Characterization of structural proteins of duck-plague virus

Abstract

Two virulent strains of duck plague virus - DPV-I (IVRI) and DPV-A (Alleppey isolate) and a vaccine strain - DPV-V (VBI Palode) were investigated for the differences in clinical manifestations in naturally and experimentally infected ducks, morphological changes in developing duck/chicken embryo (DDE/DCE) and cytopathic effects in duck embryo fibroblast/ chicken embryo fibroblast culture (DEFC/CEFC) and chicken embryo fibroblast culture (CEFC). Typical symptoms and lesions of duck plague were produced by both the virulent strains. However, in DPV-A infection, the level of mortality and severity of lesions like gizzard muscle necrosis and haemorrhagic bands in the small intestine were more pronounced. DPV-V did not produce any symptoms or lesions on experimental inoculation into ducklings. Embryonated duck eggs were used for passaging DPV-l and isolating DPV-A, while embryonated chicken eggs were used for propagating DPV-V. All the three strains produced mortality of embryos with congestion on CAM and body of the embryo. DPV-A produced more congestion on the extremities of the embryos.

Duck embryo fibroblast cultures were used for culturing the virulent strains while chicken embryo fibroblast cultures were used for culturing the vaccine strain. All the three strains produced characteristic CPE, with rounding and clumping of cells, syncytium formation, vacuolation of cytoplasm and formation of intranuclear inclusion bodies. The time for the production of CPE decreased on successive passage. Titration of the three strains of DPV was done in embryonated eggs (ELDso) and cell cultures (TCIDso). DPV-I, DPV-A and DPV-V had an ELD50 of 105.27, 104.86 and 104 per ml respectively. TCIDso of DPV-I and A in DEF culture were 105.75 per ml and 105.25 per ml respectively and that of DPV-V in CEFC was 104.5 per ml. The tissue culture system gives the best titre than the embryonating eggs for all the three DPV strains. On electron microscopy, the field isolate of DPV showed particles ranging from 170-190 nm in diameter. Protein analysis of the virulent strains viz. DPV-I and DPV-A by SDS-PAGE revealed sixteen and fourteen proteins respectively. Mild difference of two proteins (VP17 and VP22) was noticed between the two strains. DPV-A lacked the 28KD and 9KD protein bands.

The vaccine strain DPV-V on electrophoresis showed a different pattern of protein bands from the virulent strains. Eighteen proteins could be resolved in the vaccine strain. The molecular weight of sixteen proteins of DPV-I ranged from 9KD to 107KD while the fourteen proteins of DPV-A ranged from l4KD to 107KD. The proteins of the vaccine strain also ranged from 9 KD to 107 KD. The possibility of the occurrence of strain variation as indicated by the difference in the protein patterns of the DP viruses under study is discussed.