In February 1991 the causative agent of porcine reproductive and respiratory syndrome (PRRS) was isolated at the Central Veterinary Institute in the Netherlands.
The agent was characterized as a virus and designated “Lelystad virus”. Subsequent research led to the fulfilment of Koch’s postulates in July 1991.
Shortly after the isolation of LV, a virus showing resembling clinical field signs was isolated in USA and designated American Type Culture Collection VR-2332 (ATCC VR-2332), hereafter referred to as VR-2332.
PRRS virus classification
PRRSV belongs to the family Arteriviridae in the genus Arterivirus, order Nidovirales. This group also includes lactate dehydrogenase-elevating virus (LDV) of mice, equine arteritis virus (EAV) and simian hemorrhagic fever virus (SHFV). PRRSV is a small, enveloped virus of 45-70 nm in diameter. Its genome consists of a polyadenylated, single-stranded, non-segmented, positivesense RNA polycystronic molecule of 15.1-15.5 kb. It contains, in the 5’ to 3’ direction, a 5’ leader sequence, at least 10 open reading frames (ORF1a, ORF1b and ORFs 2a, 2b, 3, 4, 5, 5a, 6 and 7) and a 3’ non-translated region. ORF1a and ORF1b are located directly downstream of the 5’ leader and encode a large replicase polyprotein, which is thought to be autoproteolytically cleaved into at least 13 smaller non-structural proteins including a viral RNA-dependent RNA polymerase. Recently a transframe encoded protein (nsp2TF) resulting from a ribosomal frame shift in the region corresponding to the non structural protein 2 has been described. ORFs 2a, ORF2b and ORFs 3 to 7 encode the structural proteins GP2a, 2b (also defined as “E protein”), GP3, GP4, M and N, respectively.
These proteins are expressed from a 3’ coterminal nested set of subgenomic mRNAs. GP5, encoded by ORF5, is the main envelope protein and is thought to be involved, singly or as GP5-M heterodimers, in viral attachment and penetration of target cells. GP5 contains the main neutralizing epitope of PRRSV. The M protein, encoded by ORF6, is a non-glycosylated integral membrane protein and the N protein, encoded by ORF7, is the nucleocapsid protein.
Structure
The PRRS virus has pleomorphic morphology. The virion has a spherical to oval shape with a size ranging from about 50 to 65 nm, a hollow, layered core of around 40 nm diameter and a smooth outer surface with the envelope protein complexes embedded.
The genome is enclosed by the nucleocapsid which is constituted of a double-layered chain of nucleocapsid protein homodimers that bundle into a hollow ball. The nucleocapsid core is surrounded by a lipid membrane, the envelope where the structural proteins are embedded. The major protein components of the lipid envelope are GP5 and M, which together encompass at least half the amount of the viral proteins. GP5 and M forms a disulfide-linked heterodimer through conserved cysteine residues in both proteins. The minor structural proteins GP2, GP3, and GP4 forms a multimeric complex incorporated in the lipid envelope and for at least the Type 1 PRRS viruses E is also a part of this complex. The recently discovered ORF5a protein is believed to be the eighth structural protein of PRRSV, but its orientation in the virion particle and its interaction with the other structural proteins still needs to be clarified.
Genetic diversity of PRRSV
From early studies two different genotypes of PRRSV are recognized, designated as genotype 1 (formerly known as European (EU)) and genotype 2 (formerly known as North American). Though genotype 1 and 2 PRRSV share morphological and structural similarities, and a nearly simultaneous emergence on either continent, they display significant molecular and antigenic variation. Respective prototype virus strains (VR2332 and Lelystad Virus (LV)) share approximately 60% nucleotide identity at the genome level.
Within each genotype the virus isolates can exhibit up to 20 % variability of nucleotide sequences resulting from random mutation (RNA polymerase infidelity) and recombination. The mutation rate for genotype 1 PRRSV strains has been calculated to be between 1.4 x 10-2 base substitutions per site and year (s/s/y) and 7.7 ± 2.1 x 10-2 s/s/y. This mutation rate is similar to that described for other RNA viruses. The variation is unevenly distributed within the viral genome.
Thus, in genotype 1 viruses, the ORF1a regions encoding nsp1β and nsp2 are the most variable ones while the encoding region for nsp9 is the most conserved one. The heterogeneity appears to increase with time.
Indeed, the average similarity between early and recent isolates suggests an increase in divergence of approximately 0.5% per year.
However, the observed heterogeneity of genotype 1 PRRS isolates does not appear to be due to temporal evolution exclusively. Other findings indicate that geography-related evolution of distinct virus strains may also have contributed to the large diversity of field strains in Europe. Based on reports of major divergence of isolates from Denmark and Italy, Forsberg et al. proposed the differentiation of three European clusters (Lelystad-like, Danish-like and Italian-like). In addition, Stadejek et al. showed that at least four subtypes exist within genotype 1. There is a sharp demarcation of PRRSV diversity along the eastern Polish border with subtypes 2, 3 and 4 found only eastwards of the Eastern Polish border. As shown in Figure 1, only subtype 1 circulates in Central, Western Europe and globally.
Taken together, PRRS viruses show a high mutation rate which is similar or higher to that of other RNA viruses. Since its emergence the diversity of genotype 1 PRRSV on the European continent has increased. Temporal and geographical factors including trade of PRRSV positive animals among different countries/geographical regions have contributed to the high variability of PRRSV strains in Europe.
