Two decades of monitoring shows how migratory birds, virus reassortment, and global flyways are changing the spread of highly pathogenic avian influenza.
Study: Evolving dynamics of H5Nx avian influenza in China revealed by long-term wild bird surveillance. Image credit: GatisVil / Shutterstock
In a recent study published as a magazine article, nature communicationsA group of researchers investigated the long-term evolution, transmission, and geographic spread of H5Nx highly pathogenic avian influenza virus (HPAIV) in wild birds in China through nationwide surveillance and genomic analysis.
background
Avian influenza outbreaks affect millions of birds, disrupt global poultry production, and threaten wildlife conservation and public health. Highly pathogenic avian influenza (HPAI) H5Nx viruses spread far beyond their original geographic range by spreading across continents with the help of migratory birds, often exchanging genetic material.
Different continents are connected through migratory routes, so it is important to understand the patterns of spread and how these viruses evolve. Further research is needed to strengthen international surveillance and improve early detection of newly emerging virus strains.
About research
Researchers conducted a nationwide survey of avian influenza viruses (AIV) in wild birds across China from February 2003 to February 2023. During this period, 219,445 samples were collected from 28 provincial-level areas.
Before 2013, surveillance was mainly limited to northeastern China, especially Heilongjiang province, but the situation has changed since the establishment of the Bird Flu Monitoring and Alert System (BFMAS) in 2014.
The monitoring project included both active monitoring using live birds and passive monitoring through reports of sick and dead birds. Samples collected included fresh fecal samples, tissues, and swabs from a variety of sources.
The collected specimens were transported under temperature-controlled conditions, and virus isolation was performed in specific pathogen-free chicken embryo eggs.
Virus identification was performed through hemagglutination testing, subtyping, ribonucleic acid (RNA) extraction, amplification, and Sanger sequencing. The scientists performed a thorough analysis of the global outbreak from Food and Agriculture Organization (FAO) records, as well as obtained open-source genome sequence data from the Global Initiative for Sharing All Influenza Data (GISAID) and the National Center for Biotechnology Information (NCBI) databases.
Next, phylogenetic, phylogeographic, statistical and evolutionary analyzes were performed to assess the viral diversity, transmission routes, geographic spread, host shifts, and long-term evolutionary patterns of H5Nx viruses.
Research results
Nationwide surveillance identified significant changes in the occurrence and distribution of H5Nx HPAIV over a 20-year surveillance period. Researchers collected 219,445 bird samples in China from February 2003 to February 2023 and isolated 1,344 AIVs, with an overall isolation rate of 0.61%. Prior to 2014, monitoring was limited and H5Nx HPAIV was not detected.
After the introduction of BFMAS, surveillance expanded significantly, resulting in 368 H5Nx HPAIV isolates, including H5N1, H5N2, H5N6, and H5N8 viruses. Viral activity has a clear seasonal pattern, peaking from November to January and declining in the summer. Interestingly, despite the continued prevalence of low pathogenic avian influenza virus (LPAIV), H5Nx HPAIV was not detected from mid-2017 to late 2020, suggesting possible suppression rather than confirmation of no transmission.
Temporal changes in virus subtypes were also evident. H5N1 was predominant in 2014-2015, H5N8 became the predominant subtype in 2016-2017 and 2020, and H5N1 re-emerged in 2021-2023. These trends closely reflect global changes in clade 2.3.4.4b viruses and reflect ongoing viral evolution through antigenic drift, reassortment, and host immune pressure.
The surveillance program recorded 46 different H5Nx HPAIV detection events, revealing significant geographic and seasonal variation. Most detections occurred along major migratory bird flyways, particularly the Central Asian Flyway and the East Asian-Australasian Flyway, with approximately three-quarters of events recorded between September and February during the migration and wintering season.
Phylogenetic analysis identified three major viral clades circulating in China, with clade 2.3.4.4b being the dominant lineage and showing the greatest genetic diversity and geographic distribution. This clade continued to evolve through reassortment, creating multiple H5N1 and H5N8 genotypes over time.
The global outbreak analysis also showed that, although poultry accounted for the majority of outbreaks throughout the study period, outbreaks associated with wild birds have increased significantly since 2016 and have often exceeded outbreaks caused by poultry since fall 2020.
Phylogeographic analysis revealed that although the majority of early H5Nx transmission events occurred in East Asia, recent viruses discovered in China are genetically closer to viruses circulating in Europe and northern Asia than to regionally derived variants. Most clade 2.3.4.4b viruses detected since 2020 were consistent with repeated reintroductions via migratory birds rather than ongoing local transmission.
conclusion
The results of this study showed that long-term monitoring of wild birds contributes to a better understanding of the evolution, reassortment, and global spread of H5Nx HPAIV. Studies show that clade 2.3.4.4b has emerged as the predominant lineage for intercontinental transmission, and that the latest virus discovered in China was primarily reintroduced repeatedly through migratory birds rather than sustained regional circulation.
These observations highlight the need for continued surveillance, genomic monitoring, and international cooperation to detect new virus variants and explore their transmission routes.
Reference magazines:
- Li, X., Lv, X., Li, Y., Fei, M., Zhang, S., An, Q., Qin, S., Peng, P., Li, J., Liang, H., Xiu, Y., Li, Z., Wang, Y., Wang, Y., Zeng, X., Hou, Z., Sun, H., Xu, Y., Li, Y., and Chai, H. (2026). Evolving dynamics of H5Nx avian influenza in China revealed by long-term surveillance of wild birds. Nature Communications. Doi: 10.1038/s41467-026-76039-9, https://www.nature.com/articles/s41467-026-76039-9
