Identification of virulence determinants of porcine reproductive and respiratory syndrome virus through construction of chimeric clones.

In order to determine virulence associated genes in porcine reproductive and respiratory syndrome virus (PRRSV), a series of chimeric viruses were generated where specific genomic regions of a highly virulent PRRSV infectious clone (FL12) were replaced with their counterparts of an attenuated vaccine strain (Prime Pac). Initial genome-wide scanning using a sow reproductive failure model indicated that non-structural (ORF 1a and 1b) and structural (ORF2-7) genomic regions appear to be sites where virulence determinants of PRRSV may reside. These results thus confirm the multigenic character of PRRSV virulence. Additional chimeras containing each individual structural ORFs (2 through 7) of Prime Pac and ORF5 of Neb-1 (parental strain of Prime Pac) within the FL12 backbone were generated and tested individually for further mapping of virulence determinants. Our results allow to conclude that NSP3-8 and ORF5 are the location of major virulence determinants, while other virulence determinants may also be contained in NSP1-3, NSP10-12 and ORF2.