Avian Influenza Virus

Overview, Taxonomy, and Evolution of Avian Influenza Virus

Avian influenza viruses (AIVs) are enveloped, negative-sense, single-stranded RNA viruses belonging to the Orthomyxoviridae family. Their genome is segmented into eight RNA segments, which facilitate the process of reassortment – a form of genetic mixing critical in the emergence of novel variants. AIVs are predominantly maintained in wild aquatic birds, particularly migratory waterfowl, which act as natural reservoirs. These hosts commonly harbor low pathogenicity strains, which over time may undergo mutation and reassortment during the transmission cycle between wild species and domestic poultry, occasionally spilling over to mammalian hosts [8, 11]. The significance of these viruses is underscored by their potential to cause economically devastating outbreaks in poultry and sporadic zoonotic infections in humans, as documented by global health agencies such as the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and the World Organisation for Animal Health (WOAH).

Taxonomy

The taxonomic classification of AIVs reflects a high degree of antigenic and genetic diversity. At the highest level, AIVs are classified within the genus Influenzavirus A. Their classification into subtypes primarily relies upon two surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes and 9 NA subtypes identified in avian species, giving rise to a variety of combinations such as H5N1, H7N9, H5N8, and H9N2, among others [5, 11]. For instance, the highly pathogenic H5N1 viruses – belonging to the A/goose/Guangdong/1/96 lineage – have been extensively monitored due to their significant impacts on both poultry industries and public health. Within H5 subtypes, further classification into clades (e.g., clade 2.3.4.4b) has allowed researchers and regulatory bodies to track their geographic spread and genetic evolution meticulously [4, 6]. The ongoing refinement of nomenclature by expert groups, in line with recommendations from the WHO, OIE, and FAO, ensures that emerging genetic lineages are designated promptly, aiding in the early detection and management of outbreaks [11].

Beyond the H and N proteins, the internal gene segments also exhibit considerable genetic diversity that underlies replication efficiency and host adaptation. The polymerase subunits (PB1, PB2, and PA), nucleoprotein (NP), matrix (M), and non-structural (NS) proteins all play roles in determining host range, virulence, and the virus’s capacity for reassortment. Subtle changes or mutations in these segments, such as the mammalian adaptation signature mutations in PB2 (e.g., E627K) [7, 12], contribute significantly to the zoonotic potential of AIVs. The ability of these viruses to acquire gene segments from different strains through reassortment leads to mosaic genotypes that challenge current classification systems and complicate the prediction of outbreak dynamics.

Evolution of Avian Influenza Virus

The evolution of AIV is driven by two primary mechanisms: antigenic drift and antigenic shift. Antigenic drift refers to the gradual accumulation of genetic mutations over time, a consequence of the inherently error-prone nature of RNA-dependent RNA polymerases. This continuous evolution under immune selection pressure results in minor changes in antigenic sites, most notably within the HA and NA proteins, thereby allowing the viruses to evade host immune responses. Conversely, antigenic shift is a more abrupt process that occurs when two or more influenza viruses co-infect a single host cell, permitting the exchange of entire gene segments. This reassortment can lead to the emergence of novel viruses with entirely new antigenic profiles and host ranges [9, 13].

Recent epidemiological studies have demonstrated how these evolutionary processes can result in the generation of reassortant viruses with increased virulence and zoonotic potential. For example, clade 2.3.4.4b H5 viruses have been involved in widespread outbreaks across continents, affecting domestic birds, wild birds, and multiple mammalian species [1, 2, 4]. Their genetic plasticity has allowed these viruses to acquire internal gene segments that enhance replication in non-avian hosts, as seen in the detection of H5N1 virus in dairy cattle and domestic cats [1, 3]. Such interspecies transmissions highlight the dynamic nature of AIV evolution, where reassortment creates genetic constellations that can occasionally breach species barriers. Data from genomic surveillance underscore the fact that the persistence and spread of AIV in wild birds are intimately linked to bird migration patterns, which serve both as dissemination routes and as mixing vessels for various virus lineages [8, 14].

Molecular clock analyses have provided further insight into the evolutionary timeline of AIV. Detailed phylogenetic investigations have revealed that major gene segments of these viruses underwent a global selective sweep over the past century. This synchronized evolution of internal genes has been linked to the emergence of pandemic influenza strains, most notably the 1918 and 2009 pandemics [13]. The evolutionary pressures exerted by host immune responses, ecological changes, and anthropogenic factors such as intensive poultry farming have contributed to the rapid genetic diversification of AIVs. Such diversifications are not random but are often shaped by selective bottlenecks that facilitate the survival of variants best adapted to both avian and mammalian hosts [9].

Experimental studies using animal models have provided critical evidence on the adaptive mutations that enhance mammalian infectivity. For instance, investigations into the polymerase function and receptor binding preferences have identified a range of mutations that modulate the virus’s ability to replicate in human cells [7, 10]. These studies show that specific amino acid substitutions not only influence the efficiency of viral replication but also alter tissue tropism, thereby affecting the clinical outcomes upon infection. Moreover, the spatial and temporal analyses of viral genomes, as seen in large-scale surveillance reports, emphasize that reassortment events are frequent and can involve gene segments from both wild bird and domestic poultry reservoirs [6, 14]. This high level of genomic exchange ensures that AIV remains a moving target for vaccination and control strategies, necessitating continuous monitoring by international health organizations such as the CDC, WHO, and WOAH.

Furthermore, the interplay between viral evolution and host ecology cannot be overstated. Avian species, especially migratory waterfowl, provide a vast reservoir where low pathogenicity avian influenza viruses can circulate silently, only to acquire mutations or reassort with other viruses during co-infections in shared habitats [8]. The role of domestic ducks and other anseriform species in central and eastern Asia has been particularly highlighted in studies, where they have served as intermediary hosts that link wild avian reservoirs to commercial poultry flocks [14]. Such interactions underscore the importance of integrated surveillance systems that combine genomic, ecological, and epidemiological data, a strategy promoted by WHO and FAO guidelines for managing zoonotic and economically impactful pathogens.

In essence, the evolution of avian influenza virus represents a complex interplay of genetic mutation, ecological dynamics, and host interactions. Detailed understanding of its taxonomy and evolutionary mechanisms is crucial not only for predicting future outbreaks but also for implementing effective control measures in both animal and human populations.

Molecular Pathogenesis of Avian Influenza Virus

The molecular pathogenesis of avian influenza virus is driven by a complex interplay of viral genetic determinants and host cellular factors that govern viral entry, replication, and interspecies transmission. At the molecular level, viral pathogenesis is associated with mechanisms such as receptor binding specificity, polymerase activity modulation, host factor interactions, and genetic reassortment, all of which have critical roles in determining host tropism and pathogenicity.

Viral Entry and Receptor Binding

The initial step in avian influenza virus infection is mediated by the viral hemagglutinin (HA) protein, which binds to sialic acid receptors on host epithelial cells. Avian influenza viruses typically display a high affinity for α2,3-linked sialic acid receptors that are predominantly found in the intestinal tracts of birds. However, the potential for zoonotic transmission arises from mutations in HA that shift receptor binding preference toward α2,6-linked sialic acids, which are abundant in the human upper respiratory tract. Structural investigations have demonstrated that subtle amino acid substitutions within the receptor-binding site, such as those observed in H10N8 virus isolates, can drastically alter receptor preferences and thus influence the virus’s ability to infect mammals [17, 20]. This molecular transition is critical because it enables the virus to overcome the species barrier, which is substantiated by the identification of mutations like G228S in HA in human isolates, correlating with increased affinity for human-type receptors [21]. Regulatory bodies such as the CDC and WHO monitor these changes closely due to their implications for public health, as such modifications can herald the emergence of zoonotic or even pandemic strains.

Viral Replication and Polymerase Function

Following receptor binding and entry, the virus must efficiently replicate within host cells, a process orchestrated by the viral RNA polymerase complex, composed of PA, PB1, and PB2 proteins. Among these, mutations in the PB2 subunit, such as the E627K substitution, have been repeatedly implicated in enhanced replication efficiency in mammalian cells. Experimental work has shown that this mutation allows the polymerase complex to better function at the lower temperatures characteristic of the mammalian upper respiratory tract, significantly increasing viral replication and virulence [7, 12]. Additionally, studies have identified novel residues and combinations of mutations within the polymerase complex, even outside of PB2, that synergize to enhance polymerase activity in mammalian hosts. For example, amino acid substitutions in PA (e.g., S224P and N383D) have been demonstrated to act in concert to support mammalian adaptation, revealing that the virus can utilize multiple genetic routes for overcoming host-specific restrictions [12, 19]. This enhanced replicative capacity is one of the main drivers for the observed shift in host tropism, ultimately increasing the risk of cross-species transmission.

Host Factor Interactions and Interspecies Transmission

The efficiency of viral replication in different hosts is not solely determined by viral genetics; it also depends on host cellular factors. A notable example is the interaction between the influenza virus polymerase and host proteins such as ANP32A. In swine, for instance, ANP32A has been found to support avian influenza virus polymerase activity more robustly compared to its human counterpart, a molecular rationale underpinning the “mixing vessel” status of pigs [18]. This difference in host factor interaction explains why swine are uniquely susceptible to both avian and human strains, facilitating reassortment events that can result in novel pandemic viruses.

Moreover, host restriction factors also play a decisive role in controlling viral replication. The mitochondrial protein TUFM, which is integral to the autophagy pathway, has been identified as a host restriction factor that preferentially inhibits avian-signature viruses bearing the PB2-627E mutation. Higher levels of TUFM-dependent autophagy correlate with reduced replication of avian influenza virus in human cells, representing a crucial intrinsic defense mechanism [10]. Such host-pathogen interactions highlight the delicate balance between viral adaptations that facilitate infection and host immune responses that counteract them.

Epidemiological reports, such as those describing spillover events to mammals, including dairy cattle, domestic cats, and even marine mammals, underscore that avian influenza viruses can cross species boundaries when viral proteins acquire mutations that enhance their compatibility with mammalian cellular environments [1, 3, 7]. The detection of characteristic mutations in viruses isolated from these non-avian hosts, including those affecting both HA receptor binding and polymerase activity, illustrates the relevance of molecular pathogenesis in real-world scenarios. These events have prompted heightened surveillance by international organizations like the WOAH and FAO, in collaboration with agencies such as the CDC and WHO, to monitor potential zoonotic risks.

Genetic Reassortment and Evolution

The segmented RNA genome of avian influenza viruses enables genetic reassortment, a process whereby gene segments are exchanged between co-infecting strains. This reassortment is a critical factor in the emergence of highly pathogenic strains and plays a pivotal role in viral evolution across avian and mammalian hosts. For instance, during outbreaks involving clade 2.3.4.4 viruses, the virus has undergone multiple reassortment events that have led to the incorporation of gene segments from different low-pathogenicity avian influenza viruses [1, 6, 14-16]. Such reassortments can yield novel genetic constellations with altered virulence and host range characteristics.

Phylogenetic analyses have shed light on the interplay between reassortment events and viral dissemination. A time-resolved phylogenetic study of Eurasian H5 viruses revealed that gene segments involved in reassortment are often acquired from wild birds with migratory patterns corresponding to specific geographical intervals [14]. These findings not only illustrate the dynamic nature of viral evolution but also emphasize the importance of integrating ecological factors, such as bird migration patterns, into molecular surveillance strategies. This coupling of genetic analysis with field epidemiology is vital for assessing the public health threat posed by emerging reassortants, as noted by WHO influenza program experts.

Interactions with Host Cellular Pathways

Advances in proteomic and molecular techniques have allowed researchers to uncover the intricate details of how avian influenza viruses hijack host cellular pathways to promote infection. Beyond receptor binding and replication, the virus induces a myriad of changes within the host cell, affecting apoptosis, interferon responses, and autophagy. For example, the preferential binding of TUFM to the avian-signature PB2 variant, along with its association with increased autophagy, provides insight into how the virus attempts to modulate cellular defenses [10]. Furthermore, the cooperative effect of multiple mutations in the polymerase complex not only augments viral replication but may also interfere with normal host signaling pathways, thereby contributing to the pathogenesis observed in infected tissues.

The coordinated remodeling of host cell pathways and evasion of innate immune responses are hallmarks of severe infection. Enhanced expression of cytokines, chemokines, and other inflammatory mediators, often triggered by high viral loads in the lower respiratory tract, has been documented in both experimental models and natural outbreaks [9, 17]. This hypercytokinemia is a significant contributor to the pathology observed in human cases of zoonotic avian influenza infections, reinforcing the urgent need for comprehensive molecular surveillance. The integration of molecular data with clinical and pathological findings forms the basis for risk assessments by organizations such as the CDC, WHO, and WOAH, which continually refine guidelines for pandemic preparedness.

Implications for Zoonotic Transmission and Public Health

The multistep pathogenesis of avian influenza viruses, from receptor binding to replication, reassortment, and host interaction, explains why certain strains have a heightened potential for zoonotic transmission. Critical mutations in HA, the polymerase complex, and NP proteins not only enhance viral fitness but also determine the efficiency of cross-species spread. Spillover events to species such as dairy cattle and domestic cats, as detailed in recent investigations, provide empirical evidence that changes at the molecular level can lead to unpredictable transmission dynamics [1, 3, 4]. Such events necessitate the establishment of stringent surveillance protocols in both avian and mammalian populations, a priority echoed by international agencies like the FAO and WOAH.

Ultimately, understanding the molecular pathogenesis of avian influenza viruses, the fine balance between viral adaptation strategies and host defenses, is central to mitigating the risk of future pandemics. A thorough grasp of these mechanisms is indispensable for the development of novel antivirals and effective vaccines that target critical viral components. As research continues to elucidate the molecular determinants of virulence and host specificity, collaboration among veterinary researchers, public health organizations, and international bodies will remain essential in the global fight against these highly pathogenic pathogens.

Epidemiology and Global Dissemination of Avian Influenza Virus

The epidemiological dynamics of avian influenza virus (AIV) reflect a complex interplay of viral evolution, host species diversity, and global ecological networks. Over recent years, the spread of highly pathogenic clones, notably those belonging to clade 2.3.4.4b, has been observed not only in traditional wild and domestic avian hosts but also in several unexpected mammalian species. Surveillance and genetic analyses across continents underscore that migratory waterfowl and other wild birds act as the primary reservoirs and dissemination agents, facilitating intercontinental transfer and contributing to genetic reassortment events that spur viral adaptation and expansion across species boundaries [2, 8, 27].

Natural reservoirs, such as migratory wild birds, are instrumental in maintaining and dispersing AIV. Seasonal migratory patterns enable long-distance virus movements, as evidenced by the detection of highly pathogenic strains in regions previously considered free of such viruses, for instance, the novel introduction into sub-Antarctic areas observed in brown skuas, kelp gulls, and other avian species [2]. These migratory events not only impact avian populations but also drive virus reassortment when highly pathogenic strains interact with endemic low-pathogenic viruses during overlapping migratory cycles. Reassortment events, which have been documented in central Europe and parts of Asia, generate novel genomic constellations that may harbor increased replicative capacities in new hosts [14, 16]. Epidemiological surveillance strategies recommended by the World Organisation for Animal Health (WOAH) and guidance from the World Health Organization (WHO) stress the importance of continuous monitoring to preempt potential zoonotic spillovers and pandemic threats, particularly when reassortment leads to mutations enhancing mammalian replication [4, 23].

Avian influenza’s capacity to cross species barriers has been further emphasized by outbreaks that demonstrated spillover into non-traditional hosts such as dairy cattle, domestic cats, and even marine mammals. Studies have highlighted how viruses from clade 2.3.4.4b, originally circulating in domestic and wild birds, have infected dairy cattle across multiple states in the United States, leading to significant clinical syndromes including decreased milk production and respiratory distress [1, 3]. The detection of viral RNA in unpasteurized bovine milk raised public health concerns, prompting comprehensive investigations to ascertain that current pasteurization measures sufficiently mitigate the risk of human exposure through dairy products [22]. In other instances, mammalian species including red foxes and minks have not only acquired the virus, but in certain cases, mutations associated with enhanced replication in mammalian hosts have been observed, suggesting an adaptive trend that warrants increased vigilance [7, 24-26]. These events are critical from a One Health perspective, as underscored by international health agencies such as the US Centers for Disease Control and Prevention (CDC) and the WHO.

Importantly, the dynamics of viral spread are not merely determined by host migration but are also influenced by environmental factors and local epidemiological conditions. For example, atmospheric dispersion studies demonstrate that bioaerosols containing avian influenza viral particles can be transported over short distances from infected poultry farms, potentially exposing nearby domestic or wild birds and even humans to viral material [28]. This environmental factor highlights the need for integrated surveillance systems that combine genomic data, spatial modeling, and field investigations to accurately map dispersion routes and forecast outbreak hotspots. Such multidimensional surveillance efforts are in line with recommendations from the Food and Agriculture Organization (FAO) and the CDC, emphasizing the significance of early warning systems and synchronized global response mechanisms.

Reassortment mechanisms further complicate the epidemiological landscape. The rapid evolution of internal genes in modern AIVs has been characterized by synchronized global sweeps resulting in nearly uniform viral segments circulating among diverse host populations [13]. This genetic homogeneity among certain segments aids in rapid host adaptation, often in response to the pressures imposed by intensive poultry farming and wide-scale migratory bird aggregations. Host-specific mutations acquired during spillover events, in mammals such as cattle, minks, or even humans, highlight the virus’s capacity to exploit novel cellular environments; for instance, critical mutations in polymerase genes enhance replication efficiency in mammalian epithelial cells [12]. The interplay between the natural reservoirs in wild birds and spillover hosts in domestic settings creates a continuous cycle of adaptation and dissemination that underscores the global threat posed by AIVs.

Furthermore, live poultry markets (LPMs) have been identified as hotspots for avian influenza virus transmission, acting as critical nodes in both local and international dissemination networks. Epidemiological investigations have demonstrated that interventions such as LPM closures can drastically reduce poultry-to-human transmission, especially in areas where zoonotic strains such as H7N9 emerge [29]. These findings reinforce the idea that controlling AIV in its reservoir and interface settings is crucial to preventing subsequent pandemic spread. Such measures are strongly advocated by public health authorities including the CDC and WHO, which continuously update guidelines to manage risks associated with both seasonal and outbreak scenarios.

Another critical factor in the global spread of AIV is the role of domestic birds, particularly in regions where intensive poultry production coexists with urban expansion. In regions such as Southeast Asia and parts of Europe, close contact between wild birds and domestic poultry facilitates virus reassortment and cross-species transmission. Surveillance efforts in these areas underscore dramatic shifts in circulating subtypes, with variants like H9N2 emerging as dominant strains in live poultry markets and raising concerns due to their preferential binding to human-type receptors [5, 30]. Such epidemiological shifts underline the importance of integrating genomic surveillance with traditional ecological and clinical data to develop targeted intervention strategies.

The global dissemination of avian influenza virus is emblematic of the intricate connections between ecology, evolution, and human activity. The persistent circulation of AIV within migratory bird populations, the inherent propensity of the virus to reassort, and the documented instances of cross-species transmission collectively underscore the urgency for robust, coordinated international surveillance, vaccine development, and biosecurity measures as recommended by the WHO, CDC, and FAO.

Diagnostics and Surveillance Methodologies for Avian Influenza Virus

The diagnostic landscape for avian influenza virus (AIV) is characterized by a multifaceted approach that integrates molecular, serological, biosensor‐based, and genomic modalities, reflecting the virus’s rapid evolution and complex ecology. In the context of widespread outbreaks, including the recent incursions of clade 2.3.4.4b H5 viruses and other subtypes in both domestic and wild species, early and accurate detection is paramount. The diagnostic methods not only enable prompt clinical management of exposed populations but also play a critical role in informing surveillance strategies coordinated by agencies such as the CDC, WHO, WOAH, and FAO.

Molecular Diagnostics and Quantitative Techniques

Real-time reverse transcription polymerase chain reaction (RT-PCR) has emerged as the cornerstone for the rapid detection of AIV, especially in light of its sensitivity and specificity for various gene targets, including the matrix and hemagglutinin genes. Several studies have demonstrated the successful application of quantitative real time RT-PCR (qrRT-PCR) in detecting viral RNA in complex matrices such as raw milk and retail dairy products [22]. The sensitivity of these assays is critical when monitoring low viral loads during the early or subclinical phases of infection, which is essential in mitigating potential cross-species transmission events, such as those observed in domestic dairy cattle infected with H5N1 [1, 3]. Additionally, qrRT-PCR protocols have been crucial in distinguishing clade-specific markers that highlight the passage of genetic material between avian hosts and spillover events into mammals [17, 33]. This precise molecular discrimination aids in the rapid characterization of viral genotypes, an imperative function given the reassortment events precipitated by the mingling of domestic and wild bird populations.

Beyond standard RT-PCR methods, advanced modifications using CRISPR activation screens have also been employed to identify host restriction factors inhibitory to AIV replication [32]. These emerging techniques not only provide an alternative diagnostic modality but also contribute mechanistically to understanding viral tropism and host adaptation processes, thereby supporting both clinical and epidemiological surveillance.

Serological and Immunological Diagnostic Approaches

Serological assays, including enzyme-linked immunosorbent assays (ELISA) and hemagglutination inhibition tests, remain essential for retrospective surveillance and for establishing seroconversion in exposed animal populations. These methods are invaluable when direct viral detection is limited due to the transient nature of the viral load in clinical samples. The integration of immunoassays with molecular methods enriches the diagnostic workflow, confirming infections that might otherwise be missed during the low-viral replication phase inherent to subclinical infections in some wild waterfowl or domestic poultry [31]. Furthermore, immunohistochemistry and in situ hybridization techniques have provided histopathological context to virus localization in tissues, for example, delineating the tropism of H5N1 in mammalian mammary tissue and respiratory tracts [1, 3], thus enhancing our understanding of tissue-specific pathogenesis and supporting targeted surveillance activities.

Biosensor Technologies and Environmental Sampling

The advent of biosensor-based diagnostics, including electrochemical platforms and aptasensors, has significantly advanced the real-time detection capabilities for AIV in field settings. Electrochemical biosensors, for instance, leverage nanomaterial-enhanced detection with minimal sample processing, achieving low detection limits that are pivotal during outbreak scenarios [36]. Similarly, surface plasmon resonance (SPR) aptasensors have demonstrated rapid and specific detection of H5N1 in poultry swab samples, offering the prospect of portable, cost-effective diagnostics that can be deployed in remote or high-risk regions [37]. These platforms are particularly useful for environmental surveillance, where viral particles may be dispersed through air or water systems, a phenomenon noted in the spread of influenza via airborne particulate matter [28]. Coupling these sensor-based approaches with conventional laboratory diagnostics ensures that both on-site and centralized testing can be coordinated effectively.

Genomic Surveillance and Next-Generation Sequencing

The evolution of AIV is driven by frequent mutations, antigenic drift and shift, and reassortment events, underscoring the need for robust genomic surveillance. Next-generation sequencing (NGS) techniques have been instrumental in mapping these genetic changes across broad geographic and host-specific contexts [6, 27, 31]. Through high-throughput sequencing, researchers can discern the signatures of mammalian adaptation, such as the emergence of specific polymerase mutations, that inform on the zoonotic potential of circulating strains [4, 12, 33]. Whole-genome sequencing not only assists in real-time outbreak investigations but also facilitates phylogenetic analyses that trace the molecular epidemiology of AIV outbreaks. Detailed genomic analyses have, for example, elucidated the transcontinental movement of H5N1 viruses via migratory birds, a finding with significant implications for intercontinental surveillance efforts coordinated by global organizations like FAO and WHO [2, 27].

Moreover, sophisticated bioinformatic tools allow for the detection of reassortant strains by comparing segments from diverse viral sources. This level of genomic resolution is critical for identifying emergent viruses that possess pandemic potential, an imperative given the continuing risk of zoonotic spillover events among domestic, wild, and even human hosts [31, 33, 34]. The comprehensive analysis of viral genomes thus forms a backbone of modern surveillance strategies, helping public health authorities to implement timely interventions based on molecular risk assessments.

Integrated One Health Surveillance Strategies

Recognizing the multifaceted nature of AIV transmission, surveillance efforts are increasingly adopting a One Health framework. This integrated approach entails the pooling of data from veterinary, environmental, and human health sectors to provide a holistic view of virus circulation. Enhanced surveillance in both wildlife reservoirs, such as migratory waterfowl, and domestic settings has been critical for early detection and for guiding targeted control measures [4, 23, 35]. The use of wildlife banding data, for example, not only traces migratory routes but also aligns with genetic data from sequenced isolates to predict potential outbreak regions [6, 27]. Sampling strategies extend to environmental collections where particulate matter is analyzed to gauge virus dispersion beyond direct host-to-host contact [28].

The One Health surveillance paradigm leverages the complementary strengths of field diagnostics, advanced laboratory methodologies, and epidemiological studies to generate a high-resolution picture of AIV dynamics. Collaborative networks, supported by international bodies such as the CDC, WHO, WOAH, and FAO, use these integrated data streams to devise mitigation strategies that are tailored to the evolving risks, whether that is in response to a new reassortant emerging in live poultry markets or the spillover of viruses into mammalian populations [4, 23, 35].

Epidemiological Data Integration and Real-Time Monitoring

Epidemiological surveillance is enhanced through real-time data acquisition and modeling. Spatial and temporal data derived from field sampling, combined with phylogenetic mapping of viral sequences, create dynamic models of virus spread. This data integration supports early warning systems and risk communication, facilitating rapid responses that can include targeted culling, vaccination, or market closures as evidenced in cases of H7N9 outbreaks [29]. Real-time monitoring systems that track incidence rates and genomic changes provide a dual layer of surveillance that is critical for preemptive public health interventions.

The implementation of digital surveillance tools and statistical models also enables public health agencies to predict outbreak patterns and potentially forecast pandemic scenarios. Such methodologies are increasingly important in light of the ongoing crossover of AIV from wild birds to mammals, highlighting the critical interplay between diagnostic accuracy and surveillance precision [1, 3, 24]. The continuous evolution of diagnostic technologies, alongside integrated genomic and epidemiological monitoring, ensures that the response to AIV remains agile and data-driven in an ever-changing landscape.

Cross-Species Transmission and Zoonotic Spillover Dynamics of Avian Influenza Virus

Avian influenza viruses (AIVs) have an intrinsic capability to infect a broad array of host species, a trait that is central to their zoonotic spillover dynamics. The underlying biological mechanisms that facilitate these cross-species events are multifaceted, involving both viral evolutionary adaptations and ecological interfaces where the virus encounters different hosts. The events witnessed in domestic species such as dairy cattle and cats, alongside outbreaks in wild mammals and even certain human exposure cases, underscore the complexity of these dynamics [1, 3, 4].

Mechanisms Underpinning Interspecies Transmission

At the molecular level, a key aspect that drives the interspecies jump is the virus’s receptor binding specificity and the subsequent adaptations in the viral polymerase complex. Avian influenza viruses typically display preferential binding for α2-3 linked sialic acid receptors, which predominate in the avian respiratory and gastrointestinal tracts. However, mutations in the hemagglutinin (HA) protein may alter this specificity, allowing the virus to efficiently engage α2-6 linked receptors that are abundant in the human upper respiratory tract. Studies focusing on H10N8 and H7N9 isolates have revealed critical substitutions in the receptor-binding domain, such as those involving residues like Gln226Leu and Gly186Val, which contribute to an elevated affinity toward mammalian receptor types [17, 20]. In parallel, mutations within the polymerase basic (PB2) protein, notably the substitution of glutamic acid for lysine at residue 627, are pivotal for overcoming mammalian host restriction barriers. For instance, analyses conducted in cross-species infection models have demonstrated that viruses harboring the PB2-E627K mutation exhibit increased replication competence in mammalian cells relative to their avian counterparts [7, 10]. Such adaptive mutations usually arise during or immediately following interspecies spillover events, enhancing the virus’s capacity to replicate at the lower temperatures characteristic of the upper respiratory tract in mammals [7, 12].

Epidemiological and Contextual Drivers of Spillover

The interspecies transmission events of H5 highly pathogenic avian influenza (HPAI) viruses exemplify the compounded risk when viruses bridge the species gap. For instance, the detection of H5N1 in dairy cattle, accompanied by systemic infections and subsequent spillover to domestic cats via unpasteurized milk consumption, illustrates a non-traditional yet epidemiologically significant interface [1, 3]. This phenomenon brings to light the risk of virus dissemination through practices in food production and handling, an issue that both the U.S. Food and Drug Administration and global agencies such as the World Health Organization (WHO) monitor closely. The detection of viral RNA in retail dairy products, albeit non-infectious post-pasteurization, further reinforces the need for stringent biosafety measures across the food supply chain [22].

Wild migratory birds remain the primary reservoirs for these viruses, yet their role as vehicles for transcontinental dissemination cannot be overstated. Migratory patterns facilitate the mixing of genetically distinct viral populations, leading to reassortment events that can endow progeny viruses with novel attributes enhancing their zoonotic potential. Genetic investigations have shown that gene segments derived from wild birds, domestic poultry, and even mammals converge through reassortment events, reflecting highly dynamic evolutionary processes [2, 6, 14]. For example, during the spread of clade 2.3.4.4b viruses, reassortments have been repeatedly observed in regions ranging from North America to the Antarctic [2, 3]. These events are compounded by the fact that different host species provide unique immunological milieus, which can apply selective pressures that rapidly fix adaptive mutations within the virus.

Mammalian Adaptation and Secondary Transmission Dynamics

The consequences of spillover events are not restricted solely to isolated cases of mammalian infection; several studies have documented evidence of secondary transmission amongst mammals. Evidence from outbreaks involving farmed minks in Spain and wild red foxes in Europe suggests that once avian influenza viruses gain access to mammalian hosts, they can sometimes transmit between individuals, albeit less efficiently than in their native avian reservoir [7, 25]. In mast cases of incidental infection, adaptation markers such as the PB2-E627K mutation, often appearing as a mixed virus population initially, eventually become fixed and promote enhanced replication fitness in mammalian cells [7, 12]. Such genetic adaptations increase the risk of broader mammalian spread, raising the potential, which has been highlighted by public health authorities such as the Centers for Disease Control and Prevention (CDC) and the World Organisation for Animal Health (WOAH), for a reassortant virus to acquire sustained human-to-human transmission capabilities.

Furthermore, cross-species transmissions are not solely the preserve of highly pathogenic strains. Low pathogenic avian influenza viruses (LPAIVs) circulating in domestic poultry can also undergo genetic shifts that precipitate zoonotic spillover. Active surveillance efforts in multiple regions have reported the circulation of H9N2 viruses, which are known for their frequent genetic reassortment and ability to serve as internal gene donors to other highly pathogenic subtypes [5, 30]. The phenomenon where “mixing vessel” species such as swine or even farmed mink mediate reassortment between avian and human influenza viruses further complicates the picture. Swine, in particular, possess receptors for both avian and human influenza viruses, making them ideal incubators for novel reassortants, a finding that has significant implications for global pandemic preparedness [18, 38, 39].

Ecological Interfaces and the Role of Human Activity

Ecological interfaces where wild birds, domestic poultry, and mammals converge are critical in facilitating spillover events. Agricultural practices, live animal markets, and the improper handling of animal by-products create conduits through which viruses can traverse species boundaries. Live poultry markets, for instance, have been implicated in human infections with avian influenza viruses, as evidenced by outbreaks where the closure of such markets resulted in a dramatic reduction in transmission rates [29]. The interaction between wild birds and domestic animals in open-air markets or free-range farming systems further increases the potential for spillover, as observed during several European HPAI outbreaks where infections in wild birds directly preceded primary poultry outbreaks [35]. Additionally, the movement of live animals between farms, as observed in the case of dairy cattle infections, can inadvertently propagate the virus across geographic regions, complicating containment efforts [1, 3].

Moreover, environmental factors such as wind-mediated spread of virus-laden particulate matter from infected poultry houses have been shown to contribute to the broader dissemination of the virus [28]. This environmental dispersion underscores the complexity of transmission dynamics where aerosolized viral particles may bridge gaps between otherwise isolated host populations.

Integration of Molecular Surveillance and Global Public Health Strategies

In response to these multifaceted challenges, global institutions such as the CDC, WHO, and FAO emphasize the implementation of integrated surveillance systems that combine genetic, epidemiological, and ecological data. Enhanced diagnostic techniques, including quantitative real-time RT-PCR and CRISPR-based screens, have been pivotal in detecting early signs of cross-species transmission and identifying mutations associated with mammalian adaptation [22, 31, 32]. Whole-genome sequencing and detailed phylogenetic analyses not only help trace the pathways of interspecies virus movement but also inform strategic interventions aimed at mitigating zoonotic spillover risks. The incorporation of host factor studies further refines our understanding by elucidating how specific proteins, such as swine ANP32A, support avian virus replication in mammals, highlighting potential targets for antiviral therapeutics [18].

This comprehensive integration of surveillance data with molecular and ecological insights enhances our ability to predict and respond to spillover events. It provides an essential framework for international collaboration in outbreak response, aligning with recommendations from WHO and WOAH to prevent the emergence and global spread of potentially pandemic strains.

Clinical Manifestations and Pathological Impact on Avian and Mammalian Hosts

The clinical manifestations and pathological outcomes of avian influenza virus infections are highly variable, depending on the virus clade, the host species involved, and the exposure route. In avian hosts, including both wild birds and domestic poultry, the infection can range from asymptomatic carriage in natural reservoirs to severe systemic disease with high mortality rates. In contrast, mammalian hosts infected by spillover viruses, whether through direct contact, ingestion of contaminated products, or other pathways, typically exhibit a spectrum of clinical syndromes that involve respiratory, gastrointestinal, and neurological systems, with the severity of disease often dictated by factors such as viral adaptation mutations and host immune responses.

Pathogenesis in Avian Hosts

In domestic and wild birds, particularly poultry, highly pathogenic strains of avian influenza virus (AIV) often cause a dramatic range of clinical signs that include severe respiratory distress, sudden mortality, and sometimes neurological symptoms manifesting as tremors, lack of coordination, and altered behavior. These viruses, such as those of clade 2.3.4.4b, can trigger widespread systemic infection that involves multiple organs, often due to their capacity to disseminate via the bloodstream. In many cases, lesions in the respiratory tract, as well as necrosis in lymphoid tissues, are observed. The high mortality rate in avian populations has been a critical concern for organizations such as the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO), which monitor these outbreaks closely to mitigate economic losses in poultry sectors globally.

Wild birds, while serving as natural reservoirs with relatively benign infections in many instances, can still experience substantial morbidity and mortality when infected with clade 2.3.4.4b viruses. Outbreaks in species not evolutionarily adapted to cope with these highly pathogenic viruses can lead to mass mortality events, as documented in migratory waterfowl and other wild bird species [2, 4]. The rapid spread of such infections via migratory flyways heightens the risk that even geographically remote populations, such as those in Antarctica, may eventually be exposed and suffer significant pathological impacts.

Clinical Characteristics in Mammalian Hosts

In mammalian species, the clinical picture is made more complex by the occurrence of spillover events from avian hosts. Dairy cattle, for instance, have been documented to exhibit nonspecific clinical illness upon infection with highly pathogenic avian influenza virus. Infected cattle often show signs such as reduced feed intake, decreased rumination, and an abrupt drop in milk production accompanied by the presence of abnormal milk, which in some cases has been linked to systemic viral dissemination [1, 3]. In these cases, viral tropism extends into the mammary epithelial cells, highlighting a mechanism of direct infection of secretory tissues. Mammalian infections may further be driven by procedural lapses, such as the feeding of raw colostrum and milk from infected cows to domestic cats, which can result in fatal, multi-organ involvement in these secondary hosts.

Felines, in particular, can develop rapidly progressing systemic disease when exposed to highly pathogenic viruses through consumption of infected material. Cats have shown evidence of disseminated virus replication, with significant lesions in multiple organs including the brain, underlining the potential for neurological involvement in spillover infections [1, 4]. Similarly, outbreaks in farmed minks have revealed that highly pathogenic strains can spread efficiently among mammalian populations, sometimes associated with mutations in the viral polymerase (such as the T271A mutation in PB2) that may enhance replication in mammalian cells [25]. The clinical manifestations in these species typically present as respiratory distress, fever, and in some cases neurological deficits, thereby emphasizing the diverse tissue tropism of these viruses beyond just the respiratory system.

Wild and stray mammals, including red foxes and marine mammals like harbor seals, have also been impacted by recent spillover events. In red foxes, for example, neurological signs predominate, with significant viral loads detected primarily in the brain rather than the respiratory tract, suggesting neurotropism that could be linked to the emergence of adaptive mutations such as E627K in the PB2 protein [7, 26]. In marine mammals, particularly seals found along the northeastern seaboard or in northern regions, infections have resulted in necrotizing lesions in pulmonary tissues along with evidence of viral replication confined to the respiratory tract, a pattern that has raised concerns about potential risks to both animal health and zoonotic transmission pathways [34, 40]. These findings underscore the variable pathology driven by the virus in different mammalian hosts, with host-specific factors (such as receptor distribution and innate immune responses) playing instrumental roles.

Underlying Mechanisms and Implications

The pathobiology of avian influenza in both avian and mammalian hosts is often mediated by a complex interplay between viral molecular determinants and host-specific factors. In avian species, the presence of a polybasic cleavage site in the hemagglutinin (HA) molecule is a well-established virulence factor that facilitates systemic spread and organ tropism, thereby contributing to the high pathogenicity seen in poultry outbreaks [41]. Additionally, reassortment events that allow the mixing of gene segments from various AIV strains may introduce mutations that predispose the virus to cross species barriers, promoting infections in mammalian hosts as seen in recent outbreaks among cattle, cats, and minks [1, 3, 4].

For mammalian hosts, the adaptation process often involves acquisition of mutations improving binding to mammalian-type receptors, such as alterations in the HA receptor-binding site, and enhanced polymerase activity at the lower temperatures typical of the mammalian upper respiratory tract [7, 10, 12]. These mutations not only increase the virulence in mammals but may also alter the clinical course of the disease, resulting in a broad spectrum from mild respiratory symptoms to severe pneumonia with systemic involvement. Public health authorities such as the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) have cautioned that continuous surveillance and genetic characterization of circulating strains are essential to assess zoonotic potential and to inform preparedness strategies [4, 23].

The pathological impact in different hosts is further compounded by the virus’s dissemination routes, ranging from inhalation of aerosolized droplets to ingestion of contaminated products. For example, the detection of viral RNA in raw milk and dairy products, although not viable in pasteurized form, signals an unconventional route of exposure that may facilitate cross-species transmission, particularly in agricultural settings where biosecurity may be compromised [1, 22]. Moreover, the severity of lung damage and diffuse alveolar injury following infection with highly pathogenic strains, ranging from moderate infections such as those caused by H1N1 to the high pathogenicity of H5 variants, represents a continuum of respiratory compromise that holds significant implications for both zoonotic spillover and pandemic preparedness [42].

Overall, the intricate clinical and pathological landscape of avian influenza virus infections reflects the dynamic nature of viral evolution and host adaptation. The ongoing interplay between diverse viral clades and a wide array of avian and mammalian hosts necessitates vigilant monitoring and integrated One Health approaches to curb future outbreaks and mitigate the risks of further zoonotic emergence.

Mitigation Strategies, Vaccination, and Future Research Directions in Avian Influenza Virus Control

Mitigation Strategies in Diverse Host Populations

In light of the continual evolutionary dynamics and spillover events associated with avian influenza viruses (AIVs), comprehensive mitigation strategies are essential for both animal and public health. The widespread detection of clade 2.3.4.4b viruses in a range of host species, including poultry, wild birds, dairy cattle, and even domestic mammals such as cats and minks, necessitates an integrated One Health approach that encompasses enhanced surveillance, stringent biosecurity measures, and targeted interventions at high-risk interfaces [1, 3, 4]. For instance, detection of viral RNA in unpasteurized bovine milk raises concern for cross-species transmission, underscoring the need for strict controls in dairy production systems and transportation networks to prevent further cow-to-cow or interspecies spread [1, 3, 22]. Regulatory agencies such as the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and the World Organisation for Animal Health (WOAH) emphasize the critical role of strict movement controls, quarantine protocols, and thorough decontamination practices in outbreak hotspots.

Enhanced surveillance systems have emerged as a cornerstone of these mitigation strategies. Active monitoring of wild bird populations, as well as routine sampling in live animal markets and domestic production settings, enables early detection of novel reassortants and host-adapted mutations. Modern diagnostic techniques, including quantitative real-time RT-PCR and next-generation sequencing, have provided vital insights into outbreak dynamics by rapidly identifying viral variants and tracking reassortment events across continents [22, 30, 31]. These advanced surveillance measures not only facilitate immediate outbreak responses but also inform risk assessments and future vaccine strain selection.

Closure of live poultry markets (LPMs) and implementation of periodic market rest days have been shown to dramatically reduce poultry-to-person transmission events, as evidenced by ecological studies in China [29]. Moreover, movement restrictions on livestock, particularly in densely populated dairy and poultry regions, are critical to diminishing the inter-regional spread of highly pathogenic strains [1, 6]. These strategies must be rigorously enforced and coordinated with international guidelines from esteemed bodies such as the WHO and FAO to mitigate the risk of a pandemic.

Vaccination Strategies and Their Critical Role

Vaccination represents a central pillar in avian influenza virus control, with a dual goal of protecting animal populations and reducing the risk of zoonotic transmission to humans. Current vaccine strategies target both inactivated and recombinant modalities tailored for the predominant circulating subtypes. The rapid evolution and reassortment of influenza viruses, as notably highlighted during the resurgence events in Eurasia, pose a significant challenge to vaccine efficacy [14, 16, 43]. This antigenic drift necessitates continuous updating of vaccine strains to ensure they match the circulating isolates. In many regions, vaccination campaigns in domestic poultry are now part of an integrated control program, supported by regulatory frameworks issued by WOAH and implemented in coordination with local public health agencies.

Recent studies have provided evidence that targeted vaccination combined with improved biosecurity practices can result in significant reductions in outbreak occurrences among poultry populations. For example, in regions where H9N2 and emerging H7N9 or H5N1 variants have been detected, vaccines developed using reverse genetics and recombinant viral platforms have shown promise in inducing cross-protective immunity [5, 45]. In addition to traditional vaccine formulations, novel immunization approaches, including mRNA-based vaccines, offer an exciting avenue for rapid response to emerging strains. These platforms not only promise to shorten the vaccine development timeline but also allow for the inclusion of conserved epitopes that may provide broader cross-protection against multiple clades.

Vaccination strategies also address the unique challenges associated with interspecies transmission. In the context of the observed spillovers to mammals such as minks, cattle, and even marine mammals, ensuring that relevant animal hosts are immunized where feasible can potentially reduce the viral reservoir that poses a threat to human populations [1, 3, 25, 40]. Immunization programs in these settings, when combined with strict controls on the movement of animals and their by-products (such as unpasteurized dairy), can serve as a critical barrier along the zoonotic transmission chain. Enhanced vaccination protocols in endemic regions, guided by epidemiological data and genetic analyses from global surveillance studies, are thereby vital for both short-term outbreak control and long-term pandemic preparedness.

Future Research Directions in Virus Control and Host Adaptation

Despite the achievements in surveillance and vaccination, the dynamic nature of AIVs presents ongoing challenges that necessitate an expanded research agenda. One important area for future investigation is the elucidation of the molecular mechanisms underlying host adaptation. Studies have identified critical polymerase mutations (e.g., E627K in the PB2 protein) and alterations in receptor-binding properties that facilitate the jump from avian to mammalian hosts, a phenomenon observed in several recent outbreaks [7, 12, 17]. Research focusing on these adaptations can inform both the design of next-generation vaccines and the development of antiviral therapeutics that are less susceptible to resistance.

Emerging technologies, including CRISPR activation screens, have identified host restriction factors that modulate viral replication in human cells. For instance, the identification of B4GALNT2 as an inhibitor of avian influenza replication in numerous strains suggests that therapeutic strategies could be developed to harness or augment such host defenses [32]. Advancing our understanding of intrinsic antiviral responses in humans and animals holds promise not only for immediate therapeutic applications but also for the development of broad-spectrum antiviral agents.

Furthermore, future research must continue to refine predictive models for virus evolution and dissemination. Integrating meteorological data, migratory bird patterns, and genomic surveillance information can improve the accuracy of outbreak forecasting and risk mapping [28, 46]. Such interdisciplinary approaches, supported by large-scale genomic studies and phylogenetic analyses, are critical for anticipating reassortment events and identifying potential pandemic precursors. The integration of bioinformatics with real-time surveillance data, for instance, via platforms endorsed by the CDC and WHO, will be indispensable in bridging the gap between basic research and field applications.

Another vital area for future research is the exploration of alternative vaccine platforms that can rapidly adapt to the evolving antigenic landscape of AIVs. The development of universal or broadly-reactive influenza vaccines remains an aspirational goal, one that would mitigate the need for frequent vaccine updates. Investigations into the conserved domains of the hemagglutinin (HA) and neuraminidase (NA) proteins, as well as the internal viral proteins that are less subject to antigenic drift, are crucial in this regard [11, 13]. Such an approach could significantly bolster global preparedness, especially given the zoonotic and economic importance of pathogens like H5N1 and H7N9.

Lastly, the role of non-traditional hosts in the epidemiology of avian influenza viruses, such as swine, mink, and marine mammals, warrants extensive research. These species act as potential “mixing vessels” for the reassortment of human and avian influenza viruses, posing a significant risk for the emergence of novel pandemic strains [39, 44]. Investigating the susceptibility, receptor distribution, and immunological responses of these species will enhance our understanding of interspecies transmission dynamics. Focused studies on host-pathogen interactions at these interfaces, using both in vivo and in vitro models, will inform tailored mitigation measures and vaccination strategies aimed at preventing cross-species jump events.

Collectively, these research directions, in conjunction with robust mitigation strategies and adaptive vaccination policies, form an integrated roadmap for the control of avian influenza viruses. Aligning efforts across veterinary, human, and environmental health disciplines, as advocated by global organizations like CDC, WHO, WOAH, and FAO, will continue to be instrumental in reducing both the zoonotic and economic impacts of this ever-evolving pathogen.

References

[1] Burrough ER, Magstadt DR, Petersen B, Timmermans SJ, Gauger P, Zhang J, et al.. Highly Pathogenic Avian Influenza A(H5N1) Clade 2.3.4.4b Virus Infection in Domestic Dairy Cattle and Cats, United States, 2024. Emerging Infectious Diseases. 2024. DOI: https://doi.org/10.3201/eid3007.240508

[2] Bennison A, Byrne A, Reid SM, Lynton-Jenkins JG, Mollett B, Sliva DD, et al.. Detection and spread of high pathogenicity avian influenza virus H5N1 in the Antarctic Region. bioRxiv. 2024. DOI: https://doi.org/10.1038/s41467-024-51490-8

[3] 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

[4] Graziosi G, Lupini C, Catelli E, Carnaccini S. Highly Pathogenic Avian Influenza (HPAI) H5 Clade 2.3.4.4b Virus Infection in Birds and Mammals. Animals. 2024. DOI: https://doi.org/10.3390/ani14091372

[5] Peacock T, James J, Sealy J, Iqbal M. A Global Perspective on H9N2 Avian Influenza Virus. Viruses. 2019. DOI: https://doi.org/10.3390/v11070620

[6] Bevins S, Shriner S, Cumbee JC, Dilione KE, Douglass KE, Ellis JW, et al.. Intercontinental Movement of Highly Pathogenic Avian Influenza A(H5N1) Clade 2.3.4.4 Virus to the United States, 2021. Emerging Infectious Diseases. 2022. DOI: https://doi.org/10.3201/eid2805.220318

[7] Bordes L, Vreman S, Heutink R, Roose M, Venema S, Pritz-Verschuren S, et al.. Highly Pathogenic Avian Influenza H5N1 Virus Infections in Wild Red Foxes (Vulpes vulpes) Show Neurotropism and Adaptive Virus Mutations. bioRxiv. 2022. DOI: https://doi.org/10.1128/spectrum.02867-22

[8] Blagodatski A, Trutneva K, Glazova O, Mityaeva O, Shevkova L, Kegeles E, et al.. Avian Influenza in Wild Birds and Poultry: Dissemination Pathways, Monitoring Methods, and Virus Ecology. Pathogens. 2021. DOI: https://doi.org/10.3390/pathogens10050630

[9] Moncla L, Zhong G, Nelson CW, Dinis J, Mutschler J, Hughes A, et al.. Selective bottlenecks shape evolutionary pathways taken during mammalian adaptation of a 1918-like avian influenza virus. Cell Host and Microbe. 2016. DOI: https://doi.org/10.1016/j.chom.2016.01.011

[10] Kuo S, Chen C, Chang S, Liu T, Chen Y, Huang S, et al.. Inhibition of Avian Influenza A Virus Replication in Human Cells by Host Restriction Factor TUFM Is Correlated with Autophagy. mBio. 2017. DOI: https://doi.org/10.1128/mBio.00481-17

[11] Smith GJD, Donis R. Nomenclature updates resulting from the evolution of avian influenza A(H5) virus clades 2.1.3.2a, 2.2.1, and 2.3.4 during 2013–2014. Influenza and Other Respiratory Viruses. 2015. DOI: https://doi.org/10.1111/irv.12324

[12] Taft A, Ozawa M, Fitch A, DePasse J, Halfmann P, Hill-Batorski L, et al.. Identification of mammalian-adapting mutations in the polymerase complex of an avian H5N1 influenza virus. Nature Communications. 2015. DOI: https://doi.org/10.1038/ncomms8491

[13] Worobey M, Han G, Rambaut A. A synchronized global sweep of the internal genes of modern avian influenza virus. Nature. 2014. DOI: https://doi.org/10.1038/nature13016

[14] Lycett S, Pohlmann A, Staubach C, Caliendo V, Woolhouse M, Beer M, et al.. Genesis and spread of multiple reassortants during the 2016/2017 H5 avian influenza epidemic in Eurasia. Proceedings of the National Academy of Sciences of the United States of America. 2020. DOI: https://doi.org/10.1073/pnas.2001813117

[15] Wille M, Barr I. Resurgence of avian influenza virus. Science. 2022. DOI: https://doi.org/10.1126/science.abo1232

[16] Xie R, Edwards KM, Wille M, Wei X, Wong S, Zanin M, et al.. The episodic resurgence of highly pathogenic avian influenza H5 virus. bioRxiv. 2022. DOI: https://doi.org/10.1038/s41586-023-06631-2

[17] AbuBakar U, Amrani L, Kamarulzaman FA, Karsani SA, Hassandarvish P, Khairat J. Avian Influenza Virus Tropism in Humans. Viruses. 2023. DOI: https://doi.org/10.3390/v15040833

[18] Peacock T, Swann O, Salvesen HA, Staller E, Leung P, Goldhill DH, et al.. Swine ANP32A Supports Avian Influenza Virus Polymerase. Journal of Virology. 2020. DOI: https://doi.org/10.1128/JVI.00132-20

[19] Song J, Xu J, Shi J, Li Y, Chen H. Synergistic Effect of S224P and N383D Substitutions in the PA of H5N1 Avian Influenza Virus Contributes to Mammalian Adaptation. Scientific Reports. 2015. DOI: https://doi.org/10.1038/srep10510

[20] Wang M, Zhang W, Qi J, Wang F, Zhou J, Bi Y, et al.. Structural basis for preferential avian receptor binding by the human-infecting H10N8 avian influenza virus. Nature Communications. 2015. DOI: https://doi.org/10.1038/ncomms6600

[21] Wei S, Yang J, Wu H, Chang M, Lin J, Lin C, et al.. Human infection with avian influenza A H6N1 virus: an epidemiological analysis. The Lancet Respiratory Medicine. 2013. DOI: https://doi.org/10.1016/S2213-2600(13)70221-2

[22] Spackman E, Jones DR, McCoig AM, Colonius T, Goraichuk I, Suarez DL. Characterization of highly pathogenic avian influenza virus in retail dairy products in the US. medRxiv. 2024. DOI: https://doi.org/10.1128/jvi.00881-24

[23] Kang M, Wang L, Sun B, Wan W, Ji X, Baele G, et al.. Zoonotic infections by avian influenza virus: changing global epidemiology, investigation, and control.. Lancet. Infectious Diseases (Print). 2024. DOI: https://doi.org/10.1016/s1473-3099(24)00234-2

[24] Plaza PI, Gamarra-Toledo V, Euguí JR, Lambertucci SA. Recent Changes in Patterns of Mammal Infection with Highly Pathogenic Avian Influenza A(H5N1) Virus Worldwide. Emerging Infectious Diseases. 2024. DOI: https://doi.org/10.3201/eid3003.231098

[25] Agüero M, Monne I, Sánchez A, Zecchin B, Fusaro A, Ruano M, et al.. Highly pathogenic avian influenza A(H5N1) virus infection in farmed minks, Spain, October 2022. Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletin. 2023. DOI: https://doi.org/10.2807/1560-7917.ES.2023.28.3.2300001

[26] Rijks J, Hesselink H, Lollinga P, Wesselman R, Prins P, Weesendorp E, et al.. Highly Pathogenic Avian Influenza A(H5N1) Virus in Wild Red Foxes, the Netherlands, 2021. Emerging Infectious Diseases. 2021. DOI: https://doi.org/10.3201/eid2711.211281

[27] Caliendo V, Lewis N, Pohlmann A, Waldenstrom J, Toor MLv, Lameris T, et al.. Transatlantic spread of highly pathogenic avian influenza H5N1 by wild birds from Europe to North America in 2021. Scientific Reports. 2022. DOI: https://doi.org/10.1038/s41598-022-13447-z

[28] Jonges M, Leuken JVv, Wouters I, Koch G, Meijer A, Koopmans M. Wind-Mediated Spread of Low-Pathogenic Avian Influenza Virus into the Environment during Outbreaks at Commercial Poultry Farms. PLoS ONE. 2015. DOI: https://doi.org/10.1371/journal.pone.0125401

[29] Yu H, Wu JT, Cowling B, Liao Q, Leung G. Effect of closure of live poultry markets on poultry-to-person transmission of avian influenza A H7N9 virus: an ecological study.. The Lancet. 2014. DOI: https://doi.org/10.1016/S0140-6736(13)61904-2

[30] Bi Y, Li J, Li S, Fu G, Jin T, Zhang C, et al.. Dominant subtype switch in avian influenza viruses during 2016–2019 in China. Nature Communications. 2020. DOI: https://doi.org/10.1038/s41467-020-19671-3

[31] Fu X, Wang Q, Ma B, Zhang B, Sun K, Yu X, et al.. Advances in Detection Techniques for the H5N1 Avian Influenza Virus. International Journal of Molecular Sciences. 2023. DOI: https://doi.org/10.3390/ijms242417157

[32] Heaton B, Kennedy EM, Dumm RE, Harding AT, Sacco M, Sachs D, et al.. A CRISPR activation screen identifies a pan-avian influenza virus inhibitory host factor. Cell Reports. 2017. DOI: https://doi.org/10.1016/j.celrep.2017.07.060

[33] Eisfeld A, Biswas A, Guan L, Gu C, Maemura T, Trifkovic S, et al.. Pathogenicity and transmissibility of bovine H5N1 influenza virus. Nature. 2024. DOI: https://doi.org/10.1038/s41586-024-07766-6

[34] Bodewes R, Bestebroer T, Vries Evd, Verhagen J, Herfst S, Koopmans M, et al.. Avian Influenza A(H10N7) Virus–Associated Mass Deaths among Harbor Seals. Emerging Infectious Diseases. 2015. DOI: https://doi.org/10.3201/eid2104.141675

[35] Verhagen J, Fouchier R, Lewis N. Highly Pathogenic Avian Influenza Viruses at the Wild–Domestic Bird Interface in Europe: Future Directions for Research and Surveillance. Viruses. 2021. DOI: https://doi.org/10.3390/v13020212

[36] Grabowska I, Malecka K, Jarocka U, Radecki J, Radecka H. Electrochemical biosensors for detection of avian influenza virus--current status and future trends.. Acta Biochimica Polonica. 2014. DOI: https://doi.org/10.18388/ABP.2014_1866

[37] Bai H, Wang R, Hargis B, Lu H, Li Y. A SPR Aptasensor for Detection of Avian Influenza Virus H5N1. Italian National Conference on Sensors. 2012. DOI: https://doi.org/10.3390/s120912506

[38] Sun H, Xiao Y, Liu J, Wang D, Li F, Wang C, et al.. Prevalent Eurasian avian-like H1N1 swine influenza virus with 2009 pandemic viral genes facilitating human infection. Proceedings of the National Academy of Sciences of the United States of America. 2020. DOI: https://doi.org/10.1073/pnas.1921186117

[39] Sun H, Li F, Liu Q, Du J, Liu L, Sun H, et al.. Mink is a highly susceptible host species to circulating human and avian influenza viruses. Emerging Microbes and Infections. 2021. DOI: https://doi.org/10.1080/22221751.2021.1899058

[40] Puryear WB, Sawatzki K, Hill N, Foss AD, Stone JJ, Doughty L, et al.. Highly Pathogenic Avian Influenza A(H5N1) Virus Outbreak in New England Seals, United States. Emerging Infectious Diseases. 2023. DOI: https://doi.org/10.3201/eid2904.221538

[41] Subbarao K, Klimov A, Katz J, Regnery HL, Lim W, Hall H, et al.. Characterization of an avian influenza A (H5N1) virus isolated from a child with a fatal respiratory illness.. Science. 1998. DOI: https://doi.org/10.1126/SCIENCE.279.5349.393

[42] Brand JVDvd, Stittelaar K, Amerongen Gv, Rimmelzwaan G, Simon J, Wit Ed, et al.. Severity of Pneumonia Due to New H1N1 Influenza Virus in Ferrets Is Intermediate between That Due to Seasonal H1N1 Virus and Highly Pathogenic Avian Influenza H5N1 Virus. Journal of Infectious Diseases. 2010. DOI: https://doi.org/10.1086/651132

[43] Fusaro A, Gonzales JL, Kuiken T, Mirinaviciute G, Niqueux E, Ståhl K, et al.. Avian influenza overview December 2023–March 2024. EFSA journal. European Food Safety Authority. 2024. DOI: https://doi.org/10.2903/j.efsa.2024.8754

[44] Abdelwhab EM, Mettenleiter T. Zoonotic Animal Influenza Virus and Potential Mixing Vessel Hosts. Viruses. 2023. DOI: https://doi.org/10.3390/v15040980

[45] Chen Y, Liang W, Yang S, Wu N, Gao H, Sheng J, et al.. Human infections with the emerging avian influenza A H7N9 virus from wet market poultry: clinical analysis and characterisation of viral genome. The Lancet. 2013. DOI: https://doi.org/10.1016/S0140-6736(13)60903-4

[46] Suttie A, Deng Y, Greenhill A, Dussart P, Horwood P, Karlsson E. Inventory of molecular markers affecting biological characteristics of avian influenza A viruses. Virus genes. 2019. DOI: https://doi.org/10.1007/s11262-019-01700-z