Genetic variation and phylogenetic analyses of the ORF5 gene of acute porcine reproductive and respiratory syndrome virus isolates

Swine herds in the US have experienced recent outbreaks of a severe form of porcine reproductive and respiratory syndrome (designated acute or atypica l PRRS) characterized by abortion and high mortality in pregnant sows. Most of the affected herds had been vaccinated with modified live-vaccines (MLV s) against PRRS. To explore the possible mechanism of the emergence of acut e PRRS, the open reading frame 5 (ORF5) gene encoding the major envelope pr otein (GP5) of acute PRRSV isolates was characterized. The complete ORF5 ge ne of eight acute PRRSV isolates from herds experiencing acute PRRS outbrea ks in Iowa and North Carolina was amplified and sequenced. Sequence analyse s revealed that these acute PRRSV isolates shared 88-95% nucleotide and 88- 96% amino acid sequence identities to each other, 87-97% nucleotide and 84- 96% amino acid sequence identities with other North American PRRSV isolates and the MLVs. Most of the amino acid substitutions locate in the putative signal sequence and two short hypervariable regions at the amino terminus. The ORF5 gene sequence of the acute PRRSV isolate 98-37120-2 from a non-vac cinated swine herd in Iowa is very closely related to that of the RespPRRS MLV, with 97% nucleotide and 96% amino acid sequence identities. Phylogenet ic analysis revealed that all eight acute PRRSV isolates are clustered with in the North American genotype. Several minor branches that are not associa ted with geographic origins were also identified within the North American genotype. One acute PRRSV isolate (98-37120-2) is clustered with the RespPR RS MLV and several Danish isolates that were confirmed to be derived from t he RespPRRS MLV. The ORF5 gene sequences of other seven acute isolates are more related to those of several earlier PRRSV isolates and the PrimePac ML V than to that of the RespPRRS MLV. Our results showed that the acute PRRSV isolates analyzed in this study differed from each other in ORF5 genes, al though they all clustered within the North American genotype. The data from this study do not fully support the hypothesis that the emergence of acute PRRS is due to reversion of MLVs to a pathogenic phenotype, as only one of the eight acute isolates was shown to be very closely related to the RespP RRS MLV. (C) 2001 Elsevier Science B.V. All rights reserved.