Avian Influenza A(H9N2): Evolution, Impact and Recent Global Surveillance


Avian influenza (AI), first reported in Italy in 1878, was not identified as an influenza A virus until 1955. This group of viruses, characterized by their single-stranded, segmented RNA structure, includes the influenza A viruses with their significant impacts on both poultry and, occasionally, humans. These viruses are comprised of 8 gene segments encoding 12 proteins, with hemagglutinin (HA) and neuraminidase (NA) being critical for the virus’s ability to infect host cells. The influenza viruses are further differentiated into subtypes based on these surface antigens, with 18 HA and 11 NA subtypes identified to date from birds and bats.

Significant among the virus’s proteins are the nuclear proteins and polymerase complex proteins, which include PA, PB1, and PB2, crucial for the virus’s replication and transcription. The M gene, encoding M1 and M2 proteins, is vital for the virus’s packaging and replication, with M2 also acting as an ion channel. Research has highlighted the importance of several proteins, including NS1, NEP, and PA-X, in influencing the virus’s virulence and replication capabilities.

Evolution and Spread of H9N2-subtype AIV

The detection of the H9N2-subtype avian influenza virus in 1966 marked the beginning of its global spread, with significant epidemiological studies and phylogenetic analyses categorizing it into two main branches: the North American lineage and the Eurasian lineage. The virus’s adaptability has been demonstrated through its ability to infect not only poultry and wild birds but also mammals, including humans, with the first human case reported in China in 1998. Since then, H9N2 has been implicated in recombination events with other AIV subtypes, contributing to the emergence of new viruses capable of infecting humans.

Recent Developments and Global Impact

The ongoing avian influenza outbreaks have been a cause for concern worldwide, with recent reports emphasizing the risk these outbreaks pose to both animals and humans. The Centers for Disease Control and Prevention (CDC) provides regular updates on avian influenza, including protective actions for people to mitigate the risk of infection. These actions range from avoiding direct contact with wild birds to consuming properly handled and cooked poultry, alongside recommendations for healthcare professionals and the general public regarding vaccination and disease prevention strategies.

In a detailed analysis released by the World Health Organization (WHO) in collaboration with the Food and Agriculture Organization (FAO) and the World Organisation for Animal Health (WOAH), the current outbreaks have been highlighted as a significant risk to animal and human health. The spread of H5N1 avian influenza viruses, particularly the goose/Guangdong-lineage viruses, has led to unprecedented deaths in wild birds and poultry across multiple continents. The increase in detections among mammals has raised concerns about the potential for these viruses to adapt more easily to human hosts. With 67 countries reporting outbreaks in 2022 and additional countries in 2023, the global spread underscores the need for enhanced surveillance, biosecurity measures, and international cooperation to manage and mitigate the impact of these viruses.

The collaborative efforts of FAO, WHO, and WOAH emphasize the importance of cross-sectoral action to protect both animal and human health, highlighting the need for rapid detection, effective response strategies, and the sharing of genetic sequence data to monitor virus evolution. The ongoing research and monitoring efforts aim to understand the changing epidemiology of avian influenza and develop strategies to curb its spread, ensuring preparedness for potential pandemics.

The historical context, viral mechanics, and recent developments in avian influenza research underscore the complexity of managing and understanding this global health threat. The continued spread of avian influenza among birds and mammals, with sporadic human cases, necessitates vigilance, robust surveillance, and a coordinated international response to protect animal and human health against this ever-evolving virus.

Evolution and Host Range Expansion

H9N2 Avian Influenza Virus (AIV), known for its low pathogenicity, has a wide host range that includes humans and other mammals. This capability arises from genetic mutations and recombination, allowing the virus to overcome interspecies barriers. Poultry, particularly in live markets, serve as reservoirs for novel human influenza viruses, aiding in their evolution and spread. Surveillance in these markets is crucial for understanding and mitigating the risks associated with AIV.

Recent Trends and Governmental Measures

Between 2019 and July 2023, the global detection rate of AIV-positive cases has seen a decline, attributed to stricter management practices in live poultry markets. These measures include regular closures, along with daily cleaning and disinfection protocols, significantly reducing AIV transmission.

Molecular Insights and Public Health Implications

Phylogenetic analyses reveal that strains like ZJ81 and ML3 have undergone recombination with human influenza viruses, indicating a potential for zoonotic transfer. The presence of mutations such as Q226L in the HA protein of H9N2 strains suggests a shift in receptor preference from avian to human, raising concerns over increased human infection risks. Moreover, the analysis of key amino acid site mutations from January 2017 to July 2023 highlights a persistent threat posed by H9N2 AIV to public health, necessitating proactive prevention and control measures.

Global Surveillance and Recent Findings

Recent surveillance data underscores the global persistence of avian influenza, including H9N2. For instance, various strains of avian influenza have been detected across multiple continents, affecting a broad range of avian and mammalian species. This widespread distribution calls for continuous monitoring and research to understand the evolving epidemiology of these viruses.

In December 2023, China reported the latest human cases of H9N2, marking a continued concern for zoonotic transmission. Despite a decline in AIV-positive detection rates, the potential for H9N2 to infect humans underscores the need for vigilant surveillance and research to mitigate its public health impact.

The ongoing surveillance and research into H9N2 and other avian influenza viruses are crucial for preparing and responding to potential outbreaks. International collaborations and data sharing are essential for tracking the evolution of these viruses and devising effective control strategies.


The H9N2 Avian Influenza Virus continues to pose a zoonotic threat, with its capacity to infect a wide range of hosts and undergo genetic changes that may enhance its transmissibility and pathogenicity in humans. The global decline in detection rates, attributed to improved management practices in live poultry markets, demonstrates the effectiveness of targeted interventions. However, continuous surveillance, research, and international cooperation remain pivotal in managing the public health risks associated with H9N2 and other avian influenza viruses.


  • Centers for Disease Control and Prevention (CDC)
  • World Health Organization (WHO)
  • https://www.hindawi.com/journals/tbed/2024/9913934/#discussion
  • FAO.org
  • Canada.ca
  • WHO.int


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