where feline infection was considered a sporadic, dead-end event [5, 10, 16].

Historical Context and Emergence of Feline Influenza

The susceptibility of domestic cats to IAV infection was recognized well before the contemporary panzootic of HPAI H5N1. Early experimental studies demonstrated that cats could be infected with human influenza viruses, though clinical disease was often mild or subclinical. However, the landscape shifted dramatically in the early 2000s with the widespread circulation of HPAI H5N1 (clade 2.3.2.1 and related lineages) in Southeast Asia. Natural infections in cats were documented, often linked to the consumption of infected poultry carcasses, and these infections were characterized by severe respiratory and neurological disease, systemic viral dissemination, and high mortality [17]. Subsequent to this, the 2009 H1N1 pandemic (A(H1N1)pdm09) provided further evidence of feline susceptibility to human-adapted viruses. Experimental intratracheal inoculation of cats with A(H1N1)pdm09 resulted in diffuse alveolar damage and demonstrable cat-to-cat transmission, confirming that cats are not merely incidental hosts but can serve as competent vectors for certain IAV strains [15].

The most significant turning point in the history of feline influenza, however, occurred in 2024 with the spillover of HPAI H5N1 clade 2.3.4.4b, genotype B3.13, into dairy cattle in the United States. This event created a novel and highly efficient transmission interface for cats. The virus, exhibiting a distinct tropism for the epithelial cells lining the alveoli of the bovine mammary gland, was shed in high concentrations in raw, unpasteurized milk [5, 10]. Cats on affected dairy farms, as well as domestic cats in households of dairy workers, were exposed to this contaminated milk, leading to a cluster of severe, often fatal, infections [1, 6]. This epizootiological nexus, avian virus → bovine amplification → feline spillover, represents a previously unrecognized pathway for mammalian adaptation and transmission of HPAI, underscoring the virus's extraordinary evolutionary plasticity.

Subtypes and Strains Documented in Cats

The breadth of IAV subtypes capable of infecting cats is broader than historically appreciated. Serological and molecular surveillance studies have documented exposure to a range of subtypes, reflecting the multiple ecological niches cats occupy. These include:

Highly Pathogenic Avian Influenza (HPAI) H5N1: This subtype has been the most consequential for feline health. Since the emergence of clade 2.3.4.4b, infections have been documented across multiple continents, including North America, Europe, and Asia. In the Netherlands, a serosurvey of stray cats conducted between 2020 and 2023 revealed an HPAI H5 seroprevalence of 11.8%, indicating frequent exposure, likely through predation on infected wild birds [4]. In contrast, domestic cats in the same study showed a significantly lower seroprevalence of 0.46%, highlighting the role of foraging behavior as a primary risk factor. The clinical presentation of HPAI H5N1 in cats is often severe, progressing from fever and lethargy to acute respiratory distress, neurological signs (including seizures and ataxia), and death. Postmortem findings frequently reveal bronchointerstitial pneumonia, hepatic and lymphoid necrosis, and viral antigen in the brain, lung, and kidney [7, 17]. The isolation of infectious H5N1 virus from the urine of a surviving cat that consumed raw milk further underscores the potential for alternative routes of viral shedding and environmental contamination [6].

Low Pathogenicity Avian Influenza (LPAI) Viruses: Cats are not exclusively susceptible to HPAI strains. Experimental infection of 5-month-old cats with North American LPAI viruses of the H1N9 and H6N4 subtypes, isolated from shorebirds, resulted in productive viral replication, seroconversion, and the development of patchy bronchointerstitial pneumonia, albeit without overt clinical disease [14]. This finding is of profound significance, as it demonstrates that cats can serve as a permissive host for a wide array of avian IAVs, including those that do not cause mass mortality in poultry. Such infections, while subclinical, provide a potential arena for viral reassortment, where a cat co-infected with an LPAI virus and a mammalian-adapted virus could generate a novel, potentially zoonotic, reassortant.

Pandemic H1N1 (A(H1N1)pdm09): Following the 2009 pandemic, serological studies consistently demonstrated high rates of exposure to this subtype in domestic cats. In the Netherlands, 4.6% of domestic cats were seropositive for H1, with 26 of 40 positive samples confirmed by hemagglutination inhibition assay [4]. Similarly, a study in central Chile found that 23.3% of cats were seropositive for IAV by NP-ELISA, with two positive samples showing hemagglutination inhibition titers against A(H1N1)pdm09 [11]. These data indicate that reverse zoonosis, transmission of human influenza viruses to cats, is a frequent and ongoing event, particularly in households where cats are in close contact with infected humans.

Avian H7N2: An outbreak of LPAI H7N2 virus in a cat shelter in New York City in 2016 resulted in a rare and well-documented case of zoonotic transmission from a cat to a human [3]. The virus isolated from the infected human was closely related to the feline isolate, and both were derived from North American lineage H7N2 viruses that had circulated in poultry in the early 2000s. This event provided definitive proof that cats can act as a bridging host for the transmission of avian influenza viruses to humans, a critical consideration for public health risk assessment. The H7N2 virus involved in this outbreak is notable for lacking the 220-loop in its hemagglutinin, a structural feature that confers dual receptor specificity for both avian (α2,3-linked sialic acid) and human (α2,6-linked sialic acid) receptors, thereby facilitating cross-species infection [18].

Other Subtypes: Serological evidence from a global meta-analysis, encompassing 34 studies and 2,882 cats, estimated a pooled IAV seroprevalence of 7.39% (95% CI: 3.17–13.12) [9]. This analysis, along with other serosurveys, has detected antibodies against a variety of other subtypes, including H3N2, H3N8, and H9N2, indicating that the true diversity of IAV exposure in cats is likely underestimated [2, 13]. The use of complementary serological assays, such as ELISA against the complete HA ectodomain and nanoparticle-based hemagglutination inhibition assays, has been instrumental in revealing this broader exposure profile, as traditional assays may lack the sensitivity to detect antibodies against divergent avian strains [2].

The Role of Cats in IAV Ecology and Evolution

The accumulating evidence positions the domestic cat not merely as an accidental victim of IAV spillover but as a potentially significant participant in the virus's ecology. Several factors contribute to this role. First, cats are highly abundant in both urban and rural environments globally, living in close proximity to humans, poultry, and wildlife. Second, their predatory behavior, particularly in stray and feral populations, brings them into direct contact with infected wild birds, the primary reservoir of IAV diversity [4, 8]. Third, their susceptibility to both avian and mammalian-adapted IAVs creates the potential for co-infection and genetic reassortment. The segmented nature of the IAV genome allows for the exchange of gene segments when two different viruses infect the same cell. A cat simultaneously infected with an avian H5N1 virus and a human A(H1N1)pdm09 virus could theoretically give rise to a reassortant virus possessing the HA of H5N1 (highly pathogenic, novel to humans) and the internal genes of the pandemic H1N1 (adapted for replication and transmission in mammals). This scenario represents a credible pathway for the emergence of a pandemic virus [2, 12].

The World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) have recognized the importance of monitoring IAV in companion animals, particularly in the context of the ongoing H5N1 panzootic. The detection of HPAI H5N1 in cats on dairy farms, and the subsequent identification of multidirectional interspecies transmission (bird-to-cow, cow-to-cat, cat-to-bird) on affected premises, underscores the complexity of these spillover events [5, 10]. The U.S. Centers for Disease Control and Prevention (CDC) has issued guidance for veterinarians and public health officials, emphasizing the need for a One Health approach that integrates human, animal, and environmental health surveillance. This includes testing cats with respiratory or neurological signs for IAV, particularly if they have a history of exposure to raw milk, infected poultry, or sick wild birds [1]. The inclusion of influenza A in the differential diagnosis for cats with compatible clinical signs is no longer a matter of academic curiosity but a practical necessity for protecting both animal and human health.

Molecular Pathogenesis of HPAI H5N1 in Feline Hosts

The molecular pathogenesis of highly pathogenic avian influenza (HPAI) H5N1 virus in domestic cats represents a paradigm of cross-species spillover, systemic viral dissemination, and severe immunopathology that distinguishes it from the typically respiratory-restricted infections observed in humans and swine. Since the emergence of clade 2.3.4.4b viruses, particularly the genotype B3.13 reassortant, felines have been thrust into a critical nexus of mammalian adaptation and zoonotic risk, exhibiting a disease phenotype that is both rapidly fatal and epidemiologically alarming [1, 5]. This analysis dissects the discrete molecular mechanisms, from cell entry and cleavage activation to systemic spread and neuroinvasion, that underpin the extraordinary virulence of HPAI H5N1 in feline hosts.

Viral Entry, Receptor Specificity, and the Cleavage Activation Paradigm

The tropism of HPAI H5N1 for feline tissues is fundamentally governed by the molecular architecture of the hemagglutinin (HA) glycoprotein and its interaction with host sialic acid (SA) receptors. Unlike human-adapted influenza viruses that preferentially bind α2,6-linked SA receptors concentrated in the upper respiratory tract, the avian-origin HA of clade 2.3.4.4b viruses retains a strong affinity for α2,3-linked SA receptors, which are abundant in the lower respiratory tract of cats, particularly within bronchiolar and alveolar epithelial cells [14, 18]. Importantly, the feline respiratory tract is known to express a heterogeneous distribution of both SAα2,3 and SAα2,6 linkages, though the α2,3-glycan repertoire is dominant in the deep lung, providing a permissive environment for avian virus attachment and subsequent infection. This receptor compatibility is further refined by amino acid substitutions within the receptor-binding site (RBS) of HA, such as those observed in the H7N2 lineage during feline adaptation, where mutations like A135S or A135T introduced novel N-glycosylation sites that modulated receptor avidity [18]. In the context of H5N1 clade 2.3.4.4b, the presence of a multibasic cleavage site (MBCS) at the HA0 cleavage junction, a hallmark of high pathogenicity, is the single most critical determinant of systemic virulence. The MBCS, typically composed of multiple basic amino acid residues (e.g., -RRRKKR-), renders the HA precursor cleavable by ubiquitous furin-like proteases expressed in virtually all mammalian cell types, rather than being restricted to trypsin-like proteases localized in the respiratory tract [12]. This molecular switch liberates the virus from tissue confinement, enabling de novo replication in extrapulmonary organs such as the brain, liver, pancreas, lymphoid tissues, and myocardium [7, 17].

Systemic Dissemination and the Molecular Basis of Neurotropism

The capacity of HPAI H5N1 to cause fatal systemic disease in cats is a direct consequence of both the HA MBCS and specific adaptive mutations in the viral polymerase complex. Post-entry, the viral ribonucleoprotein (vRNP) complex must achieve efficient replication in diverse cellular environments. A paramount molecular adaptation for mammalian replication is the acquisition of a glutamic acid-to-lysine substitution at residue 627 (E627K) or, alternatively, an aspartic acid-to-asparagine at residue 701 (D701N) in the PB2 protein [5, 23]. These mutations enhance the interaction of PB2 with the host importin-α protein, facilitating nuclear import of the vRNP and optimizing replication at the lower body temperatures typical of mammalian hosts. The PB2 E627K mutation has been consistently identified in H5N1 isolates from naturally infected cats in both Poland and the United States, underscoring its role in feline adaptation [7, 17]. Once robust replication is established in the pulmonary parenchyma, typified by severe bronchointerstitial pneumonia with abundant viral antigen in type II pneumocytes and alveolar macrophages, the virus exploits a hematogenous and possibly neural route to disseminate. Viral RNA and antigen have been detected in the brain parenchyma of cats, most prominently in neurons and glial cells of the cerebrum, cerebellum, and brainstem [7, 17]. The resultant non-suppurative encephalitis, characterized by perivascular lymphohistiocytic cuffing, gliosis, and neuronal necrosis, is a directly virus-mediated cytopathic effect rather than an immunopathological bystander effect [17]. This neurotropism is further underscored by the presence of viral RNA in the urine of surviving cats, suggesting that the virus can also navigate past the blood-brain barrier and percolate through the renal system, raising critical questions about alternative routes of viral shedding and environmental contamination [6, 7].

The Feline Hemophagocytic and Hepatic Injury Response

A distinctive molecular feature of HPAI H5N1 infection in cats is the profound involvement of the reticuloendothelial system and hepatic parenchyma, which mirrors the pathology observed in severe human H5N1 cases but is far more fulminant in felines. Postmortem examinations consistently reveal hepatic necrosis and lymphoid depletion in the spleen and lymph nodes [7, 17]. The molecular driver of this injury is multi-faceted. Direct viral cytotoxicity, mediated by the induction of apoptosis via the NS1 protein's ability to inhibit the host interferon response and activate caspase cascades, leads to hepatocyte and lymphocyte destruction [12]. Concurrently, the virus triggers a hyper-inflammatory state akin to a cytokine storm, with elevated levels of TNF-α, IL-6, and IFN-γ detectable in tissue homogenates, which amplifies collateral tissue damage. In the immunocompromised cat, a scenario documented in a diabetic feline with concurrent feline infectious peritonitis (FIP), this systemic assault is particularly devastating, as the attenuated immune system is incapable of mounting a controlled antiviral response, leading to rapid multi-organ failure [7]. The presence of severe bone marrow degeneration in this case further indicates that HPAI H5N1 can infect and deplete hematopoietic progenitor cells, compounding the lymphopenia and immunosuppression that already characterize the host [7].

Viral Shedding, the Raw Milk Transmission Niche, and Zoonotic Implications

Perhaps the most consequential molecular epidemiological twist in the recent H5N1 feline story is the establishment of a lactational transmission niche through the ingestion of unpasteurized milk from infected dairy cattle. The HPAI H5N1 clade 2.3.4.4b virus, genotype B3.13, exhibits a novel and robust tropism for the mammary epithelium of dairy cows, leading to the secretion of extraordinarily high viral titers in milk, reaching 10⁷ to 10⁸ TCID₅₀/mL [5, 10]. When cats consume this contaminated raw milk, the virus is delivered directly to the gastrointestinal tract, where the low pH of the feline stomach proves an insufficient barrier against infection [6]. The virus subsequently invades the gastrointestinal mucosa and rapidly disseminates to the systemic circulation. This route bypasses the traditional respiratory portal, yet the clinical outcome is equally severe: fever, profound neurologic signs (ataxia, seizures, blindness), and death within 24–48 hours in a substantial proportion of cases [1, 6]. The mortality rate among cats infected via this route has been estimated at approximately 50%, a stark figure that underscores the virulence of the inoculum [20]. Critically, the isolation of infectious virus from the urine of a surviving cat not only confirms systemic viremia but also identifies a novel environmental shedding pathway [6]. Urine-contaminated surfaces within dairy worker households, as described in the Michigan cluster, pose a direct zoonotic risk to humans, particularly to immunocompromised individuals or those with frequent occupational exposure [1]. The CDC and WOAH have emphasized that such fomite-mediated transmission from cats to humans, though rare, represents a non-respiratory route of spillover that must be integrated into One Health surveillance frameworks.

Molecular Determinants of Disease Severity: The NS1 and PB1-F2 Axis

The difference in pathogenesis between HPAI H5N1 and low-pathogenicity or seasonal influenza infections in cats can be traced to two accessory proteins: NS1 and PB1-F2. The NS1 protein of H5N1 clade 2.3.4.4b is a potent antagonist of the host type I interferon (IFN) system. By binding to the RIG-I receptor and preventing the phosphorylation and nuclear translocation of IRF3, NS1 effectively silences the innate antiviral alarm, allowing the virus to replicate unchecked for the first 24–48 hours post-infection [12, 22]. Additionally, a PDZ domain ligand motif in the C-terminus of NS1 from avian influenza viruses has been shown to interact with cellular PDZ-domain proteins, disrupting tight junction integrity and promoting vascular leakage, a mechanism that likely contributes to the severe pulmonary edema and hemorrhagic lesions observed in feline lungs [14]. The PB1-F2 protein, while variable in sequence, is typically full-length in avian viruses and localizes to the mitochondria, where it induces apoptosis of immune effector cells such as alveolar macrophages and CD8+ T lymphocytes, further crippling the host's adaptive immune response [12]. This combined action of NS1 and PB1-F2 creates a permissive environment for the virus to reach titers sufficient for systemic dissemination and neuroinvasion, a feat rarely achieved by human-adapted subtypes such as pandemic H1N1 [15].

Histopathological Correlates and Cellular Tropism in the Lung

Detailed histopathological and immunohistochemical analyses of lung tissues from H5N1-infected cats reveal a pattern of severe, diffuse bronchointerstitial pneumonia with a molecular signature of alveolar epithelial cell necrosis. Viral antigen is localized predominantly in type II pneumocytes (the surfactant-producing cells) and alveolar macrophages, with the latter serving as both target cells and vehicles for viral transport to the draining lymph nodes [7, 14]. The detection of viral antigen within the alveolar epithelium correlates with the rapid onset of hypoxemia and respiratory distress, as surfactant production is compromised and the alveolar-capillary barrier is disrupted. Lymphoid necrosis in the mediastinal lymph nodes and splenic white pulp is a consistent feature, driven by the depletion of T and B lymphocytes through Fas/FasL-mediated apoptosis initiated by the virus [7, 17]. In cases where meningoencephalitis is present, viral antigen is most abundant in the neurons of the limbic system and brainstem, mirroring the pattern seen in ferrets and H5N1-infected humans and corroborating the neurotropic potential of the circulating clade 2.3.4.4b viruses [17, 21].

The Role of Immunological Naiveté and Pre-Existing Morbidity

The feline population as a whole is immunologically naive to H5N1 virus, with seroprevalence studies in domestic cats in Europe and North America reporting rates below 1% for H5 antibodies [4, 8, 9]. This lack of pre-existing immunity means that upon first exposure, the virus encounters a host with no memory B or T cell responses, allowing for maximal viral replication prior to the onset of the adaptive response. In stark contrast, seroprevalence for H1N1pdm09 is considerably higher in domestic cats, suggesting that prior exposure to circulating human or swine influenza viruses may provide some degree of cross-reactive immunity at the neuraminidase level [2, 4]. However, the antigenic distance between the HA of H5N1 and human H1N1 or H3N2 renders this protection insufficient. In the specific context of immunocompromised cats, such as those with diabetes mellitus, chronic viral infections (FIP), or undergoing immunosuppressive therapy, the pathogenesis is accelerated and the clinical outcome is uniformly fatal [7]. The molecular basis for this heightened susceptibility lies in the impairment of both the innate (reduced IFN-γ production by NK cells) and adaptive (poor CD8+ cytotoxic T lymphocyte expansion) arms of the immune response, which provides a lower threshold for the virus to achieve systemic dissemination and neuroinvasion.

Implications for Viral Evolution and Mammalian Adaptation

The ongoing circulation of HPAI H5N1 clade 2.3.4.4b in dairy cattle, with frequent spillover into cats, creates a high-risk environment for further mammalian adaptation. Cats serve as amplifying hosts and potential mixing vessels for reassortment, as they can be concurrently infected with avian, human, and swine influenza viruses, though the latter has not been documented in the field [2, 12]. The accumulation of PB2 E627K and HA receptor-binding mutations in feline isolates points to a trajectory of adaptation that could enhance human transmissibility [18, 23]. The isolation of viable virus from feline urine and the detection of high viral loads in fecal matter from infected animals further underscore the need to consider cats as a potential source of environmental contamination and secondary transmission to humans in occupational settings, particularly on dairy farms [1, 6]. The CDC has included domestic cats in their guidance for testing at-risk individuals, recognizing that a feline respiratory or neurologic case may be the sentinel event for a zoonotic outbreak [1].

Epidemiology and Transmission Dynamics of Avian Influenza in Cats

The emergence and sustained circulation of highly pathogenic avian influenza (HPAI) A(H5N1) virus, particularly clade 2.3.4.4b, has fundamentally altered the epidemiological landscape for feline influenza A virus (IAV) infections. Historically considered incidental or dead-end hosts, cats are now recognized as active participants in the spillover, amplification, and potential reassortment of avian influenza viruses, a role that carries profound implications for both veterinary and public health. The integration of data from active surveillance, outbreak investigations, and experimental infections reveals a complex transmission ecology driven by dietary habits, environmental contamination, occupational exposure, and the expanding host range of the virus.

Global Seroprevalence and Prevalence Patterns

Understanding the true burden of avian influenza in cats requires careful interpretation of serological and molecular detection data. A comprehensive systematic review and meta-analysis encompassing 34 studies and 2,882 cats estimated a pooled seroprevalence of 7.39% (95% CI: 3.17–13.12) for influenza A virus antibodies, while the pooled molecular prevalence was considerably lower at 1.73% (95% CI: 0.00–7.10) [9]. This disparity underscores the transient nature of active viral shedding relative to the longer-lived antibody response, but also highlights the widespread exposure that occurs even in the absence of clinical detection. Significant geographic heterogeneity exists, with higher seroprevalence reported in the Americas and Asia compared to Europe, likely reflecting regional differences in viral circulation, surveillance intensity, and ecological risk factors [9].

The serological signal becomes even more pronounced when examined at the subtype level. In a Dutch study spanning October 2020 to June 2023, 11.8% (83/701) of rural stray cats were seropositive for HPAI H5 clade 2.3.4.4 antibodies, with 65 of these findings confirmed by hemagglutination inhibition (HAI) assay [4]. In stark contrast, only 0.46% (4/871) of domestic cats in the same study were H5 seropositive, and none were confirmed by HAI [4]. This 25-fold difference in seroprevalence between stray and domestic populations is one of the most striking epidemiological findings to date and points directly to a primary transmission pathway: foraging on infected wild birds. Indeed, stray cats living in proximity to nature reserves had a 5.4-fold higher odds (95% CI: 1.5–20.1) of H5 seropositivity, and older cats, with cumulative lifetime exposure, were also at significantly increased risk (OR 3.8; 95% CI: 2.7–7.1) [4]. These data align with the massive wild bird mortalities caused by HPAI H5N1 clade 2.3.4.4b, which has devastated seabird populations in Canada and Europe [25-27] and has been documented in at least 80 wild bird species in North America alone [26].

Conversely, seroreactivity to human pandemic H1N1 influenza was more common in domestic cats (4.6%) than in stray cats, reflecting the greater proximity of pet cats to infected humans [2, 4]. This dual susceptibility, to both avian and human IAV subtypes, positions cats as potential bridging hosts in which reassortment events could occur, a scenario of considerable concern to global health authorities, including the World Health Organization (WHO) and the World Organisation for Animal Health (WOAH).

Transmission Pathways: Wild Birds and Environmental Contamination

The primary route of infection for cats with avian influenza viruses is direct or indirect contact with infected wild birds. Experimental infections have definitively demonstrated that cats are susceptible to low pathogenic avian influenza (LPAI) viruses from wild birds. In one study, 5-month-old cats inoculated with North American shorebird viruses of subtypes H1N9 and H6N4 supported viral replication, seroconverted, and developed patchy bronchointerstitial pneumonia, despite remaining clinically asymptomatic [14]. Viral antigen was localized to alveolar epithelium, and pharyngeal shedding was documented, albeit variably [14]. This finding is critical: it indicates that subclinical infections with LPAI viruses are not only possible but likely underdetected, contributing to a cryptic transmission cycle that could facilitate viral adaptation to mammalian hosts.

For HPAI viruses, the consequences of wild bird exposure are more severe. A necropsy-confirmed case in a 6-year-old outdoor cat in Poland, which was fed raw chicken meat, revealed systemic A/H5N1 infection with viral RNA detected in the brain, lungs, liver, and intestines, alongside necrotic lesions and a multidrug-resistant Enterococcus faecium coinfection [17]. Although a definitive source could not be identified, the cat’s outdoor access and raw diet were considered the most probable exposure routes [17]. This case underscores the dual risk of predation and dietary contamination, a theme that recurs across multiple investigations.

The environmental contamination of wetlands and water sources by infected birds further amplifies exposure risk. Baiting of waterfowl in Canada has been associated with a 5.4- to 8.7-fold increase in the odds of avian influenza virus detection in sediment, and HPAI H5N1 was specifically identified at baited sites [28]. Cats that frequent such contaminated environments, whether as stray animals or those allowed outdoors, face heightened risk. The broader epidemiological context is that HPAI H5N1 clade 2.3.4.4b has undergone unprecedented host range expansion, with documented infections in over 48 mammalian species, including pinnipeds, mustelids, and felids [16, 19]. The WHO and the Food and Agriculture Organization (FAO) have repeatedly warned that such expansion increases the likelihood of viral adaptation to mammals, and cats are now firmly within this risk landscape.

Spillover from Livestock: The Dairy Cattle Interface

Perhaps the most consequential epidemiological development for feline influenza has been the spillover of HPAI H5N1 clade 2.3.4.4b, genotype B3.13, to dairy cattle in the United States, first confirmed in March 2024 [5, 10, 16]. By early 2025, the virus had been detected on over 1,078 cattle farms across 17 US states [16]. Critically, genomic and epidemiological data have demonstrated multidirectional interspecies transmission, with cats becoming infected on affected farms [5, 10]. The mechanism is now well understood: raw milk from infected cows contains high titers of infectious virus, and cats that consume this milk develop severe, often fatal disease.

A landmark investigation in Michigan documented HPAI A(H5N1) infection in two exclusively indoor cats from separate households, each belonging to workers in the dairy industry [1]. The owner of one cat worked on a dairy farm and had reported a splash exposure to unpasteurized milk; the other owner transported unpasteurized milk across multiple farms [1]. Both workers declined testing, but their occupational histories provided the clear link. This study was instrumental in demonstrating that cats need not have direct contact with birds or the outdoors to become infected; indirect fomite transmission, via contaminated clothing, footwear, or equipment, can introduce the virus into the home environment [1]. The CDC, in conjunction with state health departments, has subsequently recommended that veterinarians obtain occupational information from cat owners when evaluating animals with respiratory or neurologic signs, particularly in states with confirmed livestock infections [1].

The lethality of the raw milk transmission route was tragically illustrated in California, where three domestic cats consumed raw milk contaminated with HPAI H5N1. Two died, and the surviving cat had detectable viral RNA in its urine by RT-PCR, a finding that raises the possibility of environmental contamination through excreted virus [6]. Similarly, a diabetic cat with feline infectious peritonitis (FIP) developed acute HPAI H5N1 infection linked to fomite or environmental transmission; the virus was detected in urine, lung, brain, and lymphoid tissues, and the cat was euthanized due to rapid clinical deterioration [7]. The isolation of virus from urine is particularly noteworthy, as it suggests a previously underappreciated route of potential environmental spread [6, 7]. In the Russian Federation, screening of over 8,300 cattle sera from 330 farms found no H5 antibodies, indicating that as of early 2025 the virus had not yet established a foothold in Russian dairy herds [20]; however, the authors cautioned that ongoing monitoring is essential given the migration patterns of wild birds and the movement of livestock.

Interspecies and Intraspecies Transmission Dynamics

The capacity for cat-to-cat transmission of avian influenza has been experimentally confirmed. In a seminal study, cats intratracheally infected with pandemic H1N1 2009 virus developed diffuse alveolar damage, and sentinel cats co-housed with infected animals seroconverted, providing clear evidence of direct transmission via respiratory droplets or fomites [15]. For HPAI H5N1, the evidence for efficient feline-to-feline transmission is less definitive under field conditions, but the clustering of cases on dairy farms, where multiple cats have been found dead or dying, is highly suggestive [5, 20]. The mortality rate among cats in these agricultural settings has been reported as high as 50% [20].

The role of cats in the broader ecology of influenza A virus is further illuminated by molecular evidence. A study from the Netherlands detected no influenza A RNA in pharyngeal swabs or lung tissue from 230 cats, despite a seroprevalence of 11.8% in the stray population [4]. This finding suggests that viral shedding may be transient or that the window for molecular detection is narrow, making serosurveillance a more sensitive tool for estimating cumulative exposure. However, it also indicates that acutely infected cats may shed virus for only a limited period, potentially reducing the risk of sustained intraspecies transmission unless there are repeated introductions from environmental or dietary sources.

The zoonotic potential of feline influenza is not merely theoretical. The 2016 outbreak of low pathogenic avian influenza A(H7N2) in cats in a New York City animal shelter resulted in the confirmed infection of a human veterinarian who had exposure to sick cats [3]. The virus isolated from the human was closely related to that from a cat, and both were derived from North American lineage H7N2 viruses that had circulated in wild birds since the early 2000s [3]. This event demonstrated that cats could serve as a source of zoonotic infection even with a low pathogenicity virus, and it prompted enhanced biosecurity protocols in shelter environments. The WHO has classified H7N2 as a virus with pandemic potential, and the New York outbreak remains a sentinel event in understanding the risks posed by feline influenza.

Risk Factors and Moderating Variables

A constellation of risk factors modulates the probability and severity of avian influenza infection in cats. The most consistently identified variable is dietary exposure to raw or unpasteurized animal products. Raw chicken meat, unpasteurized milk, and offal from infected birds or cattle have been implicated in multiple case reports and outbreak investigations [1, 6, 7, 17]. The feeding of raw poultry to cats has been flagged as a high-risk practice by veterinary public health authorities, and the exclusion of fresh or frozen poultry from the diet has been recommended to reduce A/H5N1 transmission risks [24].

Outdoor access is a second major determinant. Stray and free-roaming cats are exposed to infected wild birds either through direct predation, scavenging of carcasses, or contact with contaminated water and soil. The seroprevalence data from the Netherlands, which showed a stark gradient from stray to domestic cats, provides compelling evidence [4]. Occupational exposure of owners, particularly in the dairy and poultry industries, introduces a third route, fomite transmission into the home [1]. Host immune status also plays a role; the case of the immunocompromised diabetic cat with FIP, which succumbed to a particularly severe and disseminated infection, illustrates that cats with underlying chronic illness or immunosuppression are at elevated risk for lethal outcomes [7].

Age is a further factor: older cats have higher odds of H5 seropositivity in stray populations, likely reflecting cumulative lifetime exposure [4]. However, experimental data also suggest that kittens and juvenile animals may be more susceptible to severe clinical disease, as seen in the ferret model where 9-week-old juveniles developed fulminant infections [24]. The implications for population dynamics are significant, if feline populations are exposed repeatedly, older cohorts may carry a higher seroprevalence but younger animals may bear the brunt of acute mortality.

Implications for One Health Surveillance

The epidemiology of avian influenza in cats is inseparable from the broader One Health context. Cats are sentinel species that can signal the presence of HPAI in the environment before human cases are recognized. The detection of HPAI A(H5N1) in two indoor cats in Michigan was a sentinel event that prompted public health investigations of dairy workers and led to enhanced biosecurity recommendations [1]. The CDC has since emphasized the importance of joint investigations by state and federal public health and animal health officials whenever feline infections are suspected in areas with livestock outbreaks [1].

International organizations, including the WHO, WOAH, and FAO, have called for integrated surveillance systems that include companion animals. The current evidence base, spanning seroprevalence of 0.46% to 11.8% in different feline populations, molecular detection rates under 2%, and documented zoonotic transmission [3], supports the conclusion that cats are not merely passive recipients of infection but active components in the transmission web. Their close association with humans, their predatory behavior, and their susceptibility to both avian and mammalian-adapted influenza viruses make them a critical node for monitoring viral evolution and assessing pandemic risk. The ongoing expansion of HPAI H5N1 into novel mammalian hosts, including dairy cattle, and the subsequent spillover to cats underscores the urgency of sustained, high-quality epidemiological research and the integration of feline health data into national influenza preparedness plans.

Clinical Manifestations and Pathological Findings in Infected Cats

Respiratory and Systemic Clinical Syndromes

Influenza A virus (IAV) infection in domestic cats presents with a spectrum of clinical manifestations that are profoundly influenced by virus subtype, viral dose, route of exposure, and host immune status. The clinical picture ranges from subclinical infection to rapidly progressive, fatal multisystemic disease. This variability is particularly stark when comparing infections with highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b, low pathogenicity avian influenza (LPAI) viruses, and zoonotic human pandemic strains such as H1N1pdm09.

HPAI H5N1 (Clade 2.3.4.4b) Infection: The most severe and extensively documented clinical syndrome in cats arises from infection with HPAI H5N1, especially the contemporary clade 2.3.4.4b viruses of genotype B3.13 that have been circulating in North American dairy cattle and wild birds since 2024. Affected cats typically present with an acute onset of fever, profound lethargy, anorexia, and rapidly worsening respiratory distress [1, 7, 17]. Respiratory signs are a hallmark of infection and include tachypnea, dyspnea, and pronounced abdominal breathing. Lung ultrasound examinations have revealed evidence of pulmonary consolidations and the presence of numerous B-lines, indicative of severe interstitial pathology [17]. In a significant proportion of cases, respiratory distress is accompanied by neurological signs, which may manifest as ataxia, tremors, circling, nystagmus, anisocoria, and seizures that can progress to status epilepticus [1, 6, 7]. The progression from initial clinical signs to severe disease or death can be rapid, often occurring within 3 to 7 days of symptom onset. Case fatality rates are high; in one series of cats consuming raw milk from infected dairy cows, 2 of 3 cats died, and mortality rates of approximately 50% have been reported in the context of dairy farm outbreaks [6, 20]. In immunocompromised individuals, such as a diabetic cat with a history of feline infectious peritonitis (FIP), the disease can be even more fulminant, with acute fever, worsening respiratory distress, and hepatic dysfunction leading to euthanasia [7].

Other Influenza A Virus Subtypes: In contrast, infection with LPAI viruses of wild bird origin (e.g., H1N9 and H6N4) in experimentally inoculated cats resulted in no overt clinical signs, despite demonstrable viral replication in the respiratory tract and seroconversion [14]. This starkly illustrates the influence of viral pathogenicity on clinical expression. Infection with pandemic H1N1 (H1N1pdm09) virus produces an intermediate phenotype; experimentally infected cats developed diffuse alveolar damage and respiratory disease but did not exhibit the extrarespiratory lesions characteristic of H5N1 infection [15]. Similarly, during the 2016 H7N2 outbreak in a New York animal shelter, infected cats displayed primarily mild to moderate respiratory signs, including sneezing, nasal discharge, and conjunctivitis, with no reported fatalities [3]. Serosurveillance data from the Netherlands further support this spectrum: domestic cats showed a seroprevalence of 4.6% for H1 influenza viruses (indicating frequent subclinical or mild infections), while stray cats exhibited a significantly higher HPAI H5 seroprevalence (11.8%), likely reflecting more frequent exposure through predation on infected wild birds [4].

Pathological Findings

The pathological alterations in IAV-infected cats are directly correlated with the severity of clinical disease and the viral subtype. The most comprehensive descriptions derive from fatal HPAI H5N1 cases, which reveal a pattern of severe, multisystemic involvement.

Gross Pathology: At necropsy, the most consistent findings in HPAI H5N1-infected cats are confined to the respiratory tract. The lungs are often heavy, edematous, and fail to collapse, with multifocal to coalescing areas of dark red to tan consolidation, consistent with severe bronchointerstitial pneumonia [7, 17]. Pleural effusion may be present. In cases with systemic spread, necrotic lesions can be observed in the liver (pale, mottled foci) and in the intestinal mucosa [17]. The brain may appear grossly unremarkable, despite the presence of neurological signs and microscopic lesions.

Histopathology and Immunohistochemistry: The histopathological hallmarks of HPAI H5N1 infection in cats are severe bronchointerstitial pneumonia and a striking tropism for the central nervous system.

In the lung, microscopy reveals extensive necrosis and sloughing of alveolar epithelial cells, with hyaline membrane formation, alveolar edema, and a mixed inflammatory infiltrate composed of neutrophils, macrophages, and lymphocytes [7, 14]. Viral antigen is prominently detected via immunohistochemistry (IHC) within the nuclei and cytoplasm of alveolar epithelial cells (type I and type II pneumocytes) and, to a lesser extent, in bronchiolar epithelial cells [14]. This pattern of viral replication directly accounts for the profound respiratory compromise observed clinically.

In the brain, a non-suppurative meningoencephalitis is a key finding. Perivascular cuffing by lymphocytes and histiocytes, gliosis, and focal areas of neuronal necrosis have been described [7, 17]. The presence of viral antigen and RNA within neurons and glial cells of the cerebrum, cerebellum, and brainstem confirms the neurotropic nature of these HPAI viruses [7, 17]. The neuropathology explains the clinical neurological signs and is a critical factor in the high mortality associated with these strains.

Systemic spread of HPAI H5N1 is a distinguishing feature. Viral antigen and RNA have been detected in a wide array of extrarespiratory tissues, including the liver (hepatic necrosis), spleen and lymph nodes (lymphoid depletion and necrosis), pancreas, adrenal glands, and myocardium [7, 17]. Hepatic and lymphoid necrosis are particularly common, contributing to the clinical picture of liver dysfunction and immunosuppression. In one notable case, viral RNA was detected in the urine of a surviving cat, indicating potential for renal involvement and a novel route of viral shedding [6]. The bone marrow may also show degeneration [7]. This pattern of widespread viral dissemination and associated tissue damage is highly reminiscent of HPAI H5N1 infection in other mammalian species, including ferrets and humans, and underscores the virus's capacity to overcome species barriers and cause systemic disease.

Comparative Pathology and Clinical Implications

The pathological distinctions between influenza subtypes in cats carry profound clinical and epidemiological significance. HPAI H5N1, particularly the 2.3.4.4b lineage, is a pantropic virus in cats, capable of causing fatal respiratory, neurological, and systemic disease. This is in stark contrast to H1N1pdm09, where lesions are restricted to the respiratory tract [15], and LPAI viruses, where infection may be subclinical despite histopathological evidence of mild bronchointerstitial pneumonia [14]. The ability to infect and replicate in the central nervous system is a hallmark of HPAI H5N1 and a primary driver of its high lethality.

From a clinical diagnostic perspective, the presence of acute respiratory distress coupled with neurological signs in a cat, particularly one with a history of outdoor access, exposure to wild birds, consumption of raw milk or raw poultry, or residence on a farm with HPAI-positive livestock, should immediately raise suspicion for HPAI H5N1 infection [1, 17]. The World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) emphasize the importance of considering H5N1 in the differential diagnosis for cats with these clinical presentations, especially in regions with active outbreaks in birds or mammals [16, 19]. The CDC has also highlighted the need for veterinarians to obtain occupational information from cat owners to identify zoonotic risk [1]. The rapid and severe clinical course, combined with the potential for multisystemic pathology, makes prompt diagnosis and implementation of strict biosecurity measures essential for both animal and public health protection. The unique capacity of these viruses to cause brain infection in a companion animal that lives in close proximity to humans elevates the stakes of surveillance and underscores the urgency of a One Health response.

Diagnostic Approaches for Influenza A Virus Detection in Felines

The accurate and timely diagnosis of Influenza A virus (IAV) infections in domestic cats is a multifaceted challenge that requires a sophisticated, multi-platform diagnostic strategy. This necessity stems from the virus’s broad tropism, the often-subtle or overlapping clinical presentations with other feline respiratory and neurological pathogens, and the critical public health implications of zoonotic spillover events, particularly with highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b [1-3, 5]. A robust diagnostic framework must integrate molecular, serological, virological, and histopathological techniques, each with distinct sensitivities, specificities, and applications across the continuum of infection. The selection of an appropriate diagnostic modality is dictated by the clinical stage of disease, the suspected viral subtype, the purpose of testing (e.g., individual clinical diagnosis versus epidemiological surveillance), and the imperative for biosafety, given that many of these viruses are select agents requiring BSL-3 or higher containment [5, 22, 32]. The World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) have underscored the importance of standardized diagnostic protocols for IAV in companion animals to monitor interspecies transmission and assess pandemic risk [16, 32].

Clinical Suspicion and Sample Collection Fundamentals

The diagnostic process begins with a high index of clinical suspicion, which is paramount given that IAV infection can manifest as a rapidly progressive, fatal disease or as a subclinical infection [7, 14, 17]. Key historical clues include known or potential exposure to infected birds, unpasteurized dairy products, or humans with confirmed IAV infection, particularly in the context of the ongoing HPAI H5N1 outbreak in dairy cattle [1, 5, 6, 10]. Practitioners must elicit a detailed dietary history, as consumption of raw milk or raw/undercooked poultry has been directly linked to severe, often fatal, H5N1 infections in cats [6, 17]. Furthermore, the occupational exposure of owners, such as employment on dairy or poultry farms, is a critical epidemiological variable that should prompt diagnostic testing for IAV in any feline presenting with respiratory or neurological signs [1, 2].

Sample collection is the first critical step upon which all subsequent diagnostics depend. For molecular detection, the specimen of choice is an oropharyngeal (pharyngeal) swab, as IAV replication in cats is heavily concentrated in the respiratory epithelium, leading to detectable viral shedding from the upper respiratory tract [11, 14, 15]. For cats with neurological signs, collection of cerebrospinal fluid (CSF) or a deep nasopharyngeal swab may be considered, although sensitivity is lower than for oropharyngeal swabs. In cases of fatal disease, comprehensive tissue sampling is essential, including lung, trachea, brain, and lymphoid tissues (spleen, lymph nodes), as IAV exhibits a broad cellular tropism, particularly HPAI H5N1, which can invade the central nervous system and cause systemic infection [7, 17]. Notably, urine has recently emerged as a viable non-invasive specimen for detecting H5N1 viral RNA in live cats, as demonstrated by Frye et al. (2025), who detected viral RNA in the urine of a surviving cat that had consumed contaminated raw milk [6]. This finding suggests that urine may serve as an alternative diagnostic matrix when oropharyngeal swabs are negative, especially in cases with neurological involvement where the virus may have disseminated. All samples should be collected using standard infection control precautions, placed in appropriate viral transport media, and shipped on cold packs to a diagnostic laboratory capable of handling high-consequence pathogens [1, 7].

Molecular Detection by Reverse Transcription Quantitative PCR (RT-qPCR)

Reverse transcription quantitative polymerase chain reaction (RT-qPCR) targeting conserved regions of the influenza A virus matrix (M) gene or the nucleoprotein (NP) gene remains the gold standard for the initial detection of IAV RNA in feline specimens [1, 7, 11, 17]. This assay offers exceptional sensitivity and specificity, enabling the detection of viral nucleic acids even in samples with low viral loads or when the virus is no longer replication-competent. The use of RT-qPCR is critical for early diagnosis in live animals, as it can identify infection before seroconversion occurs [14, 17].

The diagnostic sensitivity of RT-qPCR is highly dependent on specimen type and collection timing. In experimentally infected cats, peak viral RNA shedding in pharyngeal swabs occurs between days 2 and 5 post-infection, closely correlating with the onset of clinical signs [14, 15]. For naturally infected cats, RT-qPCR positivity has been documented from a variety of tissues, with the lung and brain typically showing the highest viral loads in fatal H5N1 cases [7, 17]. The assay’s design has been adapted for pan-IAV detection, but specific subtyping (e.g., H5, H1, H3, H7, N1) requires additional RT-qPCR assays targeting the hemagglutinin (HA) and neuraminidase (NA) genes, or subsequent sequencing of the HA cleavage site to determine pathogenicity [5, 10, 26]. For instance, the investigation of the indoor cats in Michigan utilized a diagnostic algorithm that first screened for influenza A by RT-qPCR, followed by a specific A(H5) subtyping assay to confirm HPAI H5N1 [1]. This two-step approach is highly recommended to rapidly identify zoonotic threats. However, it must be noted that while RT-qPCR is exceptionally sensitive for detecting viral RNA, it cannot distinguish between infectious virus and non-infectious viral remnants, which is a critical consideration when interpreting positive results from environmental or processed food samples like pasteurized milk [29].

Virus Isolation and Viral Culture

Virus isolation in embryonated chicken eggs or permissive mammalian cell lines (e.g., Madin-Darby canine kidney [MDCK] cells) is the definitive method for confirming the presence of replication-competent infectious virus [5, 6, 10, 14]. This approach is indispensable for subsequent virological characterization, including antigenic analysis, antiviral susceptibility testing, and the generation of virus stocks for research and vaccine development [23, 35]. Isolation from feline specimens has been successful from a range of tissues and fluids, including lung, trachea, brain, and notably milk and urine [6, 10, 17]. The successful isolation of HPAI H5N1 from the urine of a convalescent cat [6] underscores the potential for non-respiratory routes of viral shedding and the need for stringent biosafety precautions when handling any biological material from suspect cases.

Virus isolation is a time-consuming (3–7 days) process that requires BSL-3 or BSL-3+ containment facilities, particularly for HPAI strains and other H5 or H7 subtypes, severely limiting its use as a routine diagnostic tool. Its primary value lies in confirmatory testing, outbreak investigations, and archiving for genomic surveillance. The correlation between in vitro viral titers and in vivo pathogenicity has been extensively studied in ferret models, with peak titers in human bronchial epithelial cells often correlating with viral loads in nasal wash and turbinate tissues [22, 23]. These relationships, while studied predominantly in ferrets, help inform the biological significance of successful viral isolation from feline specimens.

Antigen Detection: Point-of-Care and Immunohistochemical Assays

Rapid antigen detection tests, commonly referred to as point-of-care (POC) tests, are lateral flow immunoassays that detect the conserved NP antigen of IAV. These tests have been utilized in veterinary clinical settings for their speed (15–30 minutes) and ease of use [24]. However, their sensitivity in cats is significantly lower than RT-qPCR, making them unsuitable for definitive diagnosis. False negatives are common, particularly in samples with low viral loads or during later stages of infection when viral shedding has waned [7, 24]. In a study of H5N1-infected ferrets, POC tests yielded positive results in throat swabs from all infected animals, but this was during a period of high viral shedding in an experimental setting [24]. In clinical feline practice, reliance on a negative POC test to rule out IAV infection is inadvisable, and a negative result should always be confirmed by RT-qPCR.

In contrast, immunohistochemistry (IHC) performed on formalin-fixed, paraffin-embedded (FFPE) tissues is a highly specific and valuable technique for confirming the presence of viral antigen in situ, providing direct evidence of viral tropism and tissue-level pathogenesis [7, 14, 33]. IHC has been instrumental in demonstrating the broad tissue distribution of HPAI H5N1 in cats, including the detection of viral antigen in pneumocytes, neurons, hepatocytes, and lymphoid follicles [7, 17]. The detection of H5N1 antigen within the epithelial cells of the mammary gland in dairy cattle [5] and the oviductal epithelium in seabirds [33] highlights the unique cellular tropism of the current clade 2.3.4.4b viruses, a feature that is mirrored in feline systemic infections. IHC is not a first-line diagnostic tool due to its requirement for invasive sampling and lengthy processing times, but it is an essential component of postmortem investigations and research on viral pathogenesis.

Serological Detection of IAV Antibodies

Serological assays are critical for detecting past exposure to IAV and for conducting epidemiological serosurveys [2, 4, 8, 9, 11, 13, 30]. They measure the host’s humoral immune response, providing evidence of infection even after the virus has been cleared. The most commonly applied serologic methods include enzyme-linked immunosorbent assays (ELISA) targeting the NP or HA proteins, and the hemagglutination inhibition (HI) assay, which is subtype-specific and remains the gold standard for determining antibody titer against particular HA subtypes [2, 4, 31]. A competitive ELISA for detecting antibodies against the H5 hemagglutinin has been adapted for use in cats and has proven valuable in serosurveillance studies [8, 30].

The clinical utility of serology for diagnosing acute IAV infection in individual cats is limited. Seroconversion typically occurs 7–10 days after infection, by which time an acutely ill cat may have already died or recovered from the acute phase [14]. Therefore, a single negative serological result does not rule out active infection. Paired acute and convalescent sera (collected 2–4 weeks apart) demonstrating a four-fold or greater rise in antibody titer can provide a retrospective diagnosis of recent infection. Furthermore, serological cross-reactivity between IAV subtypes is a known limitation. For example, antibodies against pandemic H1N1 (H1N1pdm09) can cross-react with other H1 viruses and, in some cases, with H5 antigens [4, 34]. Zhao et al. (2020) demonstrated the value of using multiple complementary serological assays (e.g., HA1-specific ELISA versus whole-HA ELISA versus nanoparticle-based HI) to differentiate subtype-specific responses and gain a more accurate picture of exposure history in cats [2].

The substantial body of serosurveillance data in cats reveals marked variability in seroprevalence by geographic location and population. A meta-analysis by Ramos-Martínez et al. (2025) estimated a global pooled seroprevalence of 7.39% in cats, with significantly higher rates in the Americas and Asia compared to Europe [9]. In the Netherlands, Duijvestijn et al. (2024) found a striking contrast: 11.8% of stray cats were seropositive for HPAI H5, versus only 0.46% of domestic cats, whereas H1 seroprevalence was higher in domestic cats (4.6%) [4]. This divergence reflects the different exposure ecologies of these populations, stray cats having greater contact with wild birds, and domestic cats having closer contact with humans shedding H1N1pdm09. These serological data are foundational for understanding the true extent of IAV circulation in feline populations and informing risk assessments for zoonotic transmission [2, 4, 9].

Sequencing, Molecular Characterization, and Pathotyping

Whole-genome sequencing (WGS) of IAV from feline specimens has become an increasingly indispensable tool, providing the highest resolution for characterizing the virus and tracing its evolutionary trajectory [1, 5, 7, 10]. Sequencing reveals the specific viral subtype, clade, and genotype (e.g., clade 2.3.4.4b, genotype B3.13), providing critical information for epidemiological linkage and outbreak investigations [1, 7]. For HPAI viruses, sequencing of the HA gene cleavage site is required to confirm the highly pathogenic phenotype (multiple basic amino acids) [5, 10, 26]. Analysis of other gene segments, particularly the polymerase basic protein 2 (PB2) and hemagglutinin (HA), can identify mammalian adaptation markers (e.g., PB2 E627K, HA Q226L) that are associated with increased replication and transmission in mammals, including humans [18, 23]. The detection of such markers in feline isolates is a sentinel signal for enhanced zoonotic risk. The integration of sequencing data from cats, cattle, and birds on affected farms has shown multidirectional interspecies transmission, with cats acting as both a sentinel species and a potential mixing vessel for viral reassortment [5, 10]. The CDC and WOAH recommend WGS for all IAV isolates from unusual hosts, including cats, to support global genomic surveillance and pandemic preparedness efforts [16, 19].

Integration of Diagnostic Results with Epidemiological Context

The interpretation of any diagnostic test for IAV in cats must be framed within a comprehensive One Health epidemiological context. A positive RT-qPCR result in a cat from a household where an owner works on an H5N1-positive dairy farm is far more significant than an isolated case in a region with no known IAV activity [1, 3, 5]. The clinical presentation of the cat is also critical; a cat with acute respiratory distress and neurological signs has a high pre-test probability for HPAI H5N1, especially if there is a history of raw milk consumption [6, 7, 17]. Conversely, a subclinically infected cat with serological evidence of past H1N1pdm09 exposure may represent a dead-end spillover event with minimal public health risk [4, 15]. Therefore, diagnostic laboratories must work closely with epidemiologists and public health authorities to integrate laboratory findings with field data, enabling a dynamic risk assessment that can guide both veterinary treatment decisions and public health interventions, including post-exposure prophylaxis for exposed individuals [1, 32]. The joint One Health investigation model, as demonstrated by the Michigan Department of Health and Human Services in the 2024 cat cases, exemplifies the gold standard for diagnostic integration, where human and animal health testing are coordinated to trace the source and mitigate further transmission [1].

One Health Implications and Zoonotic Risk Assessment of Bird Flu in Cats

The emergence of highly pathogenic avian influenza (HPAI) A(H5N1) virus, particularly clade 2.3.4.4b, in domestic cats represents a critical juncture in the evolving landscape of influenza ecology and pandemic preparedness. The infection of cats, once considered an incidental and rare event, has transitioned into a recurring phenomenon with profound implications for human, animal, and environmental health. This section provides an exhaustive analysis of the One Health implications of avian influenza in cats, systematically assessing the zoonotic risk through the lens of viral adaptation, epidemiological bridging, occupational exposure, and public health policy.

The Cat as a Sentinel and Bridging Host in Influenza Ecology

Cats occupy a unique ecological niche that positions them as potent sentinels and potential bridging hosts for influenza A viruses (IAVs). Unlike many wildlife species, domestic cats (Felis catus) share an intimate, cohabitative relationship with humans, often sleeping in beds, being fed in kitchens, and interacting closely with vulnerable populations such as the elderly, infants, and immunocompromised individuals. This close proximity creates a low-barrier interface for potential zoonotic transmission. Furthermore, cats are obligate carnivores with predatory instincts, bringing them into direct contact with wild birds, the primary reservoir of IAVs, as well as with raw or undercooked poultry products and, more recently, unpasteurized dairy products from infected cattle [1, 6, 17].

The susceptibility of cats to a broad range of IAV subtypes is now well-documented. Serological surveys have demonstrated that cats are frequently exposed to both human and avian IAVs. A comprehensive meta-analysis by Ramos-Martínez et al. (2025) estimated a global seroprevalence of influenza A in cats at 7.39% (95% CI: 3.17–13.12), with significant geographic variation [9]. Zhao et al. (2020) found that cat sera collected after the 2009 H1N1 pandemic exhibited much higher seropositivity against H1 compared to pre-2009 samples, and notably, cat sera displayed higher reactivity for avian IAVs than dog sera [2]. This differential susceptibility highlights the cat’s role as a potential mixing vessel for avian-origin viruses. Experimental infections have confirmed that cats are susceptible to low pathogenic avian influenza viruses (LPAIV) from shorebirds, such as H1N9 and H6N4, resulting in subclinical infection, seroconversion, and patchy bronchointerstitial pneumonia, demonstrating that even LPAIV can establish infection in this species [14]. This capacity for subclinical infection is particularly concerning, as it allows for undetected viral circulation and potential adaptation within a permissive mammalian host.

The concept of the cat as a “bridging host” is critical. Infected cats, particularly stray or feral populations, can transport the virus from wildlife reservoirs (infected birds or contaminated environments) directly into human domiciles. Duijvestijn et al. (2024) provided compelling evidence of this dynamic in the Netherlands, reporting an HPAI H5 seroprevalence of 11.8% (95% CI: 9.5–14.5) in stray cats compared to only 0.46% (95% CI: 0.13–1.2) in domestic cats [4]. Stray cats living in nature reserves were 5.4 times more likely to be seropositive, strongly suggesting that foraging on wild birds is the primary route of exposure [4]. This creates a direct epidemiological link between the sylvatic cycle of avian influenza in wild birds and the domestic environment shared with humans.

Mechanisms of Zoonotic Transmission: From Cat to Human

The zoonotic risk posed by influenza-infected cats is not merely theoretical; it has been substantiated by documented human infections. The most definitive example occurred in 2016 in New York City, where an outbreak of low pathogenicity avian influenza A(H7N2) virus in cats at an animal shelter resulted in zoonotic transmission to a human. Marinova-Petkova et al. (2017) confirmed that the virus isolated from the infected human was closely related to the virus isolated from a cat, providing direct molecular evidence of cat-to-human transmission [3]. This event shattered the paradigm that cats were dead-end hosts for avian influenza and established them as a legitimate zoonotic source.

The biological mechanisms facilitating this transmission are multifaceted. First, cats infected with HPAI H5N1 shed virus in multiple bodily fluids, including respiratory secretions, feces, and, critically, urine. Frye et al. (2025) reported the isolation of HPAI A(H5N1) virus from the urine of a surviving cat that had consumed raw milk, demonstrating a non-respiratory route of viral excretion that could contaminate the household environment [6]. Chen et al. (2025) further confirmed the presence of H5N1 viral RNA in the urine and lymphoid tissues of an immunocompromised cat, underscoring the potential for environmental contamination [7]. This urinary shedding is a significant departure from the typical respiratory transmission of human influenza viruses and represents a novel exposure pathway for owners cleaning litter boxes or coming into contact with contaminated surfaces.

Second, the clinical presentation of HPAI in cats often includes severe neurological signs, such as seizures, ataxia, and circling, as well as respiratory distress [1, 17, 39]. A cat exhibiting these signs may bite, scratch, or salivate excessively, increasing the risk of direct exposure to infectious material for the owner or veterinary staff. The case reported by Szaluś-Jordanow et al. (2023) in Poland described a cat with severe neurological signs culminating in epileptic seizures, highlighting the aggressive and unpredictable nature of the disease in its terminal stages [17]. The presence of viral RNA in the brain and multiple organs indicates a systemic infection with high viral loads, amplifying the risk of exposure during care or necropsy [17].

Third, the virus can undergo adaptive mutations within the feline host that may enhance its zoonotic potential. The H7N2 virus that infected the New York veterinarian possessed a deletion in the hemagglutinin (HA) receptor binding site (the 220-loop), which is associated with dual receptor specificity for both avian (α2,3-linked sialic acids) and human (α2,6-linked sialic acids) receptors [18]. Lyashko et al. (2023) identified that the A135S substitution in the H7N2 HA, which is associated with adaptation to mammals (including cats and humans), reduced affinity for the avian-like receptor analog and weakened binding with monoclonal antibodies, suggesting immune evasion [18]. This demonstrates that the feline respiratory tract can serve as a selective environment for the emergence of viral variants with increased affinity for human-type receptors, a prerequisite for efficient human-to-human transmission.

Occupational and Household Risk: The Dairy Industry Nexus

The 2024 outbreak of HPAI H5N1 (clade 2.3.4.4b, genotype B3.13) in U.S. dairy cattle has created an unprecedented and complex epidemiological nexus linking cattle, cats, and humans. The investigation by Naraharisetti et al. (2025) into infected indoor cats in Michigan revealed a critical occupational link: both cat owners worked on dairy farms with confirmed HPAI-positive cattle [1]. One owner transported unpasteurized milk and reported being splashed in the face and eyes, while the other declined testing. Crucially, both cats were reported to be exclusively indoor, yet they became infected, strongly implicating fomite transmission via contaminated clothing, boots, or equipment brought home from the workplace [1]. This finding has profound implications for occupational health and safety, suggesting that dairy workers may act as mechanical vectors, transporting the virus from the farm environment into their homes and exposing their families and pets.

The role of unpasteurized (raw) milk as a vehicle for transmission to cats has been unequivocally demonstrated. Caserta et al. (2024) reported that infectious virus and viral RNA were consistently detected in milk from affected cows, and that cats on affected farms became infected [5]. Frye et al. (2025) documented a case where three domestic cats in California developed severe disease after consuming raw milk contaminated with H5N1; two died, and the surviving cat shed virus in its urine [6]. The tropism of the virus for the mammary gland epithelium in cows results in extremely high viral titers in milk, making it a potent infectious source [5, 10]. This has direct zoonotic implications: individuals who consume raw milk or raw milk products are at risk, and cats that consume such products become amplifying hosts, shedding virus into the household environment. The CDC and WOAH have since issued strong advisories against the consumption of raw milk and the feeding of raw dairy products to pets.

The scale of this risk is substantial. By 2025, HPAI H5N1 had been detected on over 1,000 cattle farms in 17 U.S. states, with documented spillover to cats, poultry, and humans [16]. Serosurveys of dairy workers in Michigan and Colorado found that 7% (8 of 115) had antibodies against H5N1, indicating that occupational exposure is far more common than clinical case reporting suggests [36]. This highlights a significant gap in surveillance and a failure of traditional public health reporting to capture the true burden of zoonotic infection. The cat, in this context, serves as a highly sensitive sentinel for the presence of the virus in the agricultural environment, often presenting with clinical disease before human cases are recognized.

Risk Assessment for Vulnerable Populations and Pandemic Potential

The zoonotic risk from feline influenza is not uniform across the human population. Certain groups are at disproportionately higher risk of severe outcomes. Immunocompromised individuals, such as those undergoing chemotherapy, organ transplant recipients, or those with chronic conditions like diabetes, are particularly vulnerable. Chen et al. (2025) described a lethal H5N1 infection in a diabetic cat with a history of feline infectious peritonitis (FIP), highlighting how immunosuppression can exacerbate viral pathogenesis and shedding [7]. The same principle applies to humans: an immunocompromised owner exposed to an infected cat could face a far more severe clinical course.

Children and the elderly are also at elevated risk. Children are more likely to engage in close, affectionate contact with pets, including kissing, sharing food, and allowing the cat to sleep on their bed. The elderly, particularly those aged ≥65 years, are at greatest risk of severe influenza requiring hospitalization, as demonstrated by O’Halloran et al. (2026) in their analysis of influenza-associated hospitalization rates [38]. The presence of underlying chronic conditions, such as heart disease, lung disease, or diabetes, further compounds this risk [37, 40]. For these populations, a zoonotic influenza infection acquired from a household cat could be a catastrophic event.

The pandemic potential of an avian influenza virus adapting to cats is a major concern. The ferret model, considered the gold standard for assessing influenza transmissibility in mammals, has shown that HPAI H5N1 viruses can replicate and transmit among ferrets under certain conditions [22, 23]. Cats, like ferrets, are mustelid-adjacent carnivores with similar distribution of sialic acid receptors in the respiratory tract. The documented cat-to-cat transmission of pandemic H1N1 in an experimental setting [15] and the epidemiological evidence of multidirectional interspecies transmission on dairy farms (cow-to-cat, cat-to-bird, bird-to-cow) [5, 10] demonstrate that cats are not dead-end hosts. They are active participants in the transmission network. If a cat were to become co-infected with a human seasonal influenza virus and an avian H5N1 virus, the potential for genetic reassortment, the shuffling of gene segments, could produce a novel virus with the antigenic novelty of H5 and the transmissibility of a human-adapted virus. This is the classic recipe for a pandemic. The World Health Organization (WHO) has classified the risk of a human pandemic from H5N1 as a persistent and significant threat, and the inclusion of cats in the transmission chain only elevates this risk.

Environmental Contamination and Public Health Surveillance

The One Health approach mandates consideration of the environmental dimension. Infected cats can contaminate the household environment through respiratory droplets, feces, and urine. The detection of H5N1 RNA in cat urine [6, 7] raises the possibility of aerosolization during litter box cleaning, a routine household chore. Furthermore, stray and feral cat populations can contaminate public spaces, parks, and water sources. The high seroprevalence in stray cats in the Netherlands [4] and the detection of HPAI in sediment from baited wetlands [28] indicate that the virus can persist in the environment and be transmitted via fomites.

Current public health surveillance systems are ill-equipped to detect zoonotic influenza transmission from cats. Human influenza surveillance relies on clinical case reporting from healthcare providers, but many mild or subclinical infections in humans go undetected. The serosurvey data from dairy workers [36] confirms this. To address this gap, the CDC, in collaboration with state and local health departments, has recommended a joint One Health investigation protocol when cats present with respiratory or neurological signs and have a known exposure to HPAI-infected livestock [1]. This protocol involves testing the cat, interviewing household members about occupational exposures, and offering human testing. This represents a paradigm shift, recognizing the veterinary clinic as a frontline sentinel for emerging zoonotic threats. Veterinarians are now being urged to obtain occupational histories from cat owners and to contact public health officials immediately when HPAI is suspected [1]. The integration of animal health surveillance into human pandemic preparedness frameworks is no longer optional; it is an operational necessity.

Regulatory and Policy Implications

The evolving situation has prompted responses from international bodies. The World Organisation for Animal Health (WOAH) has included cats in its list of species susceptible to HPAI and recommends reporting of feline cases. The Food and Agriculture Organization (FAO) has emphasized the need for enhanced biosecurity on dairy farms, including measures to prevent contact between cats and cattle, and the safe disposal of raw milk. The CDC has issued explicit guidance against feeding raw meat or unpasteurized dairy to pets and has provided recommendations for the safe handling of sick cats. However, significant policy gaps remain. There is no mandatory reporting system for feline influenza in most countries, and diagnostic testing for HPAI in cats is not routinely available in veterinary clinics. The development and deployment of a licensed vaccine for cats, such as the fowlpox-vectored H5 vaccine (TROVAC AIV-H5) that has shown immunogenicity in cats [31], could be a valuable tool for protecting high-risk pets and reducing viral shedding, but it is not currently commercially available. The economic and logistical challenges of vaccinating the global stray cat population are immense, but targeted vaccination of owned cats in high-risk areas (e.g., dairy farming regions) could be a feasible and impactful intervention.

References

[1] Naraharisetti R, Weinberg M, Stoddard B, Stobierski MG, Dodd K, Wineland N, et al.. Highly Pathogenic Avian Influenza A(H5N1) Virus Infection of Indoor Domestic Cats Within Dairy Industry Worker Households , Michigan, May 2024. MMWR. Morbidity and mortality weekly report. 2025. DOI: https://doi.org/10.15585/mmwr.mm7405a2

[2] Zhao S, Schuurman N, Tieke M, Quist B, Zwinkels S, Kuppeveld FVv, et al.. Serological Screening of Influenza A Virus Antibodies in Cats and Dogs Indicates Frequent Infection with Different Subtypes. Journal of Clinical Microbiology. 2020. DOI: https://doi.org/10.1128/JCM.01689-20

[3] Marinova-Petkova A, Laplante J, Jang Y, Lynch B, Zanders N, Rodriguez MR, et al.. Avian Influenza A(H7N2) Virus in Human Exposed to Sick Cats, New York, USA, 2016. Emerging Infectious Diseases. 2017. DOI: https://doi.org/10.3201/eid2312.170798

[4] Duijvestijn MBHM, Schuurman N, Vernooij J, Leeuwen MAJMv, Brand JVDvd, Wagenaar JA, et al.. Highly pathogenic avian influenza (HPAI) H5 virus exposure in domestic cats and rural stray cats, the Netherlands, October 2020 to June 2023. Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletin. 2024. DOI: https://doi.org/10.2807/1560-7917.ES.2024.29.44.2400326

[5] Caserta L, Frye EA, Butt SL, Laverack M, Nooruzzaman M, Covaleda LM, et al.. Spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle. Nature. 2024. DOI: https://doi.org/10.1038/s41586-024-07849-4

[6] Frye EA, Nooruzzaman M, Cronk B, Laverack M, Oliveira PSDd, Caserta L, et al.. Isolation of Highly Pathogenic Avian Influenza A(H5N1) Virus from Cat Urine after Raw Milk Ingestion, United States. Emerging Infectious Diseases. 2025. DOI: https://doi.org/10.3201/eid3108.250309

[7] Chen C, Naru A, Mareddy V, Lanka S, Olmstead CA, Revindran-Stam V, et al.. Highly pathogenic avian influenza A H5N1 virus infection in an immunocompromised domestic cat. ASM case reports. 2025. DOI: https://doi.org/10.1128/asmcr.00134-25

[8] Villanueva‐Saz S, Martínez M, Rueda P, Pérez M, Lacasta D, Marteles D, et al.. Serological exposure to influenza A in cats from an area with wild birds positive for avian influenza. Zoonoses and Public Health. 2023. DOI: https://doi.org/10.1111/zph.13085

[9] Ramos-Martínez JC, Ramos-Martínez I, Saavedra-Montañez M, Martínez-González M, Santos-Paniagua S, Martínez-Aguirre MA, et al.. Global seroprevalence and prevalence of infection of influenza in dogs and cats: A systematic review and meta-analysis.. Preventive Veterinary Medicine. 2025. DOI: https://doi.org/10.1016/j.prevetmed.2025.106716

[10] Caserta L, Frye EA, Butt SL, Laverack M, Nooruzzaman M, Covaleda LM, et al.. From birds to mammals: spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle led to efficient intra- and interspecies transmission. bioRxiv. 2024. DOI: https://doi.org/10.1101/2024.05.22.595317

[11] Jimenez-Bluhm P, Sepúlveda A, Baumberger C, Pillo FD, Ruíz S, Salazar C, et al.. Evidence of Influenza infection in dogs and cats in central Chile. Preventive Veterinary Medicine. 2021. DOI: https://doi.org/10.1016/j.prevetmed.2021.105349

[12] Short KR, Richard M, Verhagen J, Riel Dv, Schrauwen E, Brand JVDvd, et al.. One health, multiple challenges: The inter-species transmission of influenza A virus. One Health. 2015. DOI: https://doi.org/10.1016/J.ONEHLT.2015.03.001

[13] Munyua P, Onyango C, Mwasi L, Waiboci LW, Arunga G, Fields B, et al.. Identification and characterization of influenza A viruses in selected domestic animals in Kenya, 2010-2012. PLoS ONE. 2018. DOI: https://doi.org/10.1371/journal.pone.0192721

[14] Driskell EA, Driskell EA, Jones CA, Berghaus R, Stallknecht D, Howerth E, et al.. Domestic Cats Are Susceptible to Infection With Low Pathogenic Avian Influenza Viruses From Shorebirds. Veterinary Pathology-Supplement. 2012. DOI: https://doi.org/10.1177/0300985812452578

[15] Brand JVDvd, Stittelaar K, Amerongen Gv, Bildt MVDvd, Leijten L, Kuiken T, et al.. Experimental Pandemic (H1N1) 2009 Virus Infection of Cats. Emerging Infectious Diseases. 2010. DOI: https://doi.org/10.3201/eid1611.100845

[16] Krasnova EA, Korogodina E, Lunina D. Cross-species transmission of avian influenza A(H5N1) virus to mammals: lessons learnt from 2024–2025 outbreaks in cattle. Veterinary Science Today. 2026. DOI: https://doi.org/10.29326/2304-196x-2026-15-1-13-19

[17] Szaluś-Jordanow O, Golke A, Dzieciątkowski T, Chrobak-Chmiel D, Rzewuska M, Czopowicz M, et al.. A Fatal A/H5N1 Avian Influenza Virus Infection in a Cat in Poland. Microorganisms. 2023. DOI: https://doi.org/10.3390/microorganisms11092263

[18] Lyashko AV, Timofeeva T, Rudneva I, Lomakina N, Treshchalina A, Gambaryan A, et al.. Antigenic Architecture of the H7N2 Influenza Virus Hemagglutinin Belonging to the North American Lineage. International Journal of Molecular Sciences. 2023. DOI: https://doi.org/10.3390/ijms25010212

[19] Zhiltsova MV, Akimova TP, Varkentin A, Mitrofanova MN, Mazneva AV, Semakina VP, et al.. Global avian influenza situation (2019–2022). Host range expansion asevidence of high pathogenicity avian influenza virus evolution. Veterinary Science Today. 2023. DOI: https://doi.org/10.29326/2304-196x-2023-12-4-293-302

[20] Volkov M, Varvashenko DV, Irza V, Chvala I, Varkentin A, Korennoy FI. Serological screening of cattle populations in the Russian Federation to study the situation with possible introduction and spread of the H5N1 avian influenza virus in 2024-2025. Veterinariya, Zootekhniya i Biotekhnologiya. 2026. DOI: https://doi.org/10.36871/vet.zoo.bio.202601210

[21] Voss A, Günther A, Geit O, Puget C, Pauly A, Stiasny K, et al.. Spillover infections by rustrela virus, borna disease virus 1 and tick-borne encephalitis virus revealed by retrospective screening of mammalian encephalitis of unknown origin. BMC Veterinary Research. 2025. DOI: https://doi.org/10.1186/s12917-025-05132-w

[22] Creager HM, Kieran T, Zeng H, Sun X, Pulit-Penaloza J, Holmes KE, et al.. Utility of Human In Vitro Data in Risk Assessments of Influenza A Virus Using the Ferret Model. Journal of Virology. 2023. DOI: https://doi.org/10.1128/jvi.01536-22

[23] Kieran T, Sun X, Maines T, Beauchemin CAA, Belser J. Exploring associations between viral titer measurements and disease outcomes in ferrets inoculated with 125 contemporary influenza A viruses. Journal of Virology. 2024. DOI: https://doi.org/10.1128/jvi.01661-23

[24] Golke A, Jańczak D, Szaluś-Jordanow O, Dzieciątkowski T, Sapierzyński R, Moroz-Fik A, et al.. Natural Infection with Highly Pathogenic Avian Influenza A/H5N1 Virus in Pet Ferrets. Viruses. 2024. DOI: https://doi.org/10.3390/v16060931

[25] Avery‐Gomm S, Barychka T, English M, Ronconi RA, Wilhelm SI, Rail J, et al.. Wild bird mass mortalities in eastern Canada associated with the Highly Pathogenic Avian Influenza A(H5N1) virus, 2022. bioRxiv. 2024. DOI: https://doi.org/10.1101/2024.01.05.574233

[26] Giacinti J, Signore AV, Jones MEB, Bourque L, Lair S, Jardine CM, et al.. Avian influenza viruses in wild birds in Canada following incursions of highly pathogenic H5N1 virus from Eurasia in 2021–2022. mBio. 2024. DOI: https://doi.org/10.1128/mbio.03203-23

[27] Rijks J, Leopold M, Kühn S, Veld Ri‘, Schenk F, Brenninkmeijer A, et al.. Mass Mortality Caused by Highly Pathogenic Influenza A(H5N1) Virus in Sandwich Terns, the Netherlands, 2022. Emerging Infectious Diseases. 2022. DOI: https://doi.org/10.3201/eid2812.221292

[28] Andrew CL, McPhee L, Kuchinski KS, Wight J, Rahman I, Mansour SC, et al.. Bait trapping of waterfowl increases the environmental contamination of avian influenza virus (AIV). Journal of Wildlife Management. 2025. DOI: https://doi.org/10.1002/jwmg.22720

[29] Brigleb PH, Roubidoux E, Lazure L, Livingston B, Meliopoulos VA, Sharp B, et al.. Repeated oral exposure to H5N1 influenza virus in pasteurized milk does not cause adverse responses to subsequent influenza infection. Science Advances. 2025. DOI: https://doi.org/10.1126/sciadv.aeb3906

[30] Alnaeem A, Al-Shabeb A, Hemida MG. Evaluation of the immune status of birds and domestic and companion animals for the influenza A virus in Eastern Saudi Arabia. Veterinary World. 2020. DOI: https://doi.org/10.14202/vetworld.2020.1966-1969

[31] Karaca K, Swayne D, Grosenbaugh D, Bublot M, Robles A, Spackman E, et al.. Immunogenicity of Fowlpox Virus Expressing the Avian Influenza Virus H5 Gene (TROVAC AIV-H5) in Cats. Clinical Diagnostic Laboratory Immunology. 2005. DOI: https://doi.org/10.1128/CDLI.12.11.1340-1342.2005

[32] Jarynowski A, Maksymowicz S, Romanowska M, Skawina I. Avian influenza: the looming threat of Disease X and lessons from Poland and Europe. European Journal of Translational and Clinical Medicine. 2024. DOI: https://doi.org/10.31373/ejtcm/193957

[33] Lean F, Falchieri M, Furman N, Tyler G, Robinson C, Holmes P, et al.. Highly pathogenic avian influenza virus H5N1 infection in skua and gulls in the United Kingdom, 2022. Veterinary Pathology-Supplement. 2023. DOI: https://doi.org/10.1177/03009858231217224

[34] Harvey R, Cayol T, Paudyal B, Lilley A, Carr C, Hatton CF, et al.. Postinfection Pig and Ferret Antisera Show Similar Antigenic Profiles for Human Influenza A(H1N1pdm09) Viruses. bioRxiv. 2025. DOI: https://doi.org/10.1111/irv.70261

[35] Sia ZR, He X, Zhang A, Ang J, Shao S, Seffouh A, et al.. A liposome-displayed hemagglutinin vaccine platform protects mice and ferrets from heterologous influenza virus challenge. Proceedings of the National Academy of Sciences of the United States of America. 2021. DOI: https://doi.org/10.1073/pnas.2025759118

[36] Sedger LM, Ahuja V, Lau K, Byers D, Theis T, Kirkland PD, et al.. The detection of avian influenza virus in human pathology laboratories in Australia, New Zealand, and South Pacific nations. Medical Journal of Australia. 2025. DOI: https://doi.org/10.5694/mja2.70076

[37] Hitchens A, Candrilli S, Carrico J, Hicks KA, Wilson E, Mehta D, et al.. Prevalence of health conditions associated with higher risk for severe respiratory syncytial virus, influenza, or COVID-19. Current Medical Research and Opinion. 2025. DOI: https://doi.org/10.1080/03007995.2025.2535456

[38] O’Halloran A, Hood N, Ujamaa D, Merced-Morales A, Gurbaxani B, Kirley PD, et al.. Effects of age and birth cohort on influenza A virus subtype-specific hospitalization rates, United States 2010-2025. Journal of Infectious Diseases. 2026. DOI: https://doi.org/10.1093/infdis/jiag232

[39] Burke CG, Myers JR, Post C, Boulé LA, Lawrence B. DNA Methylation Patterns in CD4+ T Cells of Naïve and Influenza A Virus-Infected Mice Developmentally Exposed to an Aryl Hydrocarbon Receptor Ligand. Environmental Health Perspectives. 2021. DOI: https://doi.org/10.1289/EHP7699

[40] Rose AMC, Nicolay N, Mazagatos C, Martínez-Baz I, Launay O, Mot LD, et al.. Vaccine effectiveness against influenza A in older adults and the effect of chronic conditions: results from the I-MOVE and VEBIS multicentre European hospital case–control studies, 2015/16–2023/24. BMC Medicine. 2025. DOI: https://doi.org/10.1186/s12916-025-04426-y