Different clinical presentations of subgenotype 2.1 strain of classical swine fever infection in weaned piglets and adults, and long-term cross-protection conferred by a C-strain vaccine.

Classical swine fever is an important swine disease in China, and sporadic outbreaks with mild clinical signs despite compulsory vaccination have raised questions about the virulence and pathogenicity of prevalent subgenotype 2.1 strains, and the ability of C-strain vaccines to cross-protect against them. To investigate this, three field isolates were evaluated in experimentally infected piglets and compared with the highly virulent reference Shimen strain. Clinical signs for the field strains ranged from mild to severe, and mortality ranged from 0 to 80 %. These data show differences in virulence among the subgenotype 2.1 field isolates and support the use of field strain GD191 as a genotype 2 challenge virus to assess efficacy of C-strain vaccines. In contrast to the historical genotype 1 strain, which caused acute infection with significant virus shedding in non-vaccinated animals, the subgenotype 2.1 GD191 strain produced different clinical manifestations in weaned piglets and adults. Adult pigs showed subclinical infection with viral shedding, whereas weaned piglets showed overt signs of infection. Efficacy of, and duration of immunity conferred by a C-strain vaccine were assessed using the reference Shimen strain and field isolate GD191 at 12 and 15 months after vaccination. A robust antibody response and sterilising protection were seen in all vaccinated animals and lasted up to 15 months post-vaccination. This study confirms that C-strain vaccines confer both clinical and virological protection against the historical genotype 1 Shimen strain and cross-protection against the prevalent genotype 2 field strain.