Identification and clinical assessment of suspected vaccine-related field strains of porcine reproductive and respiratory syndrome virus

Objective-To determine the origin and clinical relevance of selected strain s of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV). Animals-38 pigs without antibodies for PRRSV. Procedure-A seemingly uncommon restriction endonuclease digestion site in a commercially available vaccine strain of attenuated PRRSV was tested for i ts stability and prevalence under defined conditions, Selected field strain s of PRRSV, with or without the restriction-site marker, were subsequently tested in pigs for virulence and for their ability to replicate competitive ly in pigs simultaneously given the vaccine. Results-Under experimental conditions, the restriction-site marker was stab le during long-term infection of pigs, It was not detected in any of the 25 field strains of PRRSV that were isolated before use of the vaccine or 21 of 25 field strains that were isolated after use of the vaccine but that, o n the basis of previous testing, were believed unrelated to the vaccine str ain. Conversely it was detected in 24 of 25 field strains that were isolate d after use of the vaccine and that, on the basis of previous testing, were believed to be direct-line descendants of the vaccine strain. Putative vac cine-related strains caused more pronounced pathologic changes than did the vaccine strain alone, and they predominated during replication in pigs als o given the vaccine strain. Conclusions-In some swine herds, the vaccine strain may have persisted and mutated to a less attenuated form. Clinical Relevance-The potential for persistence and mutation of specific s trains of virus should be an important consideration when designing vaccina tion programs involving attenuated PRRSV.