In the winter of 2021-2022, multiple subtypes (H5N8 and H5N1) of high pathogenicity avian influenza viruses (HPAIVs) were confirmed to be circulating simultaneously in Japan. Here, we phylogenetically and antigenically analyzed HPAIVs that were isolated from infected wild birds, an epidemiological investigation of affected poultry farms, and our own active surveillance study. H5 subtype hemagglutinin (HA) genes of 32 representative HPAIV isolates were classified into clade 2.3.4.4b lineage and subsequently divided into three groups (G2a, G2b, and G2d). All H5N8 HPAIVs were isolated in early winter and had HA genes belonging to the G2a group. H5N1 HPAIVs belong to the G2b and G2d groups. Although G2b viruses were widespread throughout the season, G2d viruses endemically circulated in Northeast Japan after January 2022. Deep sequence analysis showed that the four HPAIVs isolated at the beginning of winter had both N8 and N1 subtypes of neuraminidase genes. Environmental water-derived G2a HPAIV, A/water/Tottori/NK1201-2/2021 (H5N8), has unique polymerase basic protein 1 and nucleoprotein genes, similar to those of low pathogenicity avian influenza viruses (LPAIVs). These results indicate that multiple H5 HPAIVs and LPAIVs disseminated to Japan via transboundary winter migration of wild birds, and HPAIVs with novel gene constellations could emerge in these populations. Cross-neutralization test revealed that G2a H5N8 HPAIVs were antigenically distinct from a G2b H5N1 HPAIV, suggesting that antibody pressure in wild birds was involved in the transition of the HPAIV groups during the season.

In the winter of 2010-2011, Japan experienced a large outbreak of infections caused by clade 2.3.2.1 H5N1 high pathogenicity avian influenza viruses (HPAIVs) in wild birds. Interestingly, many tufted ducks (Aythya fuligula), which are migratory diving ducks, succumbed to the infection, whereas only one infection case was reported in migratory dabbling duck species, the major natural hosts of the influenza A virus, during the outbreak. To assess whether the susceptibility of each duck species to HPAIVs was correlated with the number of cases, tufted duck and dabbling duck species (Eurasian wigeon, Mareca penelope; mallard, Anas platyrhynchos; Northern pintail, Anas acuta) were intranasally inoculated with A/Mandarin duck/Miyazaki/22M807-1/2011 (H5N1), an index clade 2.3.2.1 virus previously used for experimental infection studies in various bird species. All ducks observed for 10 days post-inoculation (dpi) mostly shed the virus via the oral route and survived. The tufted ducks shed a higher titer of the virus than the other dabbling duck species, and one of them showed apparent neurological symptoms after 7 dpi, which were accompanied by eye lesions. No clinical symptoms were observed in the dabbling ducks, although systemic infection and viremia were observed in some of them sacrificed at 3 dpi. These results suggest that the susceptibility of clade 2.3.2.1 HPAIVs might differ by duck species.

The present study intended to determine the prevalence of Newcastle disease in unvaccinated backyard poultry in Africa. Using the PRISMA approach, a systematic review and meta-analysis of 107 epidemiological studies was conducted. The meta-analysis identified significant variation of both seroprevalence (I2 = 99.38, P = 0.00) and Newcastle disease virus prevalence (I2 = 99.52, P = 0.00) reported in various studies included in this review. Publication bias was not detected in either case. Seroprevalence of Newcastle disease was 40.2 (95%CI 32.9-47.8). Seroprevalence was significantly influenced by sampling frame and the African region where the studies were conducted. The prevalence of Newcastle disease virus (NDV) was 12% (95%CI 7.3-17.8), and the variation was influenced by sampling frame, diagnostic test, and regions where the studies were conducted. Also, Newcastle disease (ND) accounted for 33.1% (95%CI 11.9-58.1) of sick chickens. Results also indicated that genotypes VI and VII are widely distributed in all countries included in the study. However, genotype V is restricted in East Africa, and genotypes XIV, XVII, and XVIII are restricted in West and Central Africa. On the other hand, genotype XI occurs in Madagascar only. In addition, virulent genotypes were isolated from apparently healthy and sick birds. It is concluded that several genotypes of NDV are circulating and maintained within the poultry population. African countries should therefore strengthen surveillance systems, be able to study the viruses circulating in their territories, and establish control programs.