Effect of phylogenetic diversity of velogenic Newcastle disease virus challenge on virus shedding post homologous and heterologous DNA vaccination in chickens.

Newcastle disease (ND) is a highly devastating disease for the poultry industry as it causes high economic losses. In this present study, a DNA vaccine containing the F and HN surface antigens of a highly virulent Newcastle disease virus (NDV), NDV/1/Chicken/2005 (FJ939313), was successfully generated. Cell transfection test indicated that the vaccine expressed the F and HN genes in Hep-2 cells. The main objective of this study was to compare the extent of protection induced by DNA vaccination after homologous and heterologous NDV-challenge as determined by the amount of NDV shedding after challenge. NDV-antibody-negative chickens were vaccinated either once, twice or thrice intramuscularly at 7, 14 and 21 days old and were challenged 14 days post vaccination with either homologous virus (vaccine-matched velogenic viscerotropic Newcastle disease virus (vvNDV) strain, FJ939313), phylogenetically related to group VII, or a phylogenetically divergent heterologous virus (unmatched vvNDV strain, AY968809), which belongs to genogroup VI and shows 84.1% nucleotide similarity to the NDV-sequences of the DNA vaccine. Our data indicate that birds, which received a single dose of the DNA vaccine were poorly protected, and only 30-40% of these birds survived after challenge with high virus shedding titre. Multiple administration of the DNA vaccine induced high protection rates of 70-90% with reduced virus shedding compared to the non-vaccinated and challenged birds. Generally, homologous challenge led to reduced tracheal and cloacal shedding compared to the heterologous vvNDV strain. This study provides a promising approach for the control of ND in chickens using DNA vaccines, which are phylogenetically closely related to the circulating field strains.