Isolation and identification of a duck adenovirus B2 strain from Cherry Valley ducks

Abstract:

[Background] Pale liver disease caused by duck adenovirus B2 (DAdV B2) infection has been widely prevalent in Muscovy duck flocks in China since 2014, and no incidence has been observed in other duck breeds. In this study, two suspected cases of pale liver disease were reported in Cherry Valley ducks. [Objective] To identify the causative agent of pale liver disease in Cherry Valley ducks. [Methods] PCR or RT-PCR and pathogen isolation were carried out with the internal organ samples of diseases ducks. The isolate was identified, sequenced, and used for the challenge in ducklings. [Results] The internal organ samples were DAdV B2 positive, and a strain named DAdV B2/BG19 was successfully isolated. The LMH cells infected with BG19 developed a characteristic cytopathic effect (CPE) with cell enlargement, rounding, and final disintegration. The isolate BG19 carried two fiber genes, and its sequences of hexon, fiber-1, fiber 2, and DNA polymerase gene shared the sequence identity above 98.26% with those of the DAdV B2 isolates submitted in GenBank, whereas these genes had the sequence identity of 77.12%, 30.8%, 34%, and 99.78%, respectively, with those of the DAdV B1 isolate GR. All the strains of DAdV B1 and DAdV B2 shared the same clade in the phylogenetic tree built based on DNA polymerase gene. The mutation G133T in the ORF67 gene of BG19 resulted in a premature stop codon, which led to a truncated ORF67 protein 14 residues shorter than that of previous prevalent strain CH-GD-12-2014. BG19 was pathogenic to 2-day-old Cherry Valley ducklings and Muscovy ducklings, with the morbidities of 50.0% and 87.5% and the mortalities of 12.5% and 37.5%, respectively. The clinical symptoms and pathological changes of dead ducks were similar to those of clinical cases. [Conclusion] A highly virulent strain of DAdV B2 with truncated ORF67 protein expression was successfully isolated and identified from a case of pale liver disease in Cherry Valley ducks. This was the first report about the pathogenicity of DAdV B2 in Cherry Valley ducks.