Genetic characterization of the Korean porcine reproductive and respiratory syndrome viruses based on the nucleocapsid protein gene (ORF7) sequences

To investigate the genetic characteristics of the Korean porcine reproductive and respiratory syndrome virus (PRRSV), we determined the complete sequence of the nucleocapsid protein gene (ORF7) from 105 PRRSV isolates from all nine Korean prefectures during the years 2003 through 2006. These sequences were then analyzed along with the published ORF7 sequences for two Korean PRRS viruses (PL97-1/1997 and LMY/2002) and 36 non-Korean viruses. The ORF7 nucleotide sequence identities among the 107 Korean PRRS viruses ranged from 86.2 to 100%, corresponding to 85.4 to 100% identity at the amino acid level. All of the Korean isolates examined belonged to the North American genotype. The ORF7 gene sequence from the North American prototype virus (VR-2332) and its derived vaccine virus (Ingelvac PRRS MLV) was 90.0-100% identical to the various ORF7 sequences of the Korean isolates, with corresponding amino acid identities from 91.0 to 100%. In the phylogenetic tree obtained by neighbor-joining analysis, all of the Korean PRRSVs were divided into four groups. Our ORF7 sequence data also revealed no correlations between the date or place of collection and the distribution of PRRSV in Korea. North American genotype PRRSVs may have been introduced into Korean swine herds some time ago; these viruses apparently radiated nationwide within a relatively short period of time. Within the North American genotype PRRSVs from around the world, the Korean PRRSVs did not emerge as a single independent clade overall, and their immediate relationships with the PRRSVs from other countries could not be determined.