Chimeric (marker) C-strain viruses induce clinical protection against virulent classical swine fever virus (CSFV) and reduce transmission of CSFV between vaccinated pigs

Two live recombinant vaccines (Flc9 and Flc11) against classical swine feve r (CSF) were evaluated for their capacity to reduce transmission of virulen t CSF virus (CSFV) among vaccinated pigs. In Flc9 the 5' terminal half of t he E2 gene of the C-strain, a CSFV vaccine strain, was exchanged with the h omologous gene of the bovine viral diarrhoea virus (BVDV) strain 5250, the E-rns gene was exchanged likewise in the chimeric Flc11 virus. Both recombi nant vaccines induce an antibody response in pigs that can be distinguished from that induced after a wild-type CSFV infection. Four experiments were performed to estimate the reproduction ratio R after different vaccination- challenge intervals. Each group consisted of ten pigs [specified pathogen f ree (SPF) pigs or conventional pigs] that were vaccinated once, intramuscul arly, either with Flc9 or Flc11 virus or that were not vaccinated. Vaccinat ed and susceptible pigs were challenged intranasally with the virulent CSFV strain Brescia or Behring, 1, 2 or 4 weeks after vaccination. Whether cont act-pigs became infected was determined using a CSFV specific E2 (Flc9) or E-rns (FLc11) antibody ELISA. In the unvaccinated control groups, virus sec retion started from day 2 to 4 after inoculation and all contact pigs becam e infected. Contact pigs became infected in the group of pigs (SPF or conve ntional) vaccinated once with Flc9 virus and challenged 1-, 2- or 4-weeks l ater. The estimates of the R in the groups challenged at 1-, 2- and 4-weeks after vaccination were 0.38, 0 and 0.75, respectively. Contact infected pi gs were not detected (R = 0) in any of the groups of pigs, vaccinated with Flc11, only SPF pigs were used. In order to achieve a statistical significa nce of R within the vaccinated groups each of the experiments has to be rep eated at least once. The R of pigs vaccinated with Flc11 virus and challeng ed at 1- or 2-weeks after vaccination was however significantly lower that the reproduction ratio of the unvaccinated groups (P = 0.013). The R of pig s vaccinated with Flc9 virus and challenged at 1 (conventional pigs) or 2 w eeks (SPF pigs) after vaccination was significantly lower that the reproduc tion ratio of the unvaccinated groups (P = 0.013). In conclusion, both chim eric viruses Flc9 and Flc11 provided good clinical protection against a cha llenge with virulent CSFV at 1 or 2 weeks after vaccination. Further experi ments should be carried out to study more aspects of the efficacy of these recombinant viruses before they can be used as a marker vaccine under field circumstances. (C) 2001 Elsevier Science Ltd. All rights reserved.