In China, H9N2 avian influenza virus (AIV) has become widely prevalent in poultry, causing huge economic losses after secondary infection with other pathogens. Importantly, H9N2 AIV continuously infects humans, and its six internal genes frequently reassort with other influenza viruses to generate novel influenza viruses that infect humans, threatening public health. Inactivated whole-virus vaccines have been used to control H9N2 AIV in China for more than 20 years, and they can alleviate clinical symptoms after immunization, greatly reducing economic losses. However, H9N2 AIVs can still be isolated from immunized chickens and have recently become the main epidemic subtype. A more effective vaccine prevention strategy might be able to address the current situation. Herein, we analyze the current status and vaccination strategy against H9N2 AIV and summarize the progress in vaccine development to provide insight for better H9N2 prevention and control.

The interaction between migratory birds and domestic waterfowl facilitates viral co-infections, leading to viral reassortment and the emergence of novel viruses. In 2022, samples were collected from duck farms around Poyang Lake in Jiangxi Province, China, which is located within the East Asia–Australasia flyway. Three strains of H4N6 avian influenza virus (AIV) were isolated. Genetic and phylogenetic analyses showed that the isolated H4N6 avian influenza viruses (AIVs) belonged to new genotypes, G23 and G24. All isolated strains demonstrated dual receptor binding properties. Additionally, the isolated strains were able to replicate efficiently not only in avian cells but also in mammalian cells. Furthermore, the H4N6 AIV isolates could infect chickens, with viral replication detected in the lungs and extrapulmonary organs, and could transmit within chicken flocks through contact, with viral shedding detected only in oropharyngeal swabs from chickens in the contact group. Notably, the H4N6 AIV could infect mice without prior adaptation and replicate in the lungs with high viral titers, suggesting that it is a potential threat to humans. In conclusion, this study provides valuable insight into the characteristics of H4N6 strains currently circulating in China.

Infectious bursal disease (IBD) remains one of the most important immunosuppressive diseases in poultry and has been endemic in China for over four decades. In recent years, outbreaks characterized by markedly reduced mortality have emerged. To investigate the current prevalence and pathogenicity of infectious bursal disease virus (IBDV), strains were isolated from vaccinated poultry farms in central and eastern China, and representative isolates of distinct genotypes were evaluated for pathogenicity. Phylogenetic analysis of segment A (HVR) and segment B (B-marker) classified segment A into eight genogroups (A1-A8), with A2 further divided into A2.1 and A2.2 and A3 into six subtypes (A3.1-A3.6). Segment B was grouped into four genogroups (B1-B4), with B3 and B4 subdivided into B3.1-B3.4 and B4.1-B4.2, respectively. The 23 isolates obtained were grouped into three genotypes: A2.2B1 (14/23, 60.9 %), A3.2B3.2 (8/23, 34.8 %), and A3.5B3.3 (1/23, 4.3 %). The A2.2B1 and A3.5B3.3 strains showed only minor amino acid substitutions relative to reference strains, whereas the newly emerged A3.2B3.2 genotype displayed substantial divergence. Pathogenicity evaluation in SPF chickens revealed no mortality after infection with SD/23 (A3.2B3.2), SHX/24 (A3.5B3.3), or WD/22 (A2.2B1). However, all three strains significantly reduced the bursa-to-body weight index (BBIX) to below 0.7 at 5 days post-inoculation and caused severe bursal atrophy. Histopathological examination showed medullary necrosis and lymphocyte depletion in the bursa following infection with SD/23 and SHX/24, closely resembling the lesions induced by WD/22. Moreover, SD/23 and SHX/24 displayed replication dynamics similar to those of the reference strain LX (A3.1B2), with elevated viral detection rates and high viral loads in multiple organs. In summary, multiple IBDV genotypes are co-circulating in China. Despite the reduced pathogenicity of circulating strains, diminished attention to subclinical infections may facilitate viral transmission and cause considerable economic losses.