Phylogenetic analysis of Newcastle disease virus in Taiwan.

The virulent forms of Newcastle disease virus cause a devastating disease of poultry. Between 1998 and 2000, sporadic outbreaks of Newcastle disease occurred in Taiwan despite vaccination. The causes of the failure of the vaccination remain unclear. The purpose of this study was to investigate the possible factors causing these outbreaks by serologic and virologic methods. Anti-Newcastle disease virus hemagglutination-inhibition titers were measured for serum samples obtained from a breeder farm and a broiler farm. The serologic data showed continued presence of virulent Newcastle disease viruses in the field during inter-outbreak periods. Phylogenetic analysis demonstrated that the field virulent Newcastle disease viruses were genetically similar and were grouped into genotype VIIa. Efficacy testing by virulent Newcastle disease virus challenge revealed that the vaccines used were effective for protecting chickens from infections. This investigation demonstrated that the Newcastle disease virus strain can spread quickly and widely throughout a large geographic area, and that the sporadic cases originate from virulent Newcastle disease viruses present in the field.