Characterization of emerging European-like porcine reproductive and respiratory syndrome virus isolates in the United States

European-like field isolates of porcine reproductive and respiratory syndrome virus (PRRSV) have recently emerged in North America. The full-length genomic sequence of an index isolate characterized in 1999, strain EuroPRRSV, served as the reference strain for further studies of the evolution and epidemiology of European like isolates (type 1) in the United States. Strain EuroPRRSV shared 90.1 to 100% amino acid identity with the prototype European strain, Lelystad, within the structural and nonstructural open reading frames (ORFs) and 95.3% overall nucleotide identity. The 5' untranslated region and two nonstructural regions within ORF 1 were closely examined due to significant divergence from strain Lelystad. A 51-by deletion in a region within ORF la, coding for nonstructural protein 2 (NSP2), was observed. Sequence analysis of the structural ORFs 2 to 7 of additional European-like isolates indicated that these isolates share 93% nucleotide identity with one another and 95 to 96% identity with the Lelystad strain but only 70% identity with the North American reference strain VR-2332. Phylogenetic analysis with published PRRSV ORF 3, 5, and 7 nucleotide sequences indicated that these newly emerging isolates form a clade with the Lelystad and United Kingdom PRRSV isolates. Detailed analysis of four of these isolates with a panel of 60 monoclonal antibodies directed against the structural proteins confirmed a recognition pattern that was more consistent with strain Lelystad than with other North American isolates.