Genetic and antigenic characterization of complete genomes of Type 1 Porcine Reproductive and Respiratory Syndrome viruses (PRRSV) isolated in Denmark over a period of 10 years

Abstract

Porcine Reproductive and Respiratory Syndrome (PRRS) caused by the PRRS virus (PRRSV) is considered one of the most devastating swine diseases worldwide. PRRS viruses are divided into two major genotypes, Type 1 and Type 2, with pronounced diversity between and within the genotypes. In Denmark more than 50% of the herds are infected with Type 1 and/or Type 2 PRRSV. The main objective of this study was to examine the genetic diversity and drift of Type 1 viruses in a population with limited introduction of new animals and semen. A total of 43 ORF5 and 42 ORF7 nucleotide sequences were obtained from viruses collected from 2003 to February 2013. Phylogenetic analysis of ORF5 nucleotide sequences showed that the Danish isolates formed two major clusters within the subtype 1. The nucleotide identity to the subtype 1 protogenotype Lelystad virus (LV) spanned 84.9-98.8% for ORF5 and 90.7-100% for ORF7. Among the Danish viruses the pairwise nucleotide identities in ORF5 and ORF7 were 81.2-100% and 88.9-100%, respectively. Sequencing of the complete genomes, including the 5'- and 3'-end nucleotides, of 8 Danish PRRSV Type 1 showed that the genome lengths differed from 14,876 to 15,098 nucleotides and the pairwise nucleotide identity among the Danish viruses was 86.5-97.3% and the identity to LV was 88.7-97.9%. The study strongly indicated that there have been at least two independent introductions of Type 1 PRRSV in Denmark and analysis of the full genomes revealed a significant drift in several regions of the virus.

Efficacy and safety of simultaneous vaccination with two modified live virus vaccines against porcine reproductive and respiratory syndrome virus types 1 and 2 in pigs

“…an animal that become positive after being tested negative. The duration of nasal excretion of PRRSV was in line with the previous studies quoted above (7)(8)(9)(10)(11)(12)(13)(14). There is probably a limited risk that these weakly positive animals would transmit the virus horizontally to naïve animals [21], but the persistence of vaccine virus for two-three months after vaccination emphasize the importance of keeping vaccinated animals isolated from naïve animals for a sustained time period prior to mingling.…”

“…The strain ILI6 is a virus isolated in Russia in 2009 and belong to PRRSV-1, subtype 2 [23]. This subtype has to our knowledge never been detected in Western Europe [7].…”

“…In West-European countries, circulating PRRSV-1, subtype 1 viruses can be divided into two main clusters with one of the clusters being highly similar to the PRRSV-1 reference strain LV (>95% identity in ORF 5), while some of the isolates from the other cluster differ more than 10% from the LV strain [7]. In the present study, the PRRSV-1, subtype 1 strain 18794 was used as challenge strain.…”

“…Both PRRSV-1, subtype 1 and PRRSV-2 are circulating and causing disease in some European countries [7,8]. In contrast, PRRSV-1 strains belonging to subtypes 2, 3, and 4 have never been detected in Western Europe [23].…”

“…Only ORF5 sequences were included in the analysis. The sequencing of PRRSV-1 ORF7 for confirmation of some RT-PCR results was performed as earlier described [7].…”