Emergence and adaptive evolution of influenza D virus.

As a novel member of the Orthomyxoviridae, influenza D virus (IDV) was firstly isolated from swine. However, cattle were found to serve as its primary reservoir. The study of IDV emergence can shed light into the dynamics of zoonotic infections and interspecies transmission. Although there is an increasing number of strains and sequenced IDV strains, their origin, epidemiology and evolutionary dynamics remain unclear. In this study, we reconstruct the diversity and evolutionary dynamics of IDVs. Molecular detection of swine tissue samples shows that six IDV positive samples were identified in the Eastern China. Phylogenetic analyses suggest three major IDV lineages designated as D/Japan, D/OK and D/660 as well as intermediate lineages. IDVs show strong association with geographical location indicating a high level of local transmission, which suggests IDVs tend to establish a local lineage of in situ evolution. In addition, the D/OK lineage widely circulates in swine in Eastern China, and all of the Chinese virus isolates form a distinct sub-clade (D/China sub-lineage). Furthermore, we identified important amino acids in the HEF gene under positive selection that might affect its receptor binding cavity relevant for its broader cell tropism. The combined results highlight that more attention should be paid to the potential threat of IDV to livestock and farming in China.

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