Porcine reproductive and respiratory syndrome (PRRS) is considered one of the most economically important diseases affecting the swine industry world-wide, characterized by reproductive failure in late term gestation in sows and respiratory disease in pigs of all ages (25). Porcine reproductive and respiratory syndrome virus (PRRSV), the causative agent, has been classified, together with lactate dehydrogenase elevating virus (LDV) of mice, equine arteritis virus (EAV) and simian haemorrhagic fever virus (SHFV), into the family Arteriviridae in the order Nidovirales (8).
PRRSV, a member of the small enveloped viruses, has a single-stranded, non-segmented, positive-sense, polyadenylated RNA of approximately 15 kb in length that contains nine open reading frames (ORFs). ORF1a and ORF1b encode viral replicase polyproteins, while ORF2a, ORF2b, and ORFs 3-7 encode the viral structural proteins GP2, E, GP3, GP4, GP5, M, and N, respectively (10, 21, 23, 24, 26, 39). Sequence com-parison of PRRSV isolates from different geogra-phical regions indicated that there are two major genotypes represented by prototype viruses Lelystad and VR-2332 with differences in nucleotide sequence of approximately 40%. These viruses are referred to as the European strains (EU, or type 1 [Lelystad virus, LV]) (24) and the North American strains (NA or type 2 [VR-2332]) (27).
However, there is increasing evidence which in-dicates that these PRRSV strains are biologically, antigenically and genetically heterogeneous. They differ in virulence and are associated with different clinical signs (4). Studies have shown that ORF5 and the non-structural protein 2 (NSP2)-coding gene (nsp2) may represent the most genetically variable regions in the PRRSV genome (2, 9, 14, 19, 20, 32). It is also well documented that PRRSV strains differ greatly in their pathogenicity (5, 12, 35).
In this study, we investigated some clinical samples in Hubei province of China and isolated some genes of one emerging PRRSV variant, ZB, which was the causative agent of so-called "high fever" disease in Hubei province in 2006 similar with those PRRSV variants isolated from other provinces of China in the same period. Phylogenetic analyses of three genes, N, GP5 and NSP2, were carried out with the aim of molecular characterization of this particular PRSSV. The ZB-N is identical to the N gene from all the Chinese isolates belonging to North Amercian (NA) type. However, several mutations in GP5 and a novel deletion in NSP2 gene distinct from all of the other previous PRRSV strains in ZB were identified as same as the other PRRSV variants isolated in China in 2006, and considered to be the causes of the change in virulence of this PRRSV variant possibly.
Molecular Characterization of a Highly Pathogenetic Porcine Reproductive and Respiratory Syndrome Virus Variant in Hubei, China *
- Received Date: 01 December 2008
- Accepted Date: 10 December 2008
Abstract: Porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized as one of the most important pathogens of pigs throughout the world. In 2006, more than 10 provinces of China have experienced an epizootic outbreak of pig diseases characterized by high fever, reddened skin and high morbidity and mortality. From June 2006 to April 2007, we have investigated some clinical samples in Hubei province by RT-PCR and cloned several major genes, N, GP5 and NSP2 gene, shown in this study. Phylogenetic analysis of these genes revealed that the highly pathogenic PRRSV variant, ZB, was responsible for 2006 emergent outbreak of pig disease in Hubei province similar with those variants isolated from other provinces in China in 2006, and belongs to the NA-type PRRSV. In the PRRSV variants, the N and GP5 shear about 90% identity with prototypic ATCC VR-2332 and some typical NA-type Chinese isolates, except the 2850bp NSP2 gene (only shares 65% identity with ATCC VR-2332). But they all shear more than and 97% identity with other highly pathogenetic Chinese PRRSV strains. Additionally, there are extensive amino acid (aa) mutations in the GP5 protein and 2 deletions in the Nsp2 protein when compared with the previous isolates. Most of the variants found in 2006 epizootic outbreak of pig diseases in China were the farthest variants from the typical NA-type PRRSV in phylogenetic distance, and these diversities may be responsible for the differences in the pathogenicity observed between these variants and original Chinese PRRSV strains.