The molecular characteristic analysis of PRRSV GSWW/2015 strain and its pathogenicity to pigs.

BACKGROUND: Porcine reproductive and respiratory syndrome (PRRS) is a severe disease, causing great economic losses to the pig industry. The causative agent, porcine reproductive and respiratory syndrome virus (PRRSV) is highly variable. Since the emergence of highly pathogenic PRRSV (HP-PRRSV) in China in 2006, this virus strain has undergone extensive variation. To investigate the genetic variation and pathogenicity of currently isolated PRRSV GSWW/2015 strain, its whole genome was sequenced and analyzed for the specific variation in NSP2, GP3 and GP5 regions. Pigs were challenged with the isolated virus to investigate its pathogenicity.

METHODS: The PRRSV GSWW/2015 strain was isolated by seeding the viral material in Marc-145 cells. The virus specific cytopathic effect (CPE) was confirmed by indirect immunofluorescent assay (IFA) and PCR to detect the virus protein and RNA. Nine pairs of primers were designed to obtain the complete genome by PCR. All PCR fragments were cloned into T-vector for sequencing. The genetic variation of GSWW/2015 strain was analyzed by multiple sequence alignments. Nineteen PRRSV-free piglets were intranasally challenged with 108 copies of GSWW virus, while seven piglets were housed together as contact-infected control. Clinical signs were recorded daily after challenge. Blood samples were obtained every week and the viral titer was detected by quantitative real-time PCR (qRT-PCR). The PRRSV specific antibody was detected by LSI ELISA kit.

RESULTS: The complete genome of PRRSV GSWW/2015 strain (GenBank accession number KX767091) was obtained. The whole genome of this strain shares 88.5 and 60.6% identity with VR-2332 and LV respectively, indicating that it belongs to the North American type (NA-type). Sequence alignments revealed that GSWW/2015 strain has a discontinuous deletion of 30 amino acids in NSP2, which is similar with HP-PRRSV. Some amino acids mutations can be observed in antigenic epitope regions of GP3 and GP5 compared with earlier strains of HP-PRRSV. Some piglets showed typical clinical signs of PRRSV after challenge. Only four pigs showed viremia within 3 days after challenge, most pigs showed peaked viremia after 21-28 days including 7 contact-infected pigs. Two pigs were detected to be positive for antibody to PRRSV at 14 days post infection (DPI), and 11 pigs (11/26) show seroconversion for PRRSV at 49 DPI. Twelve piglets died of PRRSV infection within two months.

CONCLUSIONS: The genome of PRRSV GSWW/2015 strain shows the features of HP-PRRSV with 30 discontinuous amino acids deletion in NSP2 and some new amino acid mutations in epitope regions of GP5 and GP3, which might alter the antigenicity of the virus. Furthermore, the virus showed high virulence to piglets as reported in HP-PRRSV, and induced long-lasting viremia and low level of antibody responses. This work further enriched our knowledge on PRRSV evolution and pathogenicity.