Molecular Characterization of Highly Pathogenic Marek's Disease Virus Strain in Vaccinated Iraqi Pullets.

Background: Marek's disease (MD) remains a severe economic concern in poultry industry globally despite decades of vaccination strategies. Particularly challenging are viral infections due to Marek's disease virus (MDV) (Gallid alphaherpesvirus 2, GaHV-2) strains that can escape immune defenses in areas that do not have access to molecular surveillance data. Objective: This study performed molecular epidemiology of MDV in Iraqi commercial poultry based on extensive characterization of circulating strains, quantification of subclinical infection in vaccinated flocks, and phylogenetic sequencing to determine evolutionary relationships with local isolates. Materials and Methods: A cross-sectional surveillance on five commercial layer pullet flocks given the CVI988 vaccine took place from March to June 2024. A combined sample of 250 birds (50 birds per flock) was collected from Alrashdia in Baghdad Province. In this study, estimates were made for samples based on 5% expected prevalence, 95% confidence level, and 3% precision. Pulpy feather follicles and tissues were isolated for molecular detection of MDV meq oncogene by PCR amplification with known primers. Sanger sequencing was performed later and phylogenetic reconstruction carried out using the maximum likelihood technique. Results: Prevalence of subclinical MDV was 3.2% (8/250) in surveyed flocks, with significant operational heterogeneity (0%-6%). One flock had an acute clinical outbreak, and 86.7% of birds afflicted during the surveillance period showed paralysis. Molecular analysis revealed genetically homogeneous strain (Iraq/Baghdad/2024) with 99.8% nucleotide identity to Iranian very virulent isolates, with clustering in the very virulent pathotype clade and high bootstrap support (98%). Conclusion: This research describes the circulation of a very virulent, genetically uniform GaHV-2 strain capable of inducing vaccine breakthrough in Iraqi commercial poultry. The phylogenetic proximity to regional isolates suggests transboundary virus movement as a possible route of attack and highlights a gap in the level of molecular surveillance and optimization of Middle Eastern poultry production systems in immunization programs.