Repeated isolation of virulent Newcastle disease viruses of sub-genotype VIId from backyard chickens in Bulgaria and Ukraine between 2002 and 2013.

Here， we report the circulation of highly related virulent Newcastle disease viruses (NDV) in Bulgaria and Ukraine from 2002 until 2013. All of these NDV isolates have the same virulence-associated cleavage site ("RQKR↓F")， and selected ones have intracerebral pathogenicity index values ranging from 1.61 to 1.96. These isolates are most closely related to viruses circulating in Eastern Europe， followed by viruses isolated in Asia during the same period of time. Interestingly， the majority of the viruses were isolated from backyard poultry， suggesting the possibility of a "domestic" or "urban" cycle of maintenance. The molecular characterization of the nucleotide sequence of the complete fusion protein gene of the studied viruses suggests continued circulation of virulent NDV of sub-genotype VIId in Eastern Europe， with occasional introductions from Asia. Furthermore， the high level of genetic similarity among those isolates suggests that the NDV isolates of sub-genotype VIId from Bulgaria and Ukraine may have been part of a broader epizootic process in Eastern Europe rather than separate introductions from Asia or Africa. The continuous monitoring of backyard poultry flocks for the presence of circulating virulent NDV strains will allow early identification of Newcastle disease outbreaks.