Occurrence of Newcastle Disease Genotype II and VII.2 strains in Vaccinated and Unvaccinated Commercial and Backyard Poultry Flocks in Lusaka Province, Zambia

Newcastle disease (ND) remains one of the most devastating infectious diseases of poultry worldwide, causing substantial economic losses despite the widespread use of vaccination programs. In Zambia, recurrent outbreaks continue to be reported. This study aimed to characterise the pathological features and molecular diversity of Newcastle disease virus (NDV) circulating in poultry outbreaks in Lusaka Province, Zambia. A prospective and retrospective investigation of suspected ND outbreaks reported between 2020 and 2023 was conducted. A total of 89 poultry outbreaks involving indigenous chickens, broilers, layers, and pullets were investigated. Gross pathology and histopathological examinations were performed on affected birds, followed by molecular detection of NDV using reverse transcription polymerase chain reaction (RT-PCR). Partial fusion (F) gene sequencing and phylogenetic analysis were used to determine circulating genotypes. Gross lesions were consistent with ND but lacked specificity, while histopathology revealed predominantly necro-haemorrhagic lesions affecting lymphoid tissues, gastrointestinal tract, and respiratory organs, with variable severity across production systems. Newcastle disease virus was detected in both vaccinated and unvaccinated flocks, with no statistically significant difference in detection rates between the two groups (p > 0.05). Molecular analysis identified the circulation of virulent NDV genotype VII.2, alongside genotype II strains closely related to vaccine lineages. The detection of virulent NDV genotypes in vaccinated flocks highlights persistent gaps in Newcastle disease control in Zambia. These findings underscore the need for strengthened molecular surveillance, improved vaccination strategies, and enhanced biosecurity to reduce the burden of ND in poultry production systems.