Development of an improved live attenuated antigenic marker CSF vaccine strain candidate with an increased genetic stability.

Controlling classical swine fever (CSF) involves vaccination in endemic regions and preemptive slaughter of infected swine herds during epidemics. Live attenuated marker vaccines that confer effective protection against the disease and allow differentiation between infected and vaccinated animals (DIVA) could impact CSF control policies. Previously, we reported the development of FlagT4 virus (FlagT4v), a rationally designed live attenuated marker vaccine. During its vaccine assessment, FlagT4v reverted to a virulent virus during successive passages in piglets. Sequence analysis revealed deletions and substitutions almost exclusively in the areas of E1 and E2. To improve genetic stability of FlagT4v, we introduced changes in the codon usage in those areas. The newly developed virus, FlagT4Gv, was shown to retain the attenuated phenotype after successive passages in piglets. As observed with FlagT4v, the newly developed FlagT4Gv conferred effective protection against challenge with virulent CSFV at early (7 days) and at late (28 days) times post-vaccination.