Avian Influenza A Virus in Wild Birds and Poultry: Etiology, Epidemiology, Clinical Signs, Pathology, Diagnostics, Treatment, and Control
By Dr. Zubair Khalid, DVM, MS, PhD ·
Key Takeaways
- Avian Influenza A Virus (AIV) is an enveloped RNA virus of the Orthomyxoviridae family, classified by hemagglutinin (HA) and neuraminidase (NA) glycoproteins; highly pathogenic strains (HPAI) possess a multibasic cleavage site (MBCS) in HA, enabling systemic replication in poultry, while low pathogenic strains (LPAI) have a monobasic site causing milder disease.
- Wild aquatic birds, particularly Anseriformes and Charadriiformes, are the primary natural reservoirs for LPAI viruses, with global dissemination facilitated by migratory flyways; HPAI outbreaks, notably H5Nx lineages, have caused significant mortality in both wild birds and domestic poultry.
- Clinical signs of AIV infection in poultry range from severe systemic disease with high morbidity/mortality (depression, respiratory distress, cyanosis, neurological signs) in HPAI to mild respiratory signs in LPAI; wild birds, especially natural reservoirs, may shed virus asymptomatically, though HPAI can cause severe disease and mortality in species like raptors and seabirds.
- Diagnostic confirmation relies on virological methods like virus isolation, molecular techniques such as real-time RT-PCR for rapid detection and subtyping, and serological assays (HI, ELISA) for surveillance and differentiating infected from vaccinated animals (DIVA).
- Antiviral therapy for AIV in poultry is generally not recommended due to resistance concerns, though compounds like baloxavir marboxil have shown efficacy in experimental models and may be considered for valuable genetic stocks; control strategies emphasize strict biosecurity, robust surveillance, stamping out policies, and targeted vaccination programs.
- Effective control of AIV necessitates a One Health approach, integrating biosecurity measures to prevent contact between domestic poultry and wild birds, comprehensive surveillance programs for early detection, culling of infected flocks, and the strategic use of vaccines, including DIVA-compatible options, to mitigate spread and impact.
Etiology
Avian influenza A virus (AIV) is an enveloped, negative-sense, single-stranded RNA virus belonging to the family Orthomyxoviridae, genus Influenzavirus A [<a href="#ref-1">1</a>]. The viral genome comprises eight segmented RNA segments encoding at least 10 proteins, including the surface glycoproteins hemagglutinin (HA) and neuraminidase (NA) [<a href="#ref-1">1</a>]. Subtype classification is based on the antigenic properties of HA (H1 through H16) and NA (N1 through N9) [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Highly pathogenic avian influenza (HPAI) viruses are defined by the presence of a multibasic cleavage site (MBCS) in the HA protein, which allows systemic replication in poultry [<a href="#ref-1">1</a>, <a href="#ref-3">3</a>]. Low pathogenic avian influenza (LPAI) viruses possess a monobasic cleavage site and typically cause mild or subclinical infections in gallinaceous birds [<a href="#ref-3">3</a>]. The molecular basis for the cleavage site motif involves the insertion of basic amino acids (arginine and lysine) at the HA0 cleavage site, rendering the protein susceptible to ubiquitous furin-like proteases [<a href="#ref-3">3</a>]. This cleavage is essential for viral entry and fusion with host endosomal membranes [<a href="#ref-3">3</a>]. The NA protein facilitates viral release from infected cells by cleaving sialic acid residues and can also recruit plasminogen to enhance HA cleavage in LPAI strains [<a href="#ref-3">3</a>]. The internal proteins, including the polymerase basic 2 (PB2), polymerase basic 1 (PB1), polymerase acidic (PA), nucleoprotein (NP), matrix (M1 and M2), and nonstructural (NS1 and NS2) proteins, contribute to host range restriction and virulence [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>]. For example, the PB2 E627K mutation is a well-characterized mammalian adaptation marker that enhances viral replication at lower temperatures [<a href="#ref-5">5</a>]. The NS1 protein acts as an interferon antagonist, modulating the host innate immune response [<a href="#ref-4">4</a>]. Divergent immunometabolic landscapes between species, such as chickens and swan geese, have identified host restriction factors like SERPINF2 that limit AIV replication [<a href="#ref-4">4</a>].
Epidemiology
Avian influenza A virus exhibits a complex epidemiological pattern involving wild aquatic birds as the primary natural reservoir [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>, <a href="#ref-7">7</a>]. Wild birds, particularly Anseriformes (ducks, geese, swans) and Charadriiformes (gulls, terns), maintain a diverse pool of LPAI viruses [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>, <a href="#ref-8">8</a>]. These viruses are transmitted via the fecal-oral route through contaminated water and environmental surfaces [<a href="#ref-6">6</a>, <a href="#ref-8">8</a>]. Migratory flyways facilitate the global dissemination of AIV, linking continents and introducing novel subtypes into naive populations [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>, <a href="#ref-9">9</a>]. The emergence of HPAI viruses, particularly those of the H5Nx lineage (clade 2.3.4.4b), has resulted in panzootic events affecting wild birds and poultry worldwide [<a href="#ref-1">1</a>, <a href="#ref-10">10</a>, <a href="#ref-11">11</a>, <a href="#ref-12">12</a>, <a href="#ref-13">13</a>]. The 2021-2025 H5Nx clade 2.3.4.4b outbreak in North America demonstrated extensive spillover into wild bird populations, including raptors, seabirds, and waterfowl, with subsequent incursions into commercial and backyard poultry [<a href="#ref-10">10</a>, <a href="#ref-12">12</a>, <a href="#ref-13">13</a>]. In Europe, mortality surveillance in wild birds from 2016 to 2022 provided critical data on the spatial and temporal distribution of HPAI, revealing seasonal peaks associated with waterfowl aggregation and migration [<a href="#ref-8">8</a>]. Similarly, surveillance in South Korea from 2019 to 2025 identified a high prevalence of H5N1 and H5N6 subtypes in migratory wild birds, underscoring the role of East Asian flyways in viral maintenance [<a href="#ref-7">7</a>]. In Egypt, molecular surveillance combined with predictive risk modeling identified environmental and anthropogenic factors associated with AIV circulation in wild birds, including proximity to poultry farms and water bodies [<a href="#ref-6">6</a>]. The global spread of H7 subtype AIV has been driven by both wild bird migration and poultry trade networks, as demonstrated by phylogeographic analyses [<a href="#ref-9">9</a>]. Genotype diversity within the H5N1 clade 2.3.4.4b has been documented in Pennsylvania poultry, with multiple genotypes co-circulating during a single outbreak season [<a href="#ref-12">12</a>]. In Bulgaria, molecular characterization of H5N1 viruses during 2024-2025 revealed evidence of hidden circulation and the presence of zoonotic risk markers [<a href="#ref-14">14</a>]. The panzootic H5 viruses have also acquired resistance to human head interface antibodies, indicating ongoing antigenic evolution [<a href="#ref-15">15</a>]. Spatiotemporal analyses in Ireland from 1983 to 2024 demonstrated a shift in subtype dominance and host species involvement over time [<a href="#ref-16">16</a>]. In Namibia, an H5N1 outbreak among common terns (Sterna hirundo) during 2025-2026 highlighted the vulnerability of seabird colonies to HPAI [<a href="#ref-11">11</a>]. The global evolutionary landscape of H11 AIV has been characterized, with initial isolates identified in Xinjiang, China, expanding the known host range of this subtype [<a href="#ref-2">2</a>]. H3 subtype AIVs have been isolated from wild birds and poultry in Eastern China, with evidence of reassortment and mammalian adaptation markers [<a href="#ref-17">17</a>]. H3NX viruses in Guangdong province, China, from 2023 to 2025, were assessed for cross-species transmission potential [<a href="#ref-18">18</a>]. H4N6 AIV was first isolated in Iran, providing molecular insights into the diversity of this subtype in the Middle East [<a href="#ref-19">19</a>]. H1N1 AIV from wild birds in Shanghai, China, was characterized for its molecular features and genetic evolution [<a href="#ref-20">20</a>]. The role of wild boars (Sus scrofa) as sentinels for AIV exposure has been investigated in Spain, with serologic and virologic evidence of H5N1 spillover risk [<a href="#ref-21">21</a>]. Mechanistic modeling of HPAI transmission has identified critical gaps in cross-species transmission models, particularly regarding the interface between wild birds, poultry, and mammals [<a href="#ref-22">22</a>]. Machine learning-based geospatial risk modeling has been applied to predict global AIV outbreak hotspots, integrating environmental, ecological, and anthropogenic variables [<a href="#ref-23">23</a>]. High-resolution digital twin models of interacting livestock, wild birds, and human ecosystems have been developed to simulate multihost epidemic spread [<a href="#ref-24">24</a>]. The 2024-2025 HPAI outbreak in the United States was examined using multimodal data approaches, including genomic, epidemiological, and environmental data [<a href="#ref-10">10</a>].
Clinical Signs
Clinical manifestations of avian influenza A virus infection vary markedly depending on viral pathotype, host species, age, immune status, and concurrent infections [<a href="#ref-1">1</a>]. In poultry, HPAI viruses cause severe systemic disease characterized by high morbidity and mortality [<a href="#ref-1">1</a>, <a href="#ref-13">13</a>]. Clinical signs include depression, inappetence, decreased egg production, respiratory distress (coughing, sneezing, dyspnea), cyanosis of combs and wattles, edema of the head and neck, hemorrhagic lesions on shanks, and neurological signs such as ataxia, torticollis, and paralysis [<a href="#ref-1">1</a>]. Sudden death without premonitory signs is common in highly susceptible species, particularly chickens and turkeys [<a href="#ref-1">1</a>]. In contrast, LPAI viruses typically cause mild respiratory disease, decreased egg production, and increased susceptibility to secondary bacterial infections [<a href="#ref-1">1</a>]. In wild birds, clinical signs are often absent in natural reservoir hosts such as dabbling ducks, which can shed virus asymptomatically [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>]. However, HPAI infection in certain wild bird species, including swans, geese, gulls, and raptors, can result in severe disease and mortality [<a href="#ref-8">8</a>, <a href="#ref-11">11</a>]. Neurological signs, including head tremors, circling, and inability to fly, have been observed in HPAI-infected wild birds [<a href="#ref-11">11</a>]. In ducks, experimental infection with HPAI H5N1 has been characterized using systems-level analyses integrating transcriptomic, proteomic, and phosphoproteomic data, revealing dysregulation of immune and metabolic pathways [<a href="#ref-25">25</a>]. The immunometabolic response to AIV infection differs between chickens and swan geese, with the latter exhibiting a more robust interferon response and upregulation of restriction factors such as SERPINF2 [<a href="#ref-4">4</a>].
Pathology
Gross pathological findings in HPAI-infected poultry include severe congestion and edema of the comb and wattles, petechial hemorrhages on serosal surfaces (particularly the proventriculus, gizzard, and heart), multifocal necrotic foci in the liver, spleen, and pancreas, and consolidation of the lungs [<a href="#ref-1">1</a>]. Histopathological examination reveals necrotizing pancreatitis, myocarditis, encephalitis, and lymphocytic depletion in lymphoid organs [<a href="#ref-1">1</a>]. The presence of viral antigen in endothelial cells and parenchymal tissues confirms systemic dissemination [<a href="#ref-1">1</a>]. In ducks, HPAI infection induces a distinct immunometabolic response characterized by activation of interferon signaling pathways and metabolic reprogramming [<a href="#ref-25">25</a>]. The HA cleavage site sequence and NA-mediated plasminogen recruitment synergize to enhance virulence in LPAI strains, providing a mechanism for pathotype switching [<a href="#ref-3">3</a>]. In wild birds, pathological findings vary by species. In common terns, HPAI H5N1 infection resulted in severe necrotizing hepatitis, splenitis, and encephalitis [<a href="#ref-11">11</a>]. In swans and geese, gross lesions included hemorrhagic enteritis and pulmonary edema [<a href="#ref-8">8</a>].
Diagnostics
Diagnostic approaches for avian influenza A virus encompass virological, molecular, serological, and pathological methods [<a href="#ref-1">1</a>, <a href="#ref-26">26</a>]. Sample types include oropharyngeal and cloacal swabs, tracheal and lung tissue, feces, and environmental samples (water, feces) [<a href="#ref-6">6</a>, <a href="#ref-8">8</a>]. Virus isolation in embryonated chicken eggs or cell culture (e.g., MDCK cells) remains a gold standard for virus characterization [<a href="#ref-1">1</a>]. Molecular diagnostics, particularly real-time reverse transcription polymerase chain reaction (RT-PCR), are the primary tools for rapid detection and subtyping [<a href="#ref-12">12</a>, <a href="#ref-14">14</a>, <a href="#ref-26">26</a>]. RT-PCR assays targeting the matrix (M) gene provide pan-influenza A detection, while HA and NA subtype-specific assays enable differentiation [<a href="#ref-12">12</a>, <a href="#ref-14">14</a>]. High-throughput sequencing technologies, including next-generation sequencing (NGS), are increasingly used for whole-genome sequencing and phylogenetic analysis [<a href="#ref-12">12</a>, <a href="#ref-14">14</a>]. These methods allow for the identification of genetic markers associated with virulence, mammalian adaptation, and antiviral resistance [<a href="#ref-5">5</a>, <a href="#ref-14">14</a>]. Serological diagnostics, including hemagglutination inhibition (HI) and neuraminidase inhibition (NI) assays, as well as commercial enzyme-linked immunosorbent assay (ELISA) kits, are used for surveillance and to differentiate infected from vaccinated animals (DIVA) [<a href="#ref-1">1</a>, <a href="#ref-27">27</a>]. The development of DIVA vaccines, such as those using chimeric NA epitopes, enables serological differentiation between vaccinated and naturally infected birds [<a href="#ref-27">27</a>]. External quality assessment programs have demonstrated that medical laboratories can efficiently detect animal influenza A viruses using molecular methods [<a href="#ref-26">26</a>]. In the context of mixed-species holdings, molecular detection and subtype differentiation are critical for outbreak management. The use of CRISPR-based diagnostics for avian influenza represents an emerging technology with potential for field-deployable testing.
Treatment
Antiviral therapy for avian influenza A virus in poultry is not routinely recommended due to concerns about resistance development and the potential for masking clinical signs [<a href="#ref-1">1</a>]. However, experimental studies have evaluated the efficacy of antiviral compounds in avian models. Baloxavir marboxil, a cap-dependent endonuclease inhibitor, demonstrated therapeutic efficacy against HPAI virus infection in a duck model, reducing viral shedding and mortality [<a href="#ref-28">28</a>]. In ferrets infected with avian- or bovine-origin H5N1 virus, baloxavir alleviated severe disease and viremia [<a href="#ref-29">29</a>]. These findings suggest that baloxavir may have a role in therapeutic intervention in outbreak settings, particularly in valuable genetic stocks or zoological collections [<a href="#ref-28">28</a>, <a href="#ref-29">29</a>]. Supportive care, including fluid therapy, nutritional support, and management of secondary bacterial infections, may be considered in individual cases, but is impractical in commercial poultry operations [<a href="#ref-1">1</a>]. The use of antiviral drugs in poultry is subject to regulatory restrictions and should be guided by veterinary oversight and antimicrobial stewardship principles.
Control
Control of avian influenza A virus in poultry and wild birds relies on a combination of biosecurity, surveillance, stamping out, and vaccination strategies [<a href="#ref-1">1</a>, <a href="#ref-13">13</a>]. Biosecurity measures include preventing contact between domestic poultry and wild birds, implementing strict hygiene protocols (footbaths, dedicated clothing, equipment disinfection), controlling human and vehicle movements, and ensuring proper disposal of carcasses and manure [<a href="#ref-1">1</a>]. Surveillance programs, both passive (reporting of sick or dead birds) and active (routine sampling of healthy birds), are essential for early detection and rapid response [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>, <a href="#ref-8">8</a>]. In wild birds, mortality surveillance and targeted sampling of migratory populations provide early warning of HPAI incursions [<a href="#ref-8">8</a>]. Stamping out policies, involving the culling of infected and exposed flocks, are implemented in many countries to eradicate HPAI outbreaks [<a href="#ref-1">1</a>]. Compensation schemes are often used to encourage timely reporting and compliance [<a href="#ref-1">1</a>]. Vaccination is a complementary control tool, particularly in endemic settings or when stamping out is not feasible [<a href="#ref-1">1</a>, <a href="#ref-30">30</a>]. Inactivated whole-virus vaccines, recombinant vector vaccines (e.g., fowlpox virus-vectored H5), and DIVA vaccines are available [<a href="#ref-1">1</a>, <a href="#ref-27">27</a>]. Vaccination of wild king penguins against H5 HPAI resulted in a persistent immune response, suggesting that vaccination could be used to protect endangered wild bird populations [<a href="#ref-30">30</a>]. The 2024 avian influenza vaccination campaign in Finland provided lessons on vaccine acceptance and logistical challenges [<a href="#ref-31">31</a>]. Vaccination strategies must be carefully designed to avoid masking infection and to enable DIVA surveillance [<a href="#ref-27">27</a>]. In the Republic of Korea, preventive responses to human avian influenza infection included enhanced surveillance, culling, and public health measures [<a href="#ref-32">32</a>]. The World Organisation for Animal Health (WOAH) provides international standards for AIV surveillance, reporting, and trade restrictions. Control efforts must also consider the role of fomites, vectors, and environmental persistence in viral transmission. The integration of genomic epidemiology, risk modeling, and machine learning can inform targeted surveillance and control interventions [<a href="#ref-10">10</a>, <a href="#ref-23">23</a>]. Cross-species transmission models should incorporate wild bird, poultry, and mammalian hosts to predict spillover events [<a href="#ref-22">22</a>]. The development of high-resolution digital twins of interacting ecosystems can simulate outbreak scenarios and evaluate control strategies [<a href="#ref-24">24</a>]. Ultimately, a One Health approach that integrates veterinary, environmental, and public health sectors is essential for effective AIV control [<a href="#ref-1">1</a>, <a href="#ref-13">13</a>].
Diagnostic and Control Workflow
flowchart TD
A["Clinical suspicion or surveillance sampling"] --> B["Sample collection: oropharyngeal/cloacal swabs, tissues, feces"]
B --> C["Laboratory testing"]
C --> D{"Real-time RT-PCR for M gene"}
D -->|"Positive"| E["Subtype-specific RT-PCR: HA and NA"]
D -->|"Negative"| F["No AIV detected"]
E --> G{"Pathotype determination"}
G -->|"MBCS present"| H["HPAI confirmed"]
G -->|"Monobasic CS"| I["LPAI confirmed"]
H --> J["Confirmatory virus isolation and sequencing"]
I --> J
J --> K["Phylogenetic and genetic marker analysis"]
K --> L["Reporting to veterinary authorities"]
L --> M{"Control strategy"}
M -->|"Stamping out"| N["Culling, disposal, disinfection"]
M -->|"Vaccination"| O["DIVA vaccine deployment and serological monitoring"]
M -->|"Enhanced biosecurity"| P["Quarantine, movement restrictions, hygiene"]
N --> Q["Surveillance for recrudescence"]
O --> Q
P --> Q
Q -->|"Negative surveillance"| R["Restore normal status"]
Q -->|"Positive surveillance"| S["Re-evaluate control measures"]
S --> M