First European Identification of a Partial Tacheng Tick Virus 8-like Underscores the Hidden Burden of Tick-Borne Flavivirus-like Viruses.

Tick-associated viruses are an underexplored component of global viromes, and highly divergent RNA viruses often remain undetected due to low abundance and sequence divergence. Tacheng tick virus 8 (TcTV8) is a poorly characterized RNA flavivirus-like virus originally reported from Dermacentor-associated ticks in China, with few subsequent reports and no confirmed detections outside Asia. Here, we report the detection and partial genomic characterization of a TcTV8-like virus in ticks collected from Portugal. Sequence-independent (SISPA) nanopore sequencing of individual ticks recovered three partial fragments (~2.7 kb, ~14% of the reference genome; ~30% of the reference covered at ≥1×), which shared high amino-acid identity with the TcTV8 polyprotein, including a methyltransferase-region domain. Phylogenetic analysis placed the Portuguese sequence within the TcTV8 lineage. Read-level classification and coverage analysis further support the presence of this virus in the positive sample. These findings represent, to our knowledge, the first detection of a TcTV8-like virus in European ticks, extending its known geographic range beyond Asia. The detection of this cryptic viral lineage highlights the need for broader tick virome surveillance to better understand the diversity, evolution, and ecology of flavivirus-like viruses.