Identification Of The Co-contamination Of Fowl Adenovirus And Chicken Infectious Anemia Virus In Newcastle Disease Virus-Attenuated Vaccine And Pathogenicity Analysis Of Artificial Contaminated Vaccine

Newcastle disease virus(NDV)-attenuated vaccine has been widely used since the 1950 s and made great progress in preventing and controlling Newcastle disease.However,many reports mentioned exogenous virus contamination in attenuated vaccines,causing various diseases in vaccinated poultry.This study also found the co-contamination of fowl adenovirus type 4(FAdV-4)and chicken infectious anemia virus(CIAV)in NDV-attenuated vaccine using PCR-combined dot blot assay,and then isolated those viruses by serum neutralization and chicken embryo culture.After then,this study sequenced the whole genome of CIAV and partial hexon gene of FAd V-4,and homology and phylogenetic analysis showed that both of those viruses shared similar characteristics with corresponding epidemic strains in China.In order to access the pathogenicity of such co-contaminated vaccines,we prepared artificial contaminated vaccine and immunized SPF and FAd V maternal antibody-positive chickens to carry out systematic animal experiments.Results showed that no chicks died after using the NDV-attenuated vaccine without contamination or singly contaminated with CIAV,but a mortality of 75% with inclusion body hepatitis-hydropericardium syndrome(IBH-HPS)was commonly found in birds after administering the NDV-attenuated vaccine co-contaminated with FAd V-4 and CIAV,higher than mortality caused by FAd V-4 contaminated vaccine.At the same time,the exogenous virus infected by vaccine contamination can proliferate continuously in chickens,cause growth retardation and thymus atrophy,and then cause immunosuppression and significantly reduce NDV antibody level.Besides,NDV-attenuated vaccine also played an important role in the pathogenicity of contaminated exogenous viruses,while the pathogenicity of FAd V-4 and CIAV co-infection through the contaminated vaccine was significantly higher than that of direct oral infection.The synergistic reaction of these viruses and La Sota prompted their multiplication in vivo and disturbed the production of antibodies against each other.Meaning while,excessive La Sota viral load in various organs and the presence of exogenous viruses stimulated the pathogenicity of vaccine strains,which led to typical ND symptoms such as intestinal lymphoid follicle haemorrhage and swelling and glandular stomach papilla haemorrhage in immunized chickens,and eventually further aggravated the disease.Maternal antibody is an important barrier to protect chickens from viruses at early stage.However,in this study,the maternal FAd V antibody level increased gradually and peaked at three days age,but then dropped swiftly,and ultimately to the lowest level at the time of vaccination,which only inhibited FAd V infection alone but failed to protect chickens against co-contaminated vaccine.Results showed that the CIAV helped FAd V break the protection of maternal antibody by destroying the immune system,and finally led to IBH-HPS with about 15% mortality.According to clinical statistics,IBH-HPS also broke out in some farms after using NDV attenuated vaccine without exogenous virus contamination,but the incidence of IBH-HPS was lower in the same batch of non-immunized chickens.This study found that NDV-attenuated vaccine can promote the replication of FAd V and CIAV in the host,thus significantly enhance the pathogenicity of congenital infection and accelerate the course of disease,trigger obvious anemia symptoms and lead to IBH-HPS in young chickens.