Isolation and Characterization of a Moderately Virulent Classical Swine Fever Virus Emerging in China

Classical swine fever (CSF) is a devastating infectious disease of pigs caused by classical swine fever virus (CSFV). In China, CSF has been under control owing to extensive vaccination with the lapinized attenuated vaccine (C-strain) since 1950s, despite sporadic or endemic in many regions. However, recently, CSF outbreaks occurred in a large number of swine herds in China. Here, we isolated 15 CSFV strains from diverse C-strain-vaccinated pig farms in China and characterized the genetic variations and antigenicity of the new isolates. The new strains showed unique variations in the E2 protein and were clustered to the subgenotype 2.1d of CSFV recently emerging in China in the phylogenetic tree. Cross-neutralization test showed that the neutralizing titres of porcine anti-C-strain sera against the new isolates were substantially lower than those against both the highly virulent Shimen strain and the subgenotype 2.1b strains that were isolated in China in 2006 and 2009, respectively. In addition, experimental animal infection showed that the HLJZZ2014 strain-infected pigs displayed lower mortality and less severe clinical signs and pathological changes compared with the Shimen strain-infected pigs. The HLJZZ2014 strain was defined to be moderately virulent based on a previously established assessment system for CSFV virulence evaluation, and the virus shedding and the viral load in various tissues of the CSFV HLJZZ2014 strain-infected pigs were significantly lower than those of the Shimen strain-infected pigs. Taken together, the subgenotype 2.1d isolate of CSFV is a moderately virulent strain with molecular variations and antigenic alterations.