Dissecting the evolutionary forces shaping codon usage bias in SFTSV from eastern China: Insights from a previously underrepresented region

Severe fever with thrombocytopenia syndrome (SFTS), caused by <i>Dabie bandavirus</i> (SFTS virus, SFTSV), poses a growing public health concern in East Asia. Limited genomic data from Shandong Province have restricted understanding of viral evolution, while codon usage bias, a key factor in viral fitness and host adaptation, remains uncharacterized. In this study, we analyzed codon usage patterns in SFTSV strains from Shandong using newly 152 sequenced samples and publicly available genomes. Phylogenetic analysis identified seven genotypes, with genotype A being dominant. Viral genomes showed higher adenine (A) and guanine (G) content and a tendency to use G or cytosine (C) at the third codon position. Relative synonymous codon usage analysis demonstrated gene-specific preferences for codons ending in G/C. Effective number of codons values indicated weak overall codon usage bias. Further analyses suggested that both mutation pressure and natural selection influence codon usage, with natural selection playing the dominant role (52.8-91.4%), particularly in the nucleoprotein (91.4%) and nonstructural genes (89.5%). Codon adaptation index analysis across 12 host species indicated stronger codon adaptation to humans and <i>Gallus gallus</i>, implying higher viral replication efficiency in these hosts. Overall, codon usage bias is primarily driven by natural selection rather than mutation pressure. These findings improve understanding of SFTSV molecular evolution and may inform strategies for surveillance, vaccine design, and host-specific intervention.