title: Identification and clinical assessment of suspected vaccine-related field strains of porcine reproductive and respiratory syndrome virus.
abstract: OBJECTIVE
  To determine the origin and clinical relevance of selected strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV).
  
  
  METHODS
  38 pigs without antibodies for PRRSV.
  
  
  METHODS
  A seemingly uncommon restriction endonuclease digestion site in a commercially available vaccine strain of attenuated PRRSV was tested for its stability and prevalence under defined conditions. Selected field strains of PRRSV, with or without the restriction-site marker, were subsequently tested in pigs for virulence and for their ability to replicate competitively in pigs simultaneously given the vaccine.
  
  
  RESULTS
  Under experimental conditions, the restriction-site marker was stable 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, on the basis of previous testing, were believed unrelated to the vaccine strain. Conversely, it was detected in 24 of 25 field strains that were isolated 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 vaccine-related strains caused more pronounced pathologic changes than did the vaccine strain alone, and they predominated during replication in pigs also given the vaccine strain.
  
  
  CONCLUSIONS
  In some swine herds, the vaccine strain may have persisted and mutated to a less attenuated form.
  
  
  CONCLUSIONS
  The potential for persistence and mutation of specific strains of virus should be an important consideration when designing vaccination programs involving attenuated PRRSV.
authors: Virology Swine Research Unit, National Animal Disease Center, USDA, Agricultural Research Service, Ames, IA 50010, USA.
authors: W L
authors: WL
authors: Mengeling
authors: A C
authors: AC
authors: Vorwald
authors: K M
authors: KM
authors: Lager
authors: D F
authors: DF
authors: Clouser
authors: R D
authors: RD
authors: Wesley
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D016676:Macrophages, Alveolar / Q000821:virology; D008969:Molecular Sequence Data; D012150:Polymorphism, Restriction Fragment Length; D019318:Porcine Reproductive and Respiratory Syndrome / Q000821:virology*; D019316:Porcine respiratory and reproductive syndrome virus / Q000145:classification* / Q000235:genetics / Q000502:physiology; D013552:Swine / Q000821:virology*; D014765:Viral Vaccines / Q000494:pharmacology*; D014779:Virus Replication
chemical_list: D014765:Viral Vaccines