СЕРОЛОГИЧЕСКИЙ МОНИТОРИНГ И МОЛЕКУЛЯРНО-ГЕНЕТИЧЕСКАЯ ХАРАКТЕРИСТИКА ВИРУСА БОЛЕЗНИ НЬЮКАСЛА В КАЗАХСТАНЕ

Newcastle disease remains one of the most important viral diseases of poultry and requires continuous surveillance in intensive poultry production systems, including routinely vaccinated flocks. The aim of this study was to assess the serological status of poultry and to perform molecular-genetic characterization of Newcastle disease virus circulating in commercial poultry farms in Kazakhstan. Serological monitoring was carried out using the hemagglutination inhibition (HI) test; antibody titers were expressed as log₂ values, and serum samples with titers ≥3 log₂ were considered seropositive. Viral detection was performed using real-time RT-PCR followed by molecular pathotyping of positive samples. Selected samples were subjected to targeted sequencing using the Oxford Nanopore Technologies platform, followed by phylogenetic analysis of a fragment of the F gene. HI testing revealed heterogeneity in specific antibody levels and in the proportion of seropositive birds among the examined production and age groups. Molecular screening confirmed the presence of Newcastle disease virus RNA in a proportion of the tested samples and demonstrated differences in molecular pathotypes. Phylogenetic analysis showed that sequences NDV_PF08_138, PF08_143, PF08_144, and PF08_145 clustered within genotype II together with lentogenic vaccine and vaccine-like strains, whereas sequence NDV_PF03_17 clustered with representatives of genotype VII. The findings demonstrate the need for integrated interpretation of serological and molecular data when monitoring Newcastle disease in vaccinated poultry flocks. The simultaneous detection of vaccine-like genotype II variants and genotype VII highlights the importance of molecular typing of positive samples. Further studies should focus on obtaining full-length F gene sequences and whole-genome sequencing of field NDV variants to clarify their genetic relationships, transmission pathways, and circulation dynamics in Kazakhstan.