The sequences of ORFs 2 to 5 of five United States (US) porcine reproductive and respiratory syndrome virus (PRRSV) isolates with differing virulence were determined. The nucleotide and deduced amino acid sequences of these isolates were compared with those of other known PRRSV isolates. The amino acid sequence identity between seven US PRRSV isolates was 91–99% in ORF 2, 86–98% in ORF 3, 92–99% in ORF 4 and 88–97% in ORF 5. The low virulence US isolate had highest sequence variation in ORFs 2 to 4 compared to the other US isolates. A hypervariable region with antigenic potential was identified within the major envelope glycoprotein. Phylogenetic analysis of ORFs 2 to 7 indicated the existence of at least three minor genotypes within the major US genotype. The low virulence US isolate formed a branch distinct from the other US isolates. The results of this study have implications for both the taxonomy of PRRSV and vaccine development.


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