Emergence and characterization of historically extinct virulent genotype IV Newcastle disease virus in wild and domestic birds: genetic insights, pathogenicity, and vaccine efficacy.

Class II genotype IV Newcastle disease virus (NDV), a historically virulent strain responsible for the first Newcastle disease (ND) panzootic from the 1940s to 1960s, was presumed extinct after its last reported isolate in India in 2000. Here, we report the emergence of four virulent genotype IV NDV isolates from 6,731 wild birds and domestic poultry across nine provinces of China between 2021 and 2023, representing the first genetically confirmed isolation of this ancestral genotype in over two decades. These isolates exhibit remarkably high genetic similarity to ancestral strains, showing minimal divergence despite a temporal span of 50-90 years, yet they differ from the most recently reported isolate from India. Infection with the representative isolate, KS02, caused severe lethality and higher transmissibility in specific-pathogen-free chicks than a genotype VII reference virus. Notably, LaSota vaccination provided only limited protection at the conventional hemagglutination inhibition (HI) threshold and achieved complete protection only when HI titers were at least twofold higher, in contrast to the protection observed against genotype VII challenge. This unusual genetic stability raises concerns about the origin and evolutionary history of these viruses as well as highlights the urgent need to update vaccination strategies, including approaches to elicit higher HI titers through intensified immunization schedules. This study underscores the critical importance of sustained global surveillance of genotype IV NDV in domestic poultry and migratory birds to monitor its spread and evolution.IMPORTANCEVirulent Newcastle disease virus (NDV), particularly emerging isolates, poses a major threat to poultry health and production, causing severe morbidity, high mortality, and economic losses. While ancestral class II genotypes II and IX have persisted globally across various bird species, the status of genotype IV NDV-last reported in India in 2000-had been uncertain. This study documents the emergence of genotype IV isolates in wild and domestic birds across China from 2021 to 2023, marking their return after more than two decades of presumed extinction. The representative isolate, KS02, showed severe lethality and high transmissibility in chicks compared to the circulating virus. The LaSota vaccine conferred complete protection against this isolate only when HI titers were at least twofold above the conventional protective threshold. These findings underscore the significant risk posed by reemerging genotype IV NDV, highlighting the urgent need for surveillance and updated vaccination strategies.