Journal
VIRUSES-BASEL
Volume 13, Issue 1, Pages -Publisher
MDPI
DOI: 10.3390/v13010110
Keywords
NDV; evolution; virulence; host adaptation
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The hypothesis reviews the host adaptation in virulent Newcastle disease viruses and its accompanying virulence modulation. It suggests that current NDVs evolved from low virulence viruses in the 1920-1940s, giving rise to multiple virulent genotypes. Evolutionary changes have led to circulating velogenic-viscerotropic viruses being more transmissible and pathogenic in chickens compared to older velogenic-neurotropic viruses.
The hypothesis that host adaptation in virulent Newcastle disease viruses (NDV) has been accompanied by virulence modulation is reviewed here. Historical records, experimental data, and phylogenetic analyses from available GenBank sequences suggest that currently circulating NDVs emerged in the 1920-1940 ' s from low virulence viruses by mutation at the fusion protein cleavage site. These viruses later gave rise to multiple virulent genotypes by modulating virulence in opposite directions. Phylogenetic and pathotyping studies demonstrate that older virulent NDVs further evolved into chicken-adapted genotypes by increasing virulence (velogenic-viscerotropic pathotypes with intracerebral pathogenicity indexes [ICPIs] of 1.6 to 2), or into cormorant-adapted NDVs by moderating virulence (velogenic-neurotropic pathotypes with ICPIs of 1.4 to 1.6), or into pigeon-adapted viruses by further attenuating virulence (mesogenic pathotypes with ICPIs of 0.9 to 1.4). Pathogenesis and transmission experiments on adult chickens demonstrate that chicken-adapted velogenic-viscerotropic viruses are more capable of causing disease than older velogenic-neurotropic viruses. Currently circulating velogenic-viscerotropic viruses are also more capable of replicating and of being transmitted in naive chickens than viruses from cormorants and pigeons. These evolutionary virulence changes are consistent with theories that predict that virulence may evolve in many directions in order to achieve maximum fitness, as determined by genetic and ecologic constraints.
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