C-strain vaccine virus was consistently detected in tonsils of all (n = 30) animals from 3 to 77 days post vaccination (dpv) and in blood (n = 36) between 3 and 13 dpv by CSFV-specific rRT-PCR. CP7E2alf virus RNA was detected in 6 animals slaughtered between 4 and 63 dpv by a BVDV-specific rRT-PCR. The chimeric virus was not detected in blood samples.
As detected by CSFV E2-specific antibody ELISA and virus neutralisation tests, seroconversion first occurred at 11 dpv in the C-strain vaccinated group and between 11 and 15 dpv in the CP7E2alf vaccinated group. The serological response was still present at 98 dpv. The CP7E2alf serological response remained negative using the CSFV Erns ELISA whereas seroconversion occurred in the C-strain vaccinated group.
In conclusion, the primary replication site of CP7E2alf vaccine virus was found to be the tonsils as in the C-strain and virulent field strains. Persistence of CP7E2alf in the tonsils was also demonstrated up to 63 dpv. Both vaccines showed immunogenicity after oronasal administration in domestic pigs. In contrast to the C-strain, CP7E2alf vaccine allowed the use of DIVA approaches in serological tests. This study confirms CP7E2alf as a promising marker vaccine candidate for oronasal vaccination programmes to control CSF in domestic pigs and wild boar.