Complete genome sequence of attenuated low-temperature Thiverval strain of classical swine fever virus

The Thiverval vaccine strain of classical swine fever virus (CSFV) was derived from virulent Alfort strain through the serial passages in cells at 29-30 degrees C. In this study, we determined the complete genome sequence of this strain and found that its genome contains one open reading frame (ORF) that encodes a polyprotein with 3,898 amino acids. The 5'-UTR of Thiverval is 373 nt long with only one mutation at position 220. In contrast, the length of 3'-UTR is highly heterogeneous ranging from 233 to 259 bp. The heterogeneity of length of the 3'-UTR was due to an insertion of a variable length of T-rich sequence ranging from 6 to 32 nt. The insertion may change the structure and free energy of the 3'-UTR, resulting in a destabilization of the 3'-UTR. Sequence alignment of Thiverval and other CSFV strains showed 85.2-99.6% identities at the nucleotide level and 92.5-99.5% at the amino acid level. The phylogenetic tree analysis of the complete ORF, partial region of E2, and NS5B suggests that the CSFV Thiverval strain belongs to genetic group 1 and subgroup 1.1. The results from this study provide insight into the molecular mechanism of the attenuation of Thiverval vaccine strain.