Comparative pathogenicity of a genotype XXI.1.2 pigeon Newcastle disease virus isolate in pigeons and chickens

Here, we performed molecular and pathogenic characterization of a Newcastle disease virus (NDV) isolate from pigeons in Bangladesh. Molecular phylogenetic analysis based on the complete fusion gene sequences classified the three study isolates into genotype XXI (sub-genotype XXI.1.2) together with recent NDV isolates obtained from pigeons in Pakistan (2014-2018). The Bayesian Markov Chain Monte Carlo analysis revealed that the ancestor of Bangladeshi pigeon NDVs and the viruses from sub-genotype XXI.1.2 existed in the late 1990s. Pathogenicity testing using mean embryo death time pathotyped the viruses as mesogenic, while all isolates carried multiple basic amino acid residues at the fusion protein cleavage site. Experimental infection of chickens and pigeons revealed no or minimum clinical signs in chickens, while a relatively high morbidity (70%) and mortality (60%) were observed in pigeons. The infected pigeons showed extensive and systemic lesions including hemorrhagic and/or vascular changes in the conjunctiva, respiratory and digestive system and brain, and atrophy in the spleen, while only mild congestion in the lungs was noticed in the inoculated chickens. Histologically, consolidation in the lungs with collapsed alveoli and edema around the blood vessels, hemorrhages in the trachea, severe hemorrhages and congestion, focal aggregation of mononuclear cells, and single hepatocellular necrosis in the liver, severe congestion, multifocal tubular degeneration, and necrosis, as well as mononuclear cell infiltration in the renal parenchyma, encephalomalacia with severe neuronal necrosis with neuronophagia were noticed in the brain in infected pigeons. In contrast, only slight congestion was found in lungs of the infected chickens. qRT-PCR revealed the replication of the virus in both pigeons and chickens; however, higher viral RNA loads were observed in oropharyngeal and cloacal swabs, respiratory tissues, and spleen of infected pigeons than the chickens. In conclusion, genotype XXI.1.2 NDVs are circulating in the pigeon population of Bangladesh since 1990s, produce high mortality in pigeons with pneumonia, hepatocellular necrosis, renal tubular degeneration, and neuronal necrosis in pigeons, and may infect chickens without overt signs of clinical disease and are likely to shed viruses via the oral or cloacal routes.