Development and validation of a reverse transcription real-time PCR assay for specific detection of PRRSGard vaccine-like virus.

Increasing use of modified live virus (MLV) vaccines presents challenges to interpret positive results of porcine reproductive and respiratory syndrome virus (PRRSV) screening PCR that can detect both wild-type and vaccine strains. Instead, vaccine-specific PCR provides a convenient tool to detect vaccine-like virus from a sample. Here we report the development and validation of a real-time RT-PCR specific for PRRSGard® , a newly available commercial PRRSV-2 MLV vaccine. Analytical specificity, sensitivity and diagnostic performance of PRRSGard PCR were evaluated and compared to a commercial PRRSV screening PCR (reference PCR). PRRSGard and reference PCRs did not cross-react with any of the 27 non-PRRSV swine pathogens. PRRSGard PCR did not cross-react with other PRRSV-2 vaccine viruses and 31 laboratory and field PRRSV-2 isolates representing various genetic lineages of PRRSV-2. PRRSGard and reference PCRs consistently detected up to 10-6 and 10-5 dilutions of PRRSGard vaccine virus, respectively. Based on testing serial dilutions of in vitro transcribed RNA, the 95% limit of detection of PRRSGard PCR was 16 genomic copies/reaction with CT cut-off value of 36 and 7 genomic copies/reaction with CT cut-off value of 37. Diagnostic performance of PRRSGard PCR was evaluated using 846 clinical samples (684 serum and 162 oral fluid samples). Compared to the reference screening PCR, diagnostic sensitivity, specificity and agreement of PRRSGard PCR were 95.34%, 98.85% and 97.52% with cut-off CT value of 36 and 98.14%, 96.56% and 97.16% with cut-off CT value of 37. In addition, PRRSGard PCR was able to detect PRRSGard vaccine virus in a sample even with the co-presence of another PRRSV strain. In summary, in contrast to a reference screening PCR that detects both vaccine and field PRRSV strains, PRRSGard PCR provides a convenient tool to specifically detect PRRSGard vaccine-like virus and to inform PRRSV vaccination protocols.