Molecular characterization of porcine reproductive and respiratory syndrome virus (PRRSV ) strains in ORF5 and ORF7 regions

The aim of the study was to characterize PRRSV strains prevalent in Lithuanian and Latvian pig population by molecular and cell culture methods and to evaluate safety of modified live PRRSV vaccines under the field conditions. Goals of the study: 1. Characterization of PRRSV strains prevalent in Lithuanian pig farms in ORF5 and ORF7 regions and comparing them with reference sequences of other EU-genotype PRRSV strains. 2. Characterization of PRRSV strains circulating in Latvian pig herds in ORF5 and ORF7 regions and comparing nucleotide and amino acid sequences with reference sequences of other EU-genotype PRRSV strains. 3. Assessment of geographical distribution of different PRRSV subtypes prevalent in Lithuanian and Latvian pig populations. 4. Evaluation of ORF7 encoding nucleotide sequences and deduced amino acid sequences for characterization and subtype definition of PRRSV strains. 5. Evaluation of the use of antigenicity index predicted by ORF5 amino acid sequences for subtype characterization of PRRSV and determination of the potential N-glycosylation sites (N) of Lithuanian and Latvian PRRSV strains in ORF5 region. 6. Investigation of possibility to isolate Lithuanian and Latvian PRRSV strains in the culture of pulmonary alveolar macrophages and in the MARC-145 cell line. 7. Monitoring of PRRSV circulation in the farms where EU and NA genotype modified live vaccines were used and evaluating some safety aspects of this vaccination. In this study Lithuanian and Latvian PRRSV strains were characterized by molecular methods for the first time. Completely new ORF5 sequences from Lithuania, Latvia and several new sequences from Belarusian pig populations enabled to determine and to compare the antigenicity profiles of the selected PRRSV subtype 2 and new subtype 4 strains. Newly obtained ORF7 sequences from Lithuanian and Latvian pig population enabled to detect the defining characteristics of PRRSV subtypes 1, 2, 3, and 4 with higher reliability. Analysis of new PRRSV strains from Latvian pig farms resulted in classifying a completely new subtype 4 within the EU genotype PRRSV, and therefore extended the current knowledge of the previously suggested classification into the subtypes 1, 2 and 3 (Stadejek et al. 2008). ORF5 and ORF7 sequences of the new very special PRRSV strains obtained in this study will be published at the GenBank public database and will be available for other researchers. Isolation of the new very diverse Lithuanian and Latvian PRRSV strains was for the first time investigated in cell cultures and some new isolates were obtained. These isolates can be used in further studies of their immunological and virological properties in vivo and in vitro. For the first time circulation of PRRSV strains was monitored under practical conditions on Lithuanian pig farms where EU and NA genotype PRRS MLV vaccination was applied for a particular period of time. The results of this study gave precise and reliable local field data about some safety aspects of MLV use. Formerly, this knowledge was only based on some scientific references and on short-term experiments. The results of this study ensured better understanding of different spreading and persistence features of the EU and NA types of PRRSV MLV strains in the herds infected with the very diverse EU genotype field strains of PRRSV as well as of the level of heterological cross-protection between strains. These results will contribute to the development of more successful strategies and control programs against PRRS on the farms