Emergence, prevalence, and evolution of H5N8 avian influenza viruses in central China, 2020

Highly pathogenic influenza A(H5N8) viruses have caused several worldwide outbreaks in birds and are able cross the species barrier to infect humans, posing a substantial threat to public health. After the first detection of H5N8 viruses in deceased swans in Inner Mongolia, we performed early warning and active monitoring along swan migration routes in central China. We isolated and sequenced 42 avian influenza viruses, including 40 H5N8 viruses, 1 H5N2 virus, and 1 H9N2 virus, in central China. Our H5N8 viruses isolated in swan stopover sites and wintering grounds showed high nucleotide homologies in the whole genome, revealing a common evolutionary source. Phylogenetic analysis revealed that the H5 viruses of clade 2.3.4.4b prevalent in 2020 have further diverged into two sub-clades: b1 and b2. The phylogeographic analysis also showed that the viruses of sub-clade b2 most likely originated from poultry in Russia. Notably, whooper swans were found to be responsible for the introduction of sub-clade b2 viruses in central China; whooper and tundra swans play a role in viral spread in the Yellow River Basin and the Yangtze River Basin, respectively. Our findings highlight swans as an indicator species for transborder spreading and monitoring of the H5N8 virus.

高致病性禽流感A(H5N8)病毒已在全球范围内引发多起禽类疫情，并且能够跨越物种屏障感染人类，对公共卫生构成严重威胁。在内蒙地区首次检测到H5N8病毒感染死亡天鹅后，我们在中国中部天鹅迁徙路径上进行了早期预警和主动监测。在中国中部，我们分离并测序了42株禽流感病毒，其中包括40株H5N8病毒、1株H5N2病毒和1株H9N2病毒。我们在中国中部天鹅停歇地和越冬地分离的H5N8病毒在基因组全序列上显示出高度的核苷酸同源性，揭示了共同的进化来源。系统发育分析显示，2020年流行的2.3.4.4b系H5病毒进一步分化为两个亚系：b1和b2。此外，系统地理学分析亦表明，亚系b2的病毒很可能起源于俄罗斯的禽类。值得注意的是，疣鼻天鹅被发现是b2亚系病毒在中国中部引入的罪魁祸首；疣鼻天鹅和北极天鹅分别在黄河流域和长江流域中扮演病毒传播的角色。我们的研究结果表明，天鹅是H5N8病毒跨境传播和监测的重要指示物种。