Genetic change in the open reading frame of bovine viral diarrhea virus is introduced more rapidly during the establishment of a single persistent infection than from multiple acute infections

Abstract

Bovine viral diarrhea viruses (BVDV) are ubiquitous viral pathogens of cattle with a high degree of sequence diversity amongst strains circulating in livestock herds. The driving force behind change in sequence is not well established but the inaccurate replication of the genomic RNA by a viral RNA polymerase without proof-reading capabilities as well as immune pressure on immunodominant proteins are thought to play major roles. Additionally, it is not clear when the majority of changes are introduced, whether during acute infections with exposure to innate and adaptive immune responses or in establishment of persistent infections (PI) in utero. To examine which generates greater sequence diversity, two groups of viruses were compared. The first was six isolates of a single strain of BVDV-2 that were isolated over greater than a year's time. These viruses caused a series of severe acute (SA) BVD outbreaks over a large geographic area. Changes in nucleotide sequence were determined by comparison of the sequence of each strain to the six virus consensus sequence. The second group was composed of six BVDV strains isolated from PI calves whose dams were exposed to PI cattle. Changes were identified by comparison of the sequence of the progenitor PI virus to that of the progeny viruses from the single in vivo 'passage'. The open reading frames (ORF) of the six SA isolates were >99% identical at the nucleotide level with 30% of the changes being nonsynonymous changes. The amount of genetic change increased with time and distance from the original outbreak. Similarly, the PI viruses isolated from single passage PI calves had >99% identity with the progenitor virus. The number of nucleotide changes in these viruses was equal to or greater than that observed in the SA viruses. The majority of the nonsynonymous changes were found in the structural proteins, with 65% of these occurring in the immunodominant E2 protein. Antigenic mapping studies using a monoclonal antibody panel specific for the BVDV E2 protein showed no antigenic differences amongst the six SA viruses, nor between the progenitor and progeny type 1a and type 2 persistent viruses. However, antigenic differences were observed in the two type 1b progeny viruses that possessed the greatest number of amino acid changes. Two antibodies were found to have altered staining patterns. These results suggest that the establishment of a single persistent infection results in more rapid generation of genetic diversity in BVDV strains than a series of acute infections and may contribute to antigenic change in the absence of an immune response.