Changes Introduced in the Open Reading Frame of Bovine Viral Diarrhea Virus During Serial Infection of Pregnant Swine

Abstract

Bovine viral diarrhea virus (BVDV) is one of the most economically important viruses of cattle, but this pathogen is also able to infect pigs, camelids, and a wide range of domestic and wild ruminants. BVDV isolates circulating in animal populations are genetically and antigenically highly diverse. Acute BVDV infections in cattle cause the introduction of many substitutions in the viral genome. Serial infection of pregnant sheep with a BVDV-1b isolate of bovine origin was also associated with great numbers of substitutions. To our knowledge, genomic changes arising during BVDV infections in swine have not been investigated. The purpose of this study was to investigate the changes occurring in the open reading frame (ORF) of BVDV during serial infection of pregnant swine with a BVDV isolate of bovine origin. The BVDV-1b isolate AU526 was serially passaged in six pregnant gilts, two of which gave birth to live piglets congenitally infected with BVDV. The complete ORF sequences of 14 BVDV isolates obtained from pregnant gilts and their piglets were determined. Their analysis revealed that serial transmission of AU526 in pregnant swine resulted in many genomic changes. All isolates of porcine origin shared 32 nucleotide and 12 amino acid differences with the virus inoculum AU526. These changes were detected after a single passage in pregnant swine and were conserved during the subsequent five passages. Amino acid changes occurred primarily in genomic regions encoding the BVDV structural proteins E2 and E ^rns . These results suggest that BVDV infections in pregnant swine may contribute significantly to the genetic variability of BVDV and lead to the appearance of adaptive changes.

Keywords: RNA virus; bovine viral diarrhea virus; interspecies transmission; open reading frame; persistent infection; pestivirus; swine; viral genetic diversity.

FIGURE 1

Serial infection of pregnant gilts with BVDV. Gilts were between 27 and 39 days of gestation at the time of inoculation. The first gilt was inoculated intravenously with approximately 1.0 × 10⁶ TCID₅₀ of BVDV-1b AU526. The isolate AU526 was serially passaged in pregnant swine by inoculating successively the remaining gilts intravenously with serum obtained from the preceding gilt in the series. The fourth gilt (P4) was inoculated with 1 ml of serum obtained on day 5 from the third gilt (P3) but did not become infected. The fifth gilt (P5) was therefore inoculated with 1 ml of serum obtained on day 7 from P3 to continue the inoculation series.

FIGURE 2

Single nucleotide polymorphism (SNP) frequency plots of the virus inoculum AU526. All SNPs having a frequency above 1% are displayed. Nucleotide changes that were conserved during serial infection of pregnant gilts with BVDV and those that were not conserved are represented as red and blue bars, respectively. Nucleotides are numbered relative to the ORF of BVDV-1b AU526 (MG950344). More detailed information on the results of the SNP analysis can be found in the Supplementary Data Sheet.