Genetic and Pathogenicity Studies of Avian Influenza (H9N2) Virus Isolated from Chickens in Jiangsu Province, China, in 2025.

Yue Li^1,2 Xingdong Song^1,2 Tengfei Gao^1,2 Guangyuan Wang^1,2 Hongyin Wei^1,2 Jiajun Wang^1,2 Wenjun Jiang^1,2 Ruihua Zhang^1,2 Yu Meng^1,2 Shijin Jiang^1,2

Affiliations 2 institutions

Department of Preventive Veterinary Medicine, College of Veterinary Medicine, Shandong Agricultural University, Taian 271017, China.
Shandong Provincial Key Laboratory of Zoonoses, Taian 271017, China.

PMID42655013 DOI10.3390/microorganisms14081667 Year2026 Journal Microorganisms Languageeng Publication statusActive

Active Last checked 2026-09-29 03:30:36 Source: NCBI PubMed

Abstract

In this study, one H9N2 subtype avian influenza virus (AIV) was isolated in Jiangsu Province, China, in March 2025. According to phylogenetic analysis, the HA gene fragment of the H9N2 isolate was classified as the B4.7 lineage, with the remaining seven segments falling into the Eurasian lineage. This H9N2 virus underwent complex genetic reassortment across multiple regions in China during its evolution, and key amino acid substitutions were identified, which may enhance its viral polymerase activity and replication efficiency in mice. Critically, a mouse experiment demonstrated that the H9N2 virus replicated effectively in the lungs and nasal turbinates of BALB/c mice without requiring prior adaptation. In addition, poultry infection studies showed that the H9N2 virus could replicate and spread more efficiently in chickens than in ducks. Moreover, the virus could be transmitted to contacted ducks through infected chickens with greater efficiency than from ducks to ducks. Furthermore, in the lungs of inoculated chickens and ducks, the mRNA expression levels of pattern-recognition receptors (PRRs) and cytokines were upregulated, while the overall fold increase in ducks was lower than that in chickens. In conclusion, these results highlighted the potential public health risk presented by chicken-derived H9N2 AIV, emphasizing the critical need for enhanced routine surveillance of H9N2 AIVs from poultry.

Supporting text Virus Host Location

avian influenza virus 64 chicken-derived 1 H9N2 subtype 5 pathogenicity 61 phylogeny 834

Evidence records

7 total

Zoonotic Surveillance

1 records · 1 evidence types

Evidence type Virus Isolation

1 records

OVE12048 Virus Isolation

Key finding

One H9N2 avian influenza virus was isolated from chickens in Jiangsu Province, China in March 2025.

Virus

Host

Location

Supporting text

Genetic and Pathogenicity Studies of Avian Influenza (H9N2) Virus Isolated from Chickens in Jiangsu Province, China, in 2025. In this study, one H9N2 subtype avian influenza virus (AIV) was isolated in Jiangsu Province, China, in March 2025.

Experimental Infection

4 records · 3 evidence types

Evidence type Pathogenicity Experiment

2 records

OVE12051 Pathogenicity Experiment

Key finding

H9N2 replicated effectively in lungs and nasal turbinates of BALB/c mice without prior adaptation.

Virus

Host

Location

Supporting text

a mouse experiment demonstrated that the H9N2 virus replicated effectively in the lungs and nasal turbinates of BALB/c mice without requiring prior adaptation.

Method: in vivo infection of BALB/c mice | tissue-specific viral replication assessment
Experimental system: mouse model (BALB/c)

OVE12052 Pathogenicity Experiment

Key finding

Inoculated chickens and ducks showed upregulated PRR and cytokine mRNA in lungs, with lower fold increases in ducks than chickens following H9N2 infection.

Virus

Host

Location

Supporting text

in the lungs of inoculated chickens and ducks, the mRNA expression levels of pattern-recognition receptors (PRRs) and cytokines were upregulated, while the overall fold increase in ducks was lower than that in chickens.

Method: experimental inoculation of chickens and ducks | lung tissue mRNA expression analysis
Experimental system: poultry infection model (chickens and ducks)

Evidence type Host Range Experiment

1 records

OVE12053 Host Range Experiment

Key finding

H9N2 avian influenza virus replicated and spread more efficiently in chickens than in ducks in controlled poultry infection studies.

Virus

Host

Location

Supporting text

poultry infection studies showed that the H9N2 virus could replicate and spread more efficiently in chickens than in ducks.

Method: experimental infection of poultry | assessment of viral replication and spread
Experimental system: animal challenge model (poultry infection studies comparing chickens and ducks)

Evidence type Experimental Transmission

1 records

OVE12054 Experimental Transmission

Key finding

H9N2 avian influenza virus was transmitted to contact ducks more efficiently from infected chickens than from ducks to ducks under a controlled contact-transmission setup.

Virus

Host

Location

Supporting text

the virus could be transmitted to contacted ducks through infected chickens with greater efficiency than from ducks to ducks.

Method: animal contact-transmission experiment
Experimental system: poultry contact-transmission model

Functional Mechanism

1 records · 1 evidence types

Evidence type Molecular Adaptation

1 records

OVE12050 Molecular Adaptation

Key finding

The H9N2 isolate harbors key amino acid substitutions that may enhance viral polymerase activity and replication efficiency in mice.

Virus

Host

Location

Supporting text

This H9N2 virus underwent complex genetic reassortment across multiple regions in China during its evolution, and key amino acid substitutions were identified, which may enhance its viral polymerase activity and replication efficiency in mice.

Genes or proteins: polymerase
Host factors: mice
Mechanism types: replication adaptation | host-range expansion

Genomic Evolution

1 records · 1 evidence types

Evidence type Phylogenetic Evolution

1 records

OVE12049 Phylogenetic Evolution

Key finding

Phylogenetic analysis placed the H9N2 isolate’s HA in the B4.7 lineage while the other seven segments were Eurasian, indicating complex reassortment across regions in China.

Virus

Host

Location

Supporting text

Genes or proteins: HA gene | seven segments
Analysis methods: phylogenetic analysis

References

78 references

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