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Since 2016, A(H5Nx) high pathogenic avian influenza (HPAI) virus of clade 2.3.4.4b has become one of the most serious global threats not only to wild and domestic birds, but also to public health. In recent years, important changes in the ecology, epidemiology, and evolution of this virus have been reported, with an unprecedented global diffusion and variety of affected birds and mammalian species. After the two consecutive and devastating epidemic waves in Europe in 2020–2021 and 2021–2022, with the second one recognized as one of the largest epidemics recorded so far, this clade has begun to circulate endemically in European wild bird populations. This study used the complete genomes of 1,956 European HPAI A(H5Nx) viruses to investigate the virus evolution during this varying epidemiological outline. We investigated the spatiotemporal patterns of A(H5Nx) virus diffusion to/from and within Europe during the 2020–2021 and 2021–2022 epidemic waves, providing evidence of ongoing changes in transmission dynamics and disease epidemiology. We demonstrated the high genetic diversity of the circulating viruses, which have undergone frequent reassortment events, providing for the first time a complete overview and a proposed nomenclature of the multiple genotypes circulating in Europe in 2020–2022. We described the emergence of a new genotype with gull adapted genes, which offered the virus the opportunity to occupy new ecological niches, driving the disease endemicity in the European wild bird population. The high propensity of the virus for reassortment, its jumps to a progressively wider number of host species, including mammals, and the rapid acquisition of adaptive mutations make the trend of virus evolution and spread difficult to predict in this unfailing evolving scenario., Since 2016, A(H5Nx) high pathogenic avian influenza (HPAI) virus of clade 2.3.4.4b has become one of the most serious global threats not only to wild and domestic birds, but also to public health. In recent years, important changes in the ecology, epidemiology, and evolution of this virus have been reported, with an unprecedented global diffusion and variety of affected birds and mammalian species. After the two consecutive and devastating epidemic waves in Europe in 2020-2021 and 2021-2022, with the second one recognized as one of the largest epidemics recorded so far, this clade has begun to circulate endemically in European wild bird populations. This study used the complete genomes of 1,956 European HPAI A(H5Nx) viruses to investigate the virus evolution during this varying epidemiological outline. We investigated the spatiotemporal patterns of A(H5Nx) virus diffusion to/from and within Europe during the 2020-2021 and 2021-2022 epidemic waves, providing evidence of ongoing changes in transmission dynamics and disease epidemiology. We demonstrated the high genetic diversity of the circulating viruses, which have undergone frequent reassortment events, providing for the first time a complete overview and a proposed nomenclature of the multiple genotypes circulating in Europe in 2020-2022. We described the emergence of a new genotype with gull adapted genes, which offered the virus the opportunity to occupy new ecological niches, driving the disease endemicity in the European wild bird population. The high propensity of the virus for reassortment, its jumps to a progressively wider number of host species, including mammals, and the rapid acquisition of adaptive mutations make the trend of virus evolution and spread difficult to predict in this unfailing evolving scenario.

Phylogenetic evidence from the recent resurgence of high-pathogenicity avian influenza (HPAI) virus subtype H5N1, clade 2.3.4.4b, observed in European wild birds and poultry since October 2021, suggests at least two different and distinct reservoirs. We propose contrasting hypotheses for this emergence: (i) resident viruses have been maintained, presumably in wild birds, in northern Europe throughout the summer of 2021 to cause some of the outbreaks that are part of the most recent autumn/winter 2021 epizootic, or (ii) further virus variants were reintroduced by migratory birds, and these two sources of reintroduction have driven the HPAI resurgence. Viruses from these two principal sources can be distinguished by their hemagglutinin genes, which segregate into two distinct sublineages (termed B1 and B2) within clade 2.3.4.4b, as well as their different internal gene compositions. The evidence of enzootic HPAI virus circulation during the summer of 2021 indicates a possible paradigm shift in the epidemiology of HPAI in Europe.

After two decades free of Newcastle disease, Belgium encountered a velogenic avian orthoavulavirus type 1 epizootic in 2018. In Belgium, 20 cases were diagnosed, of which 15 occurred in hobby flocks, 2 in professional poultry flocks and 3 in poultry retailers. The disease also disseminated from Belgium towards the Grand Duchy of Luxembourg by trade. Independently, the virus was detected once in the Netherlands, almost simultaneously to the first Belgian detection. As such Newcastle disease emerged in the entire BeNeLux region. Both the polybasic sequence of the fusion gene cleavage site and the intracerebral pathotyping assay demonstrated the high pathogenicity of the strain. This paper represents the first notification of this specific VII.2 subgenotype in the North-West of Europe. Time-calibrated full genome phylogenetic analysis indicated the silent or unreported circulation of the virus prior to the emergence of three genetic clusters in the BeNeLux region without clear geographical or other epidemiological correlation. The Dutch strain appeared as an outgroup to the Belgian and Luxembourgian strains in the time-correlated genetic analysis and no epidemiological link could be identified between the Belgian and Dutch outbreaks. In contrast, both genetic and epidemiological outbreak investigation data linked the G.D. Luxembourg case to the Belgian outbreak. The genetic links between Belgian viruses from retailers and hobby flocks only partially correlated with epidemiological data. Two independent introductions into the professional poultry sector were identified, although their origin could not be determined. Animal experiments using 6-week- old specific pathogen-free chickens indicated a systemic infection and efficient transmission of the virus. The implementation of re-vaccination in the professional sector, affected hobby and retailers, as well as the restriction on assembly and increased biosecurity measures, possibly limited the epizootic and resulted in th