-- title: "Avian Influenza A(H5N1) in Poultry and Wild Birds: Current Epidemiology, Molecular Diagnostics, and Biosecurity" category: "avian-viruses" metaDescription: "A comprehensive review of H5N1 clade 2.3.4.4b epidemiology in poultry and wild birds, molecular diagnostic strategies including RT-qPCR and HA cleavage site sequencing, and on-farm biosecurity measures." primaryKeyword: "Avian influenza H5N1 poultry wild birds epidemiology molecular diagnostics biosecurity" secondaryKeywords: ["H5N1 clade 2.3.4.4b", "RT-qPCR M gene H5 subtype", "hemagglutinin cleavage site sequencing", "pathotyping", "poultry biosecurity", "highly pathogenic avian influenza", "wild bird surveillance"]

Avian Influenza A(H5N1) in Poultry and Wild Birds: Current Epidemiology, Molecular Diagnostics, and Biosecurity

Introduction

Highly pathogenic avian influenza (HPAI) A(H5N1) virus of the goose/Guangdong lineage continues to pose a substantial threat to global poultry production and wild bird populations. The emergence and sustained circulation of clade 2.3.4.4b viruses since the early 2020s have fundamentally altered the epizootic landscape, with unprecedented geographic spread and host species involvement [10, 13]. This article provides an exhaustive review of the current epidemiology of H5N1 clade 2.3.4.4b in poultry and wild birds, the molecular diagnostic tools that enable rapid detection and pathotyping, and the biosecurity measures essential for outbreak prevention and control. Emphasis is placed on the interplay between wild bird reservoirs and domestic poultry, the technical details of RT-qPCR and hemagglutinin (HA) cleavage site sequencing, and practical on-farm biosecurity protocols.

Current Epidemiology of H5N1 Clade 2.3.4.4b

Global Distribution and Host Range

Since the panzootic expansion of clade 2.3.4.4b, HPAI H5N1 has been detected across six continents, affecting a wide array of avian species and spilling over into mammals [13]. In North America, the incursion began in late 2021, with outbreaks in both commercial and backyard poultry flocks as well as in wild aquatic birds, raptors, and scavengers [8, 13]. Genotypic diversity within the clade has been documented; for example, Tewari et al. [8] identified multiple genotypes circulating in Pennsylvania poultry between April 2022 and March 2023, indicating ongoing reassortment with low-pathogenicity avian influenza (LPAI) viruses. Similarly, outbreaks among common terns (Sterna hirundo) in Namibia during 2025–2026 demonstrated the ability of H5N1 to cause mass mortality in colonial seabirds, highlighting the threat to biodiversity [4].

The epidemiology is further complicated by the persistent circulation of the virus in wild bird populations, which act as long-distance vectors. Environmental and ecological factors, including proximity to wetlands and migratory flyways, have been strongly associated with poultry farm spillover events, as shown by Oremush et al. [11] in British Columbia, Canada. These findings underscore the need for integrated surveillance that links wild bird ecology with poultry farm risk assessment.

Virological and Host Factors

The H5N1 virus possesses a multibasic cleavage site (MBCS) in the HA protein, a molecular hallmark of high pathogenicity in gallinaceous poultry. The MBCS allows systemic replication by enabling cleavage by ubiquitous furin-like proteases. Li et al. [5] demonstrated divergent immunometabolic landscapes between chickens and swan geese, identifying SERPINF2 as a novel restriction factor for influenza A virus. This finding provides a mechanistic basis for species-specific differences in susceptibility and disease outcome.

The continuing evolution of H5N1 clade 2.3.4.4b necessitates ongoing molecular surveillance to monitor antigenic drift and the emergence of strains with altered host tropism. Computational models integrating livestock, wild bird, and human ecosystems have been developed to forecast multihost epidemic spread, emphasizing the value of high-resolution digital analogs for preparedness planning [7].

Molecular Diagnostics for H5N1

RT-qPCR: Detection and Subtype Identification

Real-time reverse transcription polymerase chain reaction (RT-qPCR) remains the gold standard for avian influenza virus (AIV) detection in clinical specimens. The assay typically targets the matrix (M) gene, which is highly conserved across influenza A subtypes, allowing pan-influenza A screening. For H5 subtype identification, a separate RT-qPCR targeting the HA gene of the H5 lineage is performed. Multiplex formats that simultaneously detect the M gene and H5 HA are widely used in veterinary diagnostic laboratories [12].

The diagnostic workflow begins with sample collection from tracheal and cloacal swabs, or from tissues (lung, brain, pancreas) in deceased birds. RNA extraction is followed by one-step RT-qPCR using hydrolysis probe chemistry. The use of internal controls (e.g., beta-actin) is mandatory to monitor RNA integrity and inhibition. A positive M gene result triggers reflex testing for H5 (and N1) targets. Laboratories must adhere to strict quality assurance protocols, including participation in proficiency testing programs.

Hemagglutinin Cleavage Site Sequencing for Pathotyping

Confirmation of HPAI requires sequencing of the HA cleavage site to determine the presence of a multibasic motif. The cleavage site is located at the junction between HA1 and HA2 subunits; for HPAI, the motif is typically Pro-X-X-Arg-X-Arg/Lys-Arg. Sequencing is performed on a conventional thermocycler using Sanger sequencing or, in high-throughput settings, using automated capillary electrophoresis platforms. The amplicon is generated with primers flanking the cleavage site region. The resulting sequence is compared to a reference database to assign pathotype.

The decision tree for molecular diagnosis is depicted in the diagram below.

flowchart TD
    A[Sample collection: tracheal/cloacal swabs or tissues], > B[RNA extraction + internal control]
    B, > C[RT-qPCR for M gene]
    C, > D{ M gene positive? }
    D, >|Yes| E[RT-qPCR for H5 HA]
    D, >|No| F[Report negative for influenza A]
    E, > G{ H5 positive? }
    G, >|Yes| H[HA cleavage site sequencing]
    G, >|No| I[Consider other subtypes; report H5 negative]
    H, > J{ Multibasic cleavage site? }
    J, >|Yes| K[Confirm HPAI H5N1]
    J, >|No| L[LPAI or exclusion]
    K, > M[Genotypic characterization and phylogenetic analysis]

Advanced Molecular Techniques

For genotyping beyond the HA cleavage site, whole-genome sequencing followed by phylogenetic analysis is increasingly employed. This approach allows detection of reassortment events, such as the acquisition of internal gene segments from North American LPAI viruses [8]. Multiplex RT-PCR assays that simultaneously differentiate lineages (e.g., H9 subtypes) have also been developed, demonstrating the adaptability of molecular platforms for emerging diagnostic needs [12].

CRISPR-based diagnostics represent a newer modality for rapid, field-deployable detection of AIV. These assays leverage Cas12 or Cas13 nucleases to cleave a reporter molecule after target recognition, generating a fluorescent or colorimetric signal. While still under validation for H5N1, such platforms may complement RT-qPCR in resource-limited settings.

Biosecurity Measures for Poultry Operations

Core Principles

Biosecurity aims to prevent the introduction of HPAI virus into poultry premises and to limit its spread if introduced. The measures are hierarchical, encompassing physical barriers, operational protocols, and personnel training. Key components include:

• Access control: Restricted entry for vehicles, equipment, and personnel. Footbaths with disinfectant (e.g., quaternary ammonium compounds) at all entry points.
• Isolation of new birds: Quarantine for a minimum of 30 days before introduction to main flock.
• Separation of species: Avoid mixing poultry with other domestic birds or pigs, which can serve as bridging hosts.
• Feed and water security: Protected storage to prevent contamination by wild bird feces.
• Routine cleaning and disinfection: Appropriate contact time and concentration for disinfectants (e.g., peroxygen compounds).
• Rodent and insect control: Minimize vectors that can mechanically transmit virus.

On-Farm Surveillance

Early detection relies on regular clinical inspection and sampling of sick or dead birds. Any increase in mortality above a threshold should trigger immediate diagnostic testing. The use of sentinel birds (e.g., specific-pathogen-free chickens placed in known risk areas) can provide early warning. Biosecurity audits should be conducted at least quarterly, and records of all movements (people, vehicles, materials) must be maintained.

Outbreak Response

When HPAI is confirmed, control measures include stamping out (culling of infected and exposed flocks), movement restrictions, enhanced surveillance in a defined zone (typically 3–10 km radius), and cleaning and disinfection of the premises. Vaccination is a supplemental tool but must be combined with robust surveillance to avoid silent circulation. The Republic of Korea has implemented preventive response protocols that include preemptive culling and enhanced biosecurity at the national level, as described by Lee et al. [1, 2] and Kim et al. [3].

A summary of biosecurity interventions is provided in Table 1.

Table 1. Key on-farm biosecurity measures against HPAI H5N1

Surveillance and One Health Considerations

Surveillance must be integrated across wild birds, poultry, and (where relevant) mammals to detect spillover events early. Passive surveillance (testing of found-dead birds) and active surveillance (sampling of live-captured wild birds) are both essential. The genotypic diversity observed in Pennsylvania [8] and the panzootic in North America [13] underscore the need for continuous genomic monitoring. Digital surveillance tools, including the high-resolution ecosystem analogs described by Adiga et al. [7], can aid in predicting spatial risk.

The detection of H5N1 in mammals, including domestic cats and dairy cattle, has raised concerns about host adaptation. While this review focuses on avian hosts, the comparative host-range parallels are noteworthy: the same clade 2.3.4.4b virus has been associated with fatal infections in a backyard flock owner [14] and serologic evidence of infection in a veterinary professional exposed to an infected cat [15]. Such events emphasize the importance of biosecurity not only for animal health but also for reducing the risk of mammalian spillover.

Conclusion

HPAI H5N1 clade 2.3.4.4b continues to evolve and spread, causing significant mortality in poultry and wild birds worldwide. Molecular diagnostics, particularly RT-qPCR targeting the M gene and H5 subtype with confirmatory HA cleavage site sequencing, form the backbone of laboratory surveillance. Biosecurity measures tailored to each production system and integrated with wild bird ecology are critical for outbreak prevention. Future efforts should prioritize the development of rapid, field-deployable diagnostic tools and the expansion of genomic surveillance to monitor viral evolution in real time. The lessons from recent outbreaks [1, 2, 3, 4, 8, 11, 13] highlight the need for sustained investment in veterinary infrastructure and cross-sectoral collaboration.

References

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[2] Lee SH, Yeo SG, Lee S, et al. Preventive Responses to Human Avian Influenza Infection in the Republic of Korea, 2023-2024. Public Health Wkly Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/42238665/

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[8] Tewari D, Sekhwal MK, Nicholson C, et al. Genotype Diversity of Highly Pathogenic Avian Influenza H5N1 Clade 2.3.4.4b in Pennsylvania Poultry During Disease Outbreak from April 2022 to March 2023. Viruses. 2026. https://pubmed.ncbi.nlm.nih.gov/42198705/

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[11] Oremush R, Aubry P, Parmley EJ, et al. Determining the Environmental and Ecological Factors Associated With Poultry Farm Spillover of Highly Pathogenic Avian Influenza (H5N1) in British Columbia, Canada. Zoonoses Public Health. 2026. https://pubmed.ncbi.nlm.nih.gov/42136541/

[12] An SH, Heo GB, Lee B, et al. Development of multiplex real-time RT‒PCR assays for the simultaneous detection and lineage differentiation of H9 avian influenza viruses. Poult Sci. 2026. https://pubmed.ncbi.nlm.nih.gov/42127847/

[13] Giacinti JA, Signore A, Torchetti M, et al. North American perspective on the highly pathogenic avian influenza H5Nx clade 2.3.4.4b outbreak (November 2021 - March 2025). Can J Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/42114152/

[14] Kibiger L, Oltean HN, Leitz L, et al. Fatal Human Case of Highly Pathogenic Avian Influenza A(H5N5) in a Backyard Flock Owner - Washington, November 2025. MMWR Morb Mortal Wkly Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/42096351/

[15] Vaughan A, Joyce A, Traub E, et al. Serologic Evidence of Highly Pathogenic Avian Influenza A(H5N1) Virus Infection in a Veterinary Professional Exposed to an Infected Domestic Cat - Los Angeles County, California, December 2024-January 2025. MMWR Morb Mortal Wkly Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/42096344/