Classical swine fever virus is genetically stable in vitro and in vivo

Phylogenetic analyses of large numbers of classical swine fever strains hav e revealed a high degree of sequence conservation in the genomic regions ex amined, suggesting either a recent common ancestor or a low evolution rate. This low variability is in contrast to findings with other RNA viruses. To investigate the consequence of this apparent genetic stability on phylogen etic examinations, the Belgian field isolate Wingene'93 was passaged in pig s as well as in cell culture by various methods. Sequence analyses of virus es collected after various passages in three target regions proposed for ph ylogenetic studies (5' NTR, E2, and NS5B) revealed a complete sequence cons ervation. Only when the amount of passaged virus was lowered, mimicking a g enetic bottleneck, a single point mutation was observed in the E2 gene. Add itionally, only four nucleotide substitutions were observed when the genome of a virus obtained after 96 cell passages in persistently infected cells was compared with its parental virus, the recombinant virus derived from an infectious cDNA clone of CSFV strain Alfort/187. This low mutation frequen cy observed both in vitro and in vivo demonstrates that classical swine fev er virus is genetically stable. Hence, even minor mutations can be consider ed significant in molecular epidemiological studies.