Genomic sequence and virulence evaluation of MN184A-like porcine reproductive and respiratory syndrome virus in Japan.

This study characterized a porcine reproductive and respiratory syndrome virus (PRRSV) that was isolated in Japan from a 9-week-old diseased pig on a farm with an increased mortality rate during 2007–2008. This unique isolate, designated as Jpn5-37, did not have a high nucleotide identity in open reading frame 5 against any Japanese isolates. Among all available type 2 PRRSV complete genome sequences, Jpn5-37 shared the highest nucleotide identity (93.6%) with virulent strain MN184A. The genomic characteristics of Jpn5-37 were highly conserved with respect to the virulent MN184A, including a continuous 8-amino acid deletion in the nonstructural protein 2 region. Moreover, to elucidate the pathogenicity of the isolate, we determined the virus distribution, viremia, and gross and microscopic lesions in pigs studied for 10 days post-inoculation. We found that intranasal inoculation rapidly resulted in viremia and dissemination of the Jpn5-37 isolate to several tissues in a similar manner to that by EDRD1, but the average quantity of Jpn5-37 RNA in the sera was significantly higher. Similarly, the quantities of Jpn5-37 viral RNA found in all of the organs that we tested tended to be higher than those when infected with EDRD1. The mean rectal temperatures in the Jpn5-37 inoculated group were significantly higher than those in the control group at 4 and 6 dpi and in the EDRD1 inoculated group at 6 and 8 dpi. These results suggest that the Jpn5-37 strain replicates and is more efficiently distributed to the organs than EDRD1 under the same conditions.