The influence of maternal immunity on the efficacy of a classical swine fever vaccine against classical swine fever virus, genogroup 2.2, infection.

In Thailand, where vaccination is routinely employed, there has been an increased incidence of chronic classical swine fever (CSF) outbreaks during the past decade. The major causative virus has been identified to be the moderate virulence, classical swine fever virus (CSFV) of the genogroup 2.2. An investigation was made into the efficacy of a CSF vaccine against this genogroup 2.2 challenge. Five-week-old pigs, grouped by their level of passive antibody titer were immunized with lapinized Chinese-strain CSF vaccine and challenged with CSFV genogroup 2.2, 13 days after vaccination. The group containing passive titers of lower than 64 at the time of immunization, had significantly higher number of CSFV-specific IFN-gamma secreting cells and was completely protected against the challenge. Interestingly, both cellular and antibody responses were inhibited in the pigs with the higher passive titer. Furthermore, following challenge, CSFV could be isolated from 50% of the pigs in this group. It was demonstrated that the CSF vaccine could induce complete protection in pigs, provided that the maternal derived titer at the time of vaccination was lower than 64. The result implied that an increase in CSFV outbreaks might be due to the inappropriate timing of vaccination as well as the nature of the CSFV genogroup 2.2.