Novel sub-lineages, recombinants and reassortants of severe fever with thrombocytopenia syndrome virus.

Severe fever with thrombocytopenia syndrome virus (SFTSV) has been continuously circulating in Eastern Asia in recent years, without the availability of effective vaccines or antiviral drugs, posing a serious threat to public health, particularly in the central-eastern provinces of China. In this study, we isolated and sequenced four new SFTSV strains from Shandong Province identified in 2015. Phylogenetic analysis, with all publicly available L, M and S gene segments, revealed that the four newly described SFTSV strains belonged to genotype C3. In addition, our phylogenetic analyses also identified two potentially novel sub-lineages of SFTSV, tentatively named C6 and J4. Our comprehensive analysis revealed twenty recombination events in fourteen SFTSV genomes and recombination events were found in the S gene segment for the first time. A total of twenty-six strains were probable SFTSV reassortants, including sixteen which were previously unidentified. We further characterised the genetic constellation of these putative reassortants and classified them into twelve different reassortment forms. Our study revealed multiple evolutionary forces acting on SFTSV, responsible for the increasing genetic diversity of this agent, which could potentially alter the antigenicity and pathogenicity of the virus. This calls for an urgent need for intensified surveillance and the development of vaccines and antiviral drugs against this high-consequence pathogen.