Effect Of Duck Hepatitis A Virus Type 1 Attenuated Vaccine On Type 3 Virulent Virus Replication And Pathogenicity In Ducklings

To investigate the effects of DHAV-1 vaccine attenuated on the replication and pathogenicity of DHAV-3 virulent in ducklings,a series of trials were conducted to obtain the following results:1.Distribution of attenuated gene type 1 DHAV?DHAV-1?vaccine and DHAV-3virulent virus in ducklingsTotal 165 seven-day-old ducklings were randomly divided into A,B,C,and D groups.Thirty-five ducklings in group A received intramuscular injection of two DHAV-1 attenuated vaccines;Fifty ducklings in group B received intramuscular injection of two DHAV-1 vaccines and 2000LD₅₀ DHAV-3 vaccine?copy number with DHAV-1 vaccine?;Fifty ducklings in group C,each intramuscular injection of2000LD₅₀0 DHAV-3 virulent;Thirty ducklings in group D,as a blank control.At 1 h,3h,6 h,9 h,12 h,24 h,48 h,72 h,and 96 h after intramuscular injection,3 ducklings per group were taken at each time point,and liver,spleen,lung,kidney,and brain were collected.11 tissues of blood,liver,spleen,pancreas,kidney,lung,brain,thymus,Bursa of Fabricius,Harderian's glands and duodenum were detected randomly taken from three ducklings.The viral load of DHAV-1 and DHAV-3 in the sample were detected by RT-PCR.The results showed that DHAV-1 attenuated mainly in the duckling liver,kidney,spleen,thymus and bursa of Fabricius.DHAV-1 can be detected in all tissues and organs at 1h after the virus was infected,the virus content was about 10⁶copies/g.The content of virus decreased after 1⁶h,and the virus content gradually increased and reached its peak at 9⁷2h after the virus was inhaled(the highest virus content in the liver was 10^8.43copies/g),and the virus content decreased after 96 hours.DHAV-3 showed different degrees of proliferation in various tissues and organs;it was detected in tissues and organs at 1 h after challenge,in which the virus content in the liver,spleen,and kidney(10^6.34,10^5.56,and 10^5.03copies/g,respectively)was obvious.Compared with other tissues and organs,the virus content gradually increased from 1 to 24 h after challenge,and peaked at 24 to48 h after the challenge(the top three organs with higher virus content were liver,spleen,and kidney,which were 10^11.15,10^10.37and 10^10.30copies/g,respectively)then the virus content slowly decreased.This shows that the strong toxicity of DHAV-3 to duckling tissue is stronger than that of DHAV-1 vaccine,and the DHAV-3 virulent virus has higher proliferation efficiency in ducklings.2.Effect of attenuated DHAV-1 vaccine on DHAV-3 virulent replication in ducklingsThe proliferation of DHAV-3 virulent was similar in the two groups of ducklings.There was no significant difference in the content of DHAV-3 in all tissues and organs at most time points.It was shown that the DHAV-1 vaccine had only a slight effect on the proliferation of DHAV-3.3.Effect of DHAV-3 virulent virus on the replication of DHAV-1 vaccine in ducklingsThe attenuated DHAV-1 vaccine showed no significant proliferation in ducklings in the mixed infection group.The DHAV-1 content in the infected group was significantly higher than that in the mixed infection group.This shows that DHAV-3virulent can significantly inhibit the attenuated DHAV-1 vaccine proliferation in ducklings.4.Effect of attenuated DHAV-1 vaccine on the virulent virulence of DHAV-3There was no significant difference in clinical symptoms and lesions between DHAV-3 virulent and infected ducklings alone.The death of mixed infected ducks was concentrated in 24 to 30 hours,and the liver damage repair was better in surviving ducks 72 to 96 hours than in infected ducks alone.The expression levels of IL-1?/6 and IFN-?/?in liver were significantly higher than the level of those in infected ducks alone;IFN-?was higher in ducklings infected alone;IL-2 was higher in mixed ducks 6 to 12 hours after infection.Therefore,attenuated DHAV-1 vaccine may regulate the expression of cytokines in the liver,accelerate the repair process of liver injury,and then reduce the pathogenicity of DHAV-3 virulent on ducklings.The DHAV-1 vaccine immunized one-day-old ducklings 7 days after infection with DHAV-3 virulent,and the protection rate was 10%to 30%.5.Effect of age on the pathogenesis of DHAV-3Artificial infection of ducklings with DHAV-3 revealed that the mortality rate gradually decreased as the young ducklings increased in age.The mortality rate of ducklings on days 1,4 and 7 was 100%,70%and 40%,respectively.The LD₅₀ of the DHAV-3 QL strain was 10^-4.13/0.2 ml for 4-day old ducklings.