CONCLUSION

This study provides the first comprehensive molecular, pathological, and phylogenetic characterization of naturally circulating lentogenic genotype II NDV in young ostriches in Egypt. Molecular analysis identified genotype II, class II NDV carrying the characteristic lentogenic F protein cleavage-site motif (¹¹²GRQGRL¹¹⁷), which was further confirmed by biological pathotyping (MDT = 96 h and ICPI = 0.4). Despite its lentogenic nature, the virus produced evident clinicopathological changes, including respiratory, hepatic, gastrointestinal, and lymphoid lesions, demonstrating that lentogenic NDV strains can induce clinically relevant disease in susceptible young ostriches under field conditions. Phylogenetic analysis revealed that the isolate was closely related to genotype II vaccine-like strains, with 97%–99% sequence identity, yet genetically distinct from the virulent genotype VII strains currently circulating in Egyptian poultry. In addition, no evidence of recombination was detected within the analyzed F gene fragment.

These findings have important practical implications for disease surveillance and control in the expanding ostrich industry. The close genetic relationship between the identified isolate and vaccine-related strains highlights the need for continuous molecular surveillance to differentiate field and vaccine-derived viruses, evaluate vaccine performance, and improve biosecurity practices in ostrich production systems. Enhanced monitoring of interactions among ostriches, commercial poultry, backyard flocks, and wild birds is also essential to reduce the risk of virus dissemination.

A major strength of this study is the integration of virus isolation, biological pathotyping, molecular detection, phylogenetic characterization, recombination analysis, and detailed histopathological evaluation, providing a comprehensive understanding of NDV infection in naturally infected ostriches. Nevertheless, the study was limited by the relatively small number of sequenced isolates and the use of partial F gene sequencing rather than complete genome analysis. Future studies incorporating whole-genome sequencing, larger sample sizes, longitudinal surveillance, and investigation of vaccine-derived virus transmission dynamics will further clarify the epidemiology and evolution of NDV in ostriches.

Overall, the present findings improve the current understanding of NDV infection in ostriches and provide valuable baseline information for molecular epidemiological surveillance, vaccine evaluation, and the development of effective prevention and control strategies to support the sustainable growth of the ostrich industry in Egypt.