Date Full Report Received04/03/2012
Date Abstract Report Received04/03/2012
Funded ByIowa Pork Producers Association
Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) infections are among the most important swine viruses facing the industry today. Although vaccines have been highly effective overall, cases of porcine circovirus associated disease (PCVAD) have re-emerged in the United States in recent years. The outbreaks have raised concerns over introduction of a new more virulent PCV2-variant (PCV2b) into North America. Simultaneous with the PCVAD increase, we have observed a marked increase in submissions of PRRSV-associated pneumonia to the Veterinary Diagnostic Laboratory at Iowa State University. PRRSV, an RNA virus, is capable of continuous genomic changes through innate error of RNA polymerase or PRRSV recombination. The goal of the study was to investigate the effect of PCV2 infection on genetic and pathogenic evolution of PRRSV during serial passages in pigs. We hypothesized that concurrent PCV2b infection during passage of PRRSV in pigs would result in a substantial increase in mutations of the PRRSV genome compared to concurrent PCV2a infection or singular PRRSV infection. Eight, 2-week-old conventional PRRSV and PCV2 free pigs were randomly divided into 4 groups and rooms of 2 pigs each. Group 4 served as the negative control group. Pigs in groups 1, 2 and 3 were challenged intranasally with PRRSV VR2385. In addition, pigs in groups 2 and 3 were inoculated intranasally and intramuscularly with PCV2a or PCV2b. Forty-two days after inoculation, the pigs were euthanized and lungs and lymphoid tissues were collected, tissue homogenate was produced which was utilized to inoculated the next set of pigs. This process was repeated seven times to account for a total of eight in vivo passages. PRRSV open reading frames (ORF) 5, 6, 7 as well as a portion of ORF4 were sequenced and compared at the end of each passage. In this study, overall 13 nucleotide mutations were detected in the PRRSV only group 1, 4 nucleotide mutations were detected in the PRRSV/PCV2a coinfected group and 2 nucleotide mutations were detected in the PRRSV/PCV2b coinfected group 3 after eight consecutive passages in pigs. Seven of these mutations resulted into amino acid alterations. One of the amino acid substitutions in the PRRSV only group 1 resulted in a change of polarity, that is, the hydrophilic 16Ser in ORF4 mutated into hydrophobic 16Tyr. An influence of PCV2 on the PRRSV mutation rate was not identified based on sequencing of ORF5, ORF6, ORF7 and partial ORF4.