Potency of two chimeric vaccine candidates derived from the classical swine fever GPE(-) vaccine strain against a circulating virus strain isolated in Japan.

A classical swine fever (CSF) vaccine combining high potency and an immunological marker for differentiating infected animals from vaccinated animals facilitates disease control and provides proof of eradication to promote international pig trade. Previously, CSF virus (CSFV) recombinant live vaccine strains, guinea-pig exaltation of Newcastle disease virus-negative strain vaccine [vGPE^- (genotype 1.1)], were developed with the E^rns gene replaced by non-CSF pestiviruses (Pronghorn antelope or Phocoena pestiviruses). We evaluated the potency of these marker vaccines against a Japanese circulating CSFV strain (genotype 2.1), which is genetically distant from the vaccine strain. Pigs were experimentally vaccinated with the vGPE^- and two marker vaccine strains. All vaccinated and unvaccinated pigs were challenged with CSFV JPN/1/2018 at 26 days post-vaccination. The clinical signs and viral titers in blood and oral swabs were monitored for three weeks post-challenge, and antibodies against CSFV E2 and E^rns were detected using commercial enzyme-linked immunosorbent assay kits. Unvaccinated pigs showed typical CSF clinical signs and viremia, and one pig died at 19 days post-challenge. Meanwhile, none of the vaccinated pigs showed any clinical signs, and the replication of infectious virus was substantially suppressed. Both vGPE^--vaccinated and unvaccinated pigs had CSFV E2 and E^rns antibodies after vaccination and virus challenge; meanwhile, notably, marker-vaccinated pigs had only E2 antibodies, while both E2 and E^rns antibodies detected only after the challenge. In conclusion, the marker vaccine strains provided protective immunity to suppress clinical signs, viremia, and virus excretion, comparable to the GPE^- live vaccine, and successfully differentiated infection from vaccination.