Genetic Stability And Protective Efficacy Of Recombinant Fowlpox Virus Vaccines Expressing F And HN Genes Of Newcastle Disease Virus

Newcastle disease (ND) is one of the most important avian diseases that can cause severe economic losses in the poultry industry the world over. The causative agent of the disease, Newcastle disease virus (NDV), isamember of the Paramyxovirus and contains a non-segmented negative strand RNA genome, which encodes two envelop glycoproteins, the fusion(F) protein and the hemagglutinin-neuraminidase(HN) protein. The F protein is required for virus-cell fusion in the process of infection and has been shown to be capable of inducing protective immunity to NDV, first in a vaccina virus vector and more recently in a fowlpox virus vector. Vaccines against Newcastle disease in current use include live vaccines consisting of avirulent or attenuate strains of the virus and inactivated vaccines comprising killed virus from avirulent or pathogenic strains. Although these vaccines have largely controlled the disease in the field, they inherit disadvantages of interfering serological surveillance or incomplete inactivation. Hence there is currently great interest in assessing the possibility of developing a new generation of vaccines based on expression of virus components in live vaccine vectors. In this study we described the genetic stability and protective efficacy of the recombinant fowlpox virus vaccines expressing F and HN gene of NDV.These two recombinant fowlpox virus vaccines were serially passaged to the 20th on specific-pathogen-free (SPF) chickens embryo fibroblast (CEF) monolayers. The F and HN genes from passages 0, 10 and 20 were amplified bypolymerase chain reaction (PCR) and sequenced. The expression of F and HN genes from passages 0,10 and 20 was identified by indirect-immunofluo-rescence assay (IF). The results showed that these two vaccines were genetically stable for at least 20 passages.To evaluate their protective efficacy against Newcastle disease, these two vaccines were administered to specific-pathogen-free (SPF) chickens at 1 day old. Though the mean titer of hemagglutinin-inhibition test of each rFPV-HN vaccinated group was lower than that of inactivated oil vaccine group before challenging, nearlly all chickens vaccinated with rFPV were protected from intramuscular challenge of virulent NDV F48E821 days post-vaccination. Levels of antibody to NDV-F protein detected by indirect-ELISA and competitive-ELISA showed that these two assays were simple, specific, rapid and the result could be judged accurately. To examine these two vaccines were also immunogenic in commercial chickens with maternal antibody, Commercial broiler chickens and layers were vaccinated with rFPV-F and rFPV-HN and tested for the protective efficacy against NDV. Only low levels of antibody were induced with some protection when challenged with the virulent NDV strain. Maternal antibodies against NDV may decrease the amount of rFPV in immunized commercial chickens through antibody neutralization or antibody- dependent cell-mediated cytotoxicity. Chickens vaccinated with wild type FPV or recombinant FPV-NDV had lower body weight, whereas chickens vaccinated with mixture of rFPVs expressing IFN- n and NDV gene had significantly higher weight. This is in agreement with the observations of Other workers that FPV causes body weight losses in chickens and rFPV-IFN may act as adjuvant to reduce harmful side effects. In order to obtain better protection in commercial chickens, research aiming at improving the present rFPVs is now under way.