Abstract
BACKGROUND: Porcine reproductive and respiratory syndrome virus (PRRSV) exhibits a marked geographical clustering, with PRRSV-2 being predominantly found in North America and Asia. Its presence in Europe remains extremely limited and mainly represented by vaccine-like strains, belonging to sublineage 5.1. While the use of this vaccine is possible and applied in Northern Europe, its detection in Southern European countries has been largely anecdotal. RESULTS: In the present study, we report the first confirmed case of PRRSV-2 detection in Italy, identifying two independent introduction events with no clear epidemiological link. A deeper evaluation of the overall epidemiological end evolutionary patterns of MLV-derived strains was performed to elucidate the present study findings. The phylogenetic and molecular analyses revealed significant genetic variability among vaccine-derived strains, with pairwise genetic distances exceeding 8% and an estimated evolutionary rate of ~10(- 3) substitutions/site/year, comparable to field strains. These findings suggested ongoing viral evolution and persistent circulation since the vaccine introduction. Signatures of diversifying selection, particularly in ORF5, indicated adaptation to different host populations or immune environments. Moreover, phylogeographic analysis supported multiple independent introduction events of independently evolved strains rather than a single introduction followed by local evolution. CONCLUSIONS: Contrary to expectations, PRRSV-2 vaccine-like strains exhibited transmission dynamics comparable to field strains. Their divergence, potential for adaptation, and reversion to virulence raise concerns about their long-term epidemiological and clinical impact. Given the scarcity of PRRSV-2 field strain reports in Europe, further surveillance and sequencing efforts are crucial to assess its true prevalence, evolutionary potential, and implications for swine health.