Molecular epidemiology of Newcastle disease virus isolates from vaccinated commercial poultry farms in non-epidemic areas of Japan.

BACKGROUND: Newcastle Disease (ND) is a highly contagious and economically devastating disease of poultry. At present, limited molecular epidemiological data are available regarding the causes of ND outbreaks in vaccinated commercial poultry farms. Knowing the genomic characteristics of Newcastle disease virus (NDV) infecting commercial poultry operations in spite of vaccination might give important insights on the infection dynamics of these viruses. In addition, molecular analyses at the subgenotype level and studies on the relationship of Japanese NDVs with other isolates from around the world are lacking. Therefore, in the present study, a molecular epidemiological investigation was conducted to characterize nine NDVs isolated from vaccinated commercial poultry flocks in five different Prefectures in non-epidemic areas of Japan between 1969 and 2002.

METHODS: Nucleotide sequencing and phylogenetic studies were performed to characterize the complete fusion (F)-protein gene, 3-prime end of the nucleoprotein (NP)-gene and 5-prime end of the RNA dependent RNA polymerase (L)-gene. Sequence data were compared with 180 NDV strains from GenBank representing different NDV genotypes and subgenotypes from different regions of the world at different time periods. Deduced amino acids were analyzed for homologies, recombination and mutation. Recombination events were estimated using Recombination Detection Program (RDP) version 3.44. Phylogenetic trees were constructed to determine evolutionary relationships among strains.

RESULTS: Mean death time (MDT: 48-56 hr), Intracerebral Pathogenicity Index (ICPI: 1.7-1.9) and deduced amino acid sequences of the F0 proteolytic cleavage site (112RRQKR116) revealed that all nine field isolates were velogenic. Phylogenetic analysis showed that these isolates could be classified into two genetic lineages and three sublineages namely genotypes VIa (lineage 4a), VId (lineage 4d) and VIId (lineage 5d). No recombination events were observed but a point mutation in one of the neutralizing epitope of the F-protein was identified in the field isolates from Japan.

CONCLUSIONS: All field isolates from vaccinated commercial poultry in non-epidemic areas of Japan were part of much bigger outbreaks in provinces and regions and, in some cases, continents. In general, four ND panzootics occurred in Japan and that these outbreaks were mostly characterized by co-circulation of genetically distinct virus lineages due to involvements of infected wild birds. The point mutation identified in the field isolates from Japan may be due to escape from vaccine pressure. The identification of such mutation may be useful for future site-directed mutagenesis to understand the dynamics of NDV infection in vaccinated chickens.