Genetic divergence with emergence of novel phenotypic variants of equine arteritis virus during persistent infection of stallions

The persistently infected carrier stallion is the critical natural reservoi r of equine arteritis virus (EAV), as venereal infection of mares frequentl y occurs after breeding to such stallions. Two Thoroughbred stallions that were infected during the 1984 outbreak of equine viral arteritis in central Kentucky subsequently became long-term EAV carriers. EAV genomes amplified from the semen of these two stallions were compared by sequence analysis o f the six 3' open reading frames (ORFs 2 through 7), which encode the four known structural proteins and two uncharacterized glycoproteins. The major variants of the EAV population that sequentially arose within the reproduct ive tract of each carrier stallion varied by approximately 1% per year, and the heterogeneity of the viral quasispecies increased during the course of long-term persistent infection. The various ORFs of the dominant EAV varia nts evolved independently, and there was apparently strong selective pressu re on the uncharacterized GP3 protein during persistent infection. Amino ac id changes also occurred in the V1 variable region of the G(L), protein. Th is region has been previously identified as a crucial neutralization domain , and selective pressures exerted on the V1 region during persistent EAV in fection led to the emergence of virus variants with distinct neutralization properties. Thus, evolution of the EAV quasispecies that occurs during per sistent infection of the stallion clearly can influence viral phenotypic pr operties such as neutralization and perhaps virulence.