Highly Pathogenic Avian Influenza (H5N1) in Poultry and Wild Birds: Clinical Signs, Transmission Dynamics, and Surveillance Maps

Introduction

Highly pathogenic avian influenza (HPAI) H5N1 viruses of the A/goose/Guangdong/1/1996 lineage, particularly those belonging to clade 2.3.4.4b, have caused a protracted global panzootic in poultry and wild birds since 2020 [1, 2, 3]. This clade has demonstrated remarkable genetic plasticity, frequent reassortment, and an expanding host range that includes numerous avian taxa and an increasing number of mammalian species [4, 70, 71]. The virus is characterized by a multibasic cleavage site in the hemagglutinin (HA) protein, conferring systemic tropism and high mortality in gallinaceous poultry. The sustained circulation of H5N1 clade 2.3.4.4b in wild bird populations complicates eradication efforts and drives continuous spillover into domestic poultry holdings [55, 63]. Understanding the clinical presentation, transmission ecology, and spatial epidemiology of this pathogen is critical for veterinary diagnostics and control planning. This article provides a detailed reference on clinical signs, transmission dynamics, and surveillance mapping approaches for H5N1 HPAI in poultry and wild birds.

Clinical Signs in Poultry

The clinical presentation of H5N1 clade 2.3.4.4b infection in poultry varies by species, age, immune status, and viral genotype [5, 84, 91]. In chickens and turkeys, peracute disease is common, with sudden death as the first indicator of an outbreak [6, 7]. Surviving birds may exhibit severe depression, anorexia, ruffled feathers, and a marked drop in egg production [8]. Respiratory signs include dyspnea, rales, and conjunctivitis. Edema of the head, wattles, and comb is frequently observed, along with cyanosis of the comb and shanks. Neurologic signs such as ataxia, torticollis, and paralysis occur in a subset of cases due to viral neurotropism [84, 91]. Hemorrhagic lesions on the shanks and petechiae on visceral organs are common postmortem findings.

Table 1 summarizes the key clinical signs observed across different poultry species.

Table 1. Clinical Signs of H5N1 Clade 2.3.4.4b Infection in Poultry

Ducks frequently act as silent carriers, shedding virus without overt clinical disease, which complicates early detection in mixed poultry operations [8, 9, 77]. Pigeons (Columba livia) exhibit low susceptibility and poor transmission capacity for clade 2.3.4.4b viruses. The innate immune response, including interferon regulatory factor 1 (IRF1) signaling, influences clinical outcome in chickens. Host genetics and laying activity also modulate disease severity. Pathological findings include necrotizing pancreatitis, myocarditis, and encephalitis.

Clinical Signs in Wild Birds

Wild birds, particularly waterfowl (Anatidae) and gulls (Laridae), are primary reservoirs of H5N1 clade 2.3.4.4b [79, 85]. Many dabbling ducks (e.g., mallards) can carry the virus asymptomatically, but some species develop clinical disease [10, 8]. Mortality events in wild birds have been documented in terns (Sterna hirundo) in Namibia, with birds presenting with severe neurologic signs including circling, head tilt, and inability to fly [6]. Similar neurologic presentations have been reported in gulls and raptors. Infected mute swans and geese may show lethargy, swimming in circles, and respiratory distress. Seabirds and marine mammals in South America have experienced spillback events, suggesting persistent circulation in coastal ecosystems. Surveillance of wild bird mortality is a cornerstone of early warning systems [50, 55]. In France, systematic collection of dead wild birds and RT-qPCR testing provided key insights into the spatiotemporal spread of H5N1 from 2016 to 2022.

Transmission Dynamics

Transmission of H5N1 HPAI occurs through multiple pathways: direct bird-to-bird contact, inhalation of aerosolized respiratory secretions, ingestion of contaminated feed or water, and fomite transport [11, 12, 82]. The virus is shed in high titers in respiratory and fecal material, particularly from infected ducks. Environmental persistence of H5N1 in water and organic matter facilitates indirect transmission. Acidic conditions and pH affect virus stability in milk and other matrices, though relevance for poultry is indirect. Ultraviolet germicidal irradiation can reduce airborne viral loads in poultry houses.

Wild birds introduce H5N1 into poultry flocks through direct contact or contamination of shared water sources and feed [13, 55, 63]. Once introduced, within-flock transmission is rapid, driven by high stocking densities and limited biosecurity [11]. Farm size, outdoor access, and farm density are significant risk factors for epidemic spread [13, 11]. In the Netherlands, modeling studies demonstrated that outdoor farming increases the probability of H5N1 introduction [11]. Partial culling strategies, as applied in Lower Saxony, Germany, may reduce within-flock transmission but are less effective than full depopulation [14]. The role of fomites and contaminated equipment is well documented; live bird markets are particularly high-risk interfaces [15, 76]. Mechanistic models of HPAI transmission often fail to capture cross-species dynamics, limiting predictive accuracy [16].

Figure 1 illustrates the major transmission pathways between wild birds, poultry, and the environment.

graph TD
 A[Wild Bird Reservoir] -->|Fecal shedding, respiratory droplets| B[Surface Water / Feed]
 A -->|Direct contact| C[Commercial Poultry Farm]
 A -->|Aerosol| D[Backyard Flocks]
 B --> C
 B --> D
 C -->|Intensive within-flock spread| E[High Mortality]
 C -->|Fomites, personnel| F[Other Farms]
 D --> F
 D -->|Reversion to wild birds| A
 E -->|Culling| G[Outbreak Control]
 F --> H[Regional Epidemic]
 H -->|Environmental contamination| A

Figure 1. Schematic of H5N1 transmission pathways in the poultry-wild bird interface.

Mammalian spillover events, including infections in dairy cattle, domestic cats, and wild carnivores, have raised concerns about sustained transmission outside avian hosts [17, 18, 65, 85, 90, 92]. In Germany, H5N1 was detected in domestic cats on infected poultry premises [7]. Serologic evidence of infection was found in a veterinary professional exposed to an infected cat [19]. Alpacas in the United States have also tested positive for H5N1 clade 2.3.4.4b. These events underscore the plasticity of H5N1 and the need for One Health surveillance [17, 92]. However, experimental studies in ferrets indicate limited respiratory droplet transmission of recent human isolates, suggesting that airborne spread among mammals may not be efficient.

Surveillance Maps and Risk Modeling

Spatial surveillance mapping integrates epidemiological data with environmental and ecological variables to identify high-risk areas for H5N1 introduction and spread [20, 21, 60, 70]. Geographic information system (GIS) based risk mapping has been applied in Morocco using multi-criteria decision analysis that considered waterbody proximity, poultry density, migratory bird routes, and biosecurity indicators. Similarly, district-level risk assessments have been performed for live bird markets in Bogor, Indonesia [15]. Continuous phylogeography using Bayesian inference has revealed shifting environmental drivers of H5 spread in Italy between 2016 and 2023, showing that temperature and precipitation patterns influence viral dispersal [21].

Machine learning algorithms have been employed to optimize wild bird fecal surveillance sites by predicting HPAI occurrence probability based on habitat features and bird movements [20]. The European Food Safety Authority (EFSA) has produced periodic overview maps of H5N1 detections in Europe, incorporating both wild bird mortality and poultry outbreaks [3]. Global risk mapping efforts have highlighted that regions with high waterfowl abundance and intensive poultry production are at greatest risk [70, 79]. Integration of ENSO based forecasts with regional modeling has shown promise for predicting H5N1 emergence in Asia. In the United States, temporal analysis of H5N1 outbreaks in commercial and non-commercial flocks between 2022 and 2025 identified seasonal peaks and geographic clustering [22].

Risk modeling also informs sample size calculations for emergency surveillance in secondary infected farms [23]. Environmental metagenomics from live poultry markets enhances detection of circulating viruses and provides a supplementary tool for spatial surveillance. The use of phylogeographic models in France and Spain has confirmed that wild birds are the primary drivers of viral incursion [50, 55]. In Japan, inter-species transmission dynamics were elucidated through phylogenetic analysis of H5N1 sequences from poultry, wild birds, and the environment.

Diagnostic Considerations

Accurate diagnosis of H5N1 HPAI relies on molecular detection methods, principally real-time reverse transcription polymerase chain reaction (RT-qPCR) targeting the matrix gene or H5-specific assays [89, 97, 99]. Subtyping RT-qPCR assays have been validated for H5 detection in respiratory and fecal samples. Virus isolation in embryonated chicken eggs or cell culture remains the gold standard for strain characterization, though isolation success rates can be low from wild bird samples. Biosensor technologies, including electrochemical and optical platforms, are emerging as rapid, field-deployable alternatives. Nanobody-based detection systems have been developed for H9N2 and may be adapted for H5N1 [24]. External quality assessment programs in the Netherlands confirm that diagnostic laboratories can reliably detect avian influenza A viruses using established molecular protocols. Sequencing of the HA and NA genes is essential for clade assignment and genotyping, as demonstrated in Pennsylvania outbreaks where multiple genotypes of clade 2.3.4.4b co-circulated [5, 52]. Serologic surveillance using hemagglutination inhibition assays provides evidence of past infection, but may not detect active cases [8, 65].

Control and Prevention

Control of H5N1 HPAI in poultry relies on rapid outbreak detection, strict biosecurity, stamping out policies, and movement restrictions [25, 82]. In the United States, commercial flocks have remained unvaccinated against H5, a policy driven by trade and surveillance considerations [25]. Partial culling has been explored as a welfare-sparing alternative in certain European regions [14]. Vaccination strategies include inactivated whole-virus vaccines and recombinant viral vectors expressing H5 hemagglutinin [26, 27, 66, 73, 101]. Trimeric HA protein vaccines have provided complete protection against lethal H5N1 clade 2.3.4.4b challenge in chickens [26]. Recombinant Newcastle disease virus based vaccines co-expressing H5 have shown efficacy against both HPAI and velogenic Newcastle disease. Mucosal vaccination approaches are under investigation for enhanced respiratory tract immunity. In turkeys, inactivated H5 vaccines have demonstrated protective efficacy. Antiviral compounds such as baloxavir marboxil have reduced viremia and disease severity in ferret models. However, prophylactic antiviral use in poultry is not currently recommended. Biosecurity measures, including disinfection protocols, rodent control, and limiting farm access, remain the primary defense. Smallholder poultry producers often face barriers to implementing biosecurity due to cost and trust issues. Public perception and misinformation on social media can undermine control efforts [29].

Conclusion

Highly pathogenic avian influenza H5N1 clade 2.3.4.4b continues to pose a significant threat to poultry production and wild bird conservation globally. The virus causes a spectrum of clinical signs ranging from sudden death in chickens to asymptomatic carriage in ducks. Transmission dynamics are driven by wild bird movements, environmental contamination, and poultry farming practices. Surveillance maps and risk models, incorporating GIS, phylogeography, and machine learning, are essential tools for targeting early detection and response. Continued investment in diagnostics, vaccine development, and biosecurity research is necessary to mitigate the panzootic impact.

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.