Genetic Determinants Analysis of PB1 and PB2 Genes in H5N1 Avian Influenza Strains from Romania.

Avian influenza is a highly contagious viral disease affecting domestic and wild birds, caused by influenza A viruses of the family Orthomyxoviridae. Continuous genetic surveillance is essential for monitoring viral evolution and identifying molecular determinants associated with pathogenicity and host adaptation. In this study, the PB1 and PB2 polymerase genes of highly pathogenic avian influenza (HPAI) H5N1 viruses detected in Romania between 2017 and 2024 were genetically characterized. A total of 18 organ samples collected from domestic and wild birds in several Romanian counties were analyzed by real-time RT-PCR, followed by conventional RT-PCR, Sanger sequencing, mutational analysis, and phylogenetic reconstruction. High-quality sequence data were obtained for six PB1 and eight PB2 gene fragments. Sequence analysis revealed a high degree of conservation within the PB1 gene, particularly across regions involved in viral replication, indicating strong evolutionary constraints. In contrast, the PB2 gene exhibited several amino acid substitutions, including changes at positions 464 and 678 in selected isolates. Notably, all analyzed strains lacked the mammalian adaptation marker E627K. Phylogenetic analysis clustered the Romanian isolates within Eurasian H5N1 lineages and demonstrated close genetic relationships with contemporary strains circulating in Eastern Europe and Russia. These findings highlight the genetic stability of PB1, the ongoing diversification of PB2, and the importance of continuous molecular surveillance for the early detection of viral variants with potential implications for host adaptation and zoonotic risk.