Quantitative polymerase chain reaction: another tool to evaluate viable virus content in live attenuated orf vaccine

A probe-based real-time polymerase chain reaction (PCR) assay based on the highly conserved DNA polymerase gene of orf virus (ORFV) for the quality control of attenuated orf vaccine is reported. Primary lamb testis (PLT) cells were infected with orf vaccine virus and harvested at a critical time point to obtain maximum viable virus content as determined by real-time PCR. DNA extracted from these harvests was subjected to real-time PCR. A critical time point for the harvesting of PLT cells infected with various log(10) dilutions of vaccine virus was found to be 42 h (highest slope of 3.335), which was obtained by comparing the slopes of standard curves of different time intervals. The assay was employed to evaluate viable virus content in different batches of orf vaccine. The titres estimated by real-time PCR and conventional TCID(50) were comparable with a correlation of 0.8169. Thus, the real-time PCR assay could provide an alternative method or supplementary tool to estimate live ORFV particles in attenuated orf vaccine.