Abstract
Investigation of a suspected case of hog cholera led to the isolation of bovine viral diarrhoea viruses from a persistently infected steer and from five sibling piglets. The persistently infected steer was able to infect pigs by indirect contact under controlled laboratory conditions and the bovine virus was able to cause congenital porcine disease. Pestiviruses isolated from the steer and from the pigs infected on the farm were compared with one another and with the bovine virus after experimental pig passage. Antigenic and genetic analyses of the gp53 proteins and genes of these viruses revealed that the bovine virus had only three amino acid changes consequent upon experimental passage in pigs, but was distinct from the porcine field isolates. The initial assumption that the persistently infected steer had infected the farm pigs was therefore not confirmed by experimentation. A panel of BVDV mAbs that included some of those used to discriminate between the above and other pestivirus strains was characterised with respect to protein specificity. Most of the mAbs recognised either the major envelope glycoprotein (gp53) of BVDV or an immunodominant non-structural protein (p80). Competitive binding studies demonstrated four domains on p80 and two on gp53. Conservation of epitopes was much greater for p80 than for gp53. Sequence comparison between mAb neutralization-escape mutants and parental viruses indicated the possible location of antibody binding sites on gp53 and suggested two domains, one of which was defined by relatively conserved epitopes. The pattern of amino acid differences comparing escape mutant viruses with both parental viruses and others of known mAb reactivity suggested that gp53 epitopes may be non-linear.