The use of chimeric venezuelan equine encephalitis viruses as an approach for the molecular identification of natural virulence determinants

Venezuelan equine encephalitis (VEE) virus antigenic subtypes and varieties are considered either epidemic/ epizootic or enzootic. In addition to epid emiological differences between the epidemic and enzootic viruses, several in vitro and in vivo laboratory markers distinguishing the viruses have bee n identified, including differential plaque size, sensitivity to interferon (IFN), and virulence for guinea pigs, These observations have been shown t o be useful predictors of natural, equine virulence and epizootic potential . Chimeric viruses containing variety LAB (epizootic) nonstructural genes w ith variety IE (enzootic) structural genes (VE/IAB-IE) or IE nonstructural genes and IAB structural genes (IE/IAB) were constructed to systematically analyze and map viral phenotype and virulence determinants. Plaque size ana lysis showed that both chimeric viruses produced a mean plaque diameter tha t was intermediate between those of the parental strains. Additionally, bot h chimeric viruses showed intermediate levels of virus replication and viru lence for guinea pigs compared to the parental strains. However, IE/IAB pro duced a slightly higher viremia and an average survival time 2 days shorter than the VE/IAB-IE virus. Finally, IFN sensitivity assays revealed that on ly one chimera, VE/IAB-IE, was intermediate between the two parental types. The second chimera, containing the IE nonstructural genes, was at least fi ve times more sensitive to IFN than the IE parental virus and greater than 50 times more sensitive than the IAB parent, These results implicate viral components in both the structural and nonstructural portions of the genome in contributing to the epizootic phenotype and indicate the potential for e pidemic emergence from the IE enzootic VEE viruses.