Comparison of the pathogenicity of two strains (wild type and vaccine-like) of porcine reproductive and respiratory syndrome virus (PRRSV) in experimentally infected sows

The objective of this study was to compare two Korean strains of porcine reproductive and respiratory syndrome virus (PRRSV), namely a wild type (WT) strain and a vaccine-like (VL) strain, in respect of pathogenicity and viral distribution in the tissues. Both strains were of the North American genotype. Two groups of five pregnant sows were infected with either the WT or the VL strain 2 weeks before their expected farrowing date. The WT strain-inoculated sows showed abortion and premature farrowing, whereas the VL strain-inoculated sows remained clinically normal and did not farrow prematurely. Of the 18 liveborn piglets from the WT strain-inoculated sows, 14 had interstitial pneumonia. Of the 60 liveborn piglets from the VL strain-inoculated sows, only six had interstitial pneumonia. PRRSV antigen or nucleic acid was detected in 48/65 (73.8%) of stillborn and liveborn piglets from the WT strain-inoculated sows, but in only 12/64 (18.8%) of stillborn and liveborn piglets from the VL strain-inoculated sows.