Evolutionary changes affecting rapid identification of 2008 Newcastle disease viruses isolated from double-crested cormorants.

A morbidity-mortality event involving virulent Newcastle disease virus (NDV) in wild double-crested cormorants (Phalacrocorax auritus) occurred in North America in the summer of 2008. All 22 viruses isolated from cormorants were positively identified by the USDA-validated real-time reverse transcription-PCR assay targeting the matrix gene. However, the USDA-validated reverse transcription-PCR assay targeting the fusion gene that is specific for virulent isolates identified only 1 of these 22 isolates. Additionally, several of these isolates have been sequenced, and this information was used to identify genomic changes that caused the failure of the test and to revisit the evolution of NDV in cormorants. The forward primer and fusion probe were redesigned from the 2008 cormorant isolate sequence, and the revised fusion gene test successfully identified all 22 isolates. Phylogenetic analyses using both the full fusion sequence and the partial 374-nucleotide sequence identified these isolates as genotype V, with their nearest ancestor being an earlier isolate collected from Nevada in 2005. Histopathological analysis of this ancestral strain revealed morphological changes in the brain consistent with that of the traditional mesogenic pathotypes in cormorants. Intracerebral pathogenicity assays indicated that each of these isolates is virulent with values of >0.7 but not more virulent than earlier isolates reported from Canada.