Pathogenicity Of Avian Group Ⅰ Adenovirus Serotype 8b In Ducks

Fowl adenovirus（FAd V）is an unenveloped double-stranded DNA virus belonging to the adenoviridae,avian adenovirus,which can be divided into three subgroups:I,II and III.Among them,the I subgroup FAd V can be divided into five subgroups with 12 serotype,A（1 type）,B（5 type）,C（4,10 type）,D（2,3,9,11 type）,E（6,7,8a,8b type）.From the current epidemic form,there are many types of FAd V serotypes in the I subgroup,and the current study has found that its pathogenic mechanism is not clear,and there are many problems in the study of pathogenicity on ducks.FAd V in subgroup I can cause duck pericardial effusion-hepatitis syndrome,which is characterized by sudden death in meat ducks around 20 days of age,pericardial hydrops,slow growth,and strong morbidity and mortality.In this experiment,135 healthy 1-day-old ducklings were randomly divided into 9 groups of 15 ducklings in each group,and they were kept in isolation.Artificial infection using FAd V-8b virus was administered orally and subcutaneously in 10-day-old,15-day-old,and 20-day-old meat ducks,each vaccinated with 1 m L of cytotoxicity containing 10^5.5 TCID₅₀,and the control group was injected with normal saline in the same proportion.After vaccination,we continued to observe for 12 days and recorded the incidence of ducks in each group.After observation at 3,5,7,9 and 12 dpi,the ducks were observed in detail,their weight was also measured,and their onset,mental status,clinical manifestations,autopsy results,and histopathological changes were recorded.The detection of cytokines in each group of blood,the detection of viral load in each tissue by fluorescence PCR detection method,the determination of the titer of antibodies by enzyme-linked immunosorbent assay and other indicators were analyzed for pathogenicity.Specific results showed that the virus was widely distributed in ducks,could invade multiple organs,10-day-old ducks were infected with FAd V-8b,some ducks showed obvious clinical responses,poor mental state,decreased appetite,slower growth rate and death in ducks in the 5 dpi muscle group,with a mortality rate of 20%,while the mortality rate in the oral group was about 6.6%.Ducks aged 15 and 20 days old were infected with FAd V-8b,and the clinical symptoms were milder than those in the 10-day-old challenge group,and the mortality rate was lower than that in the 10-day-old challenge group.After autopsy of the lesions,the color change of the liver of the dead duck was obviously earthy yellow,swollen and brittle,and the lung congestion and bleeding were serious.At 15 days of age,the 15-day-old attack group had obvious typical lesions such as pericardial effusion.The autopsy changes in the 20-day-old challenge group showed that the symptoms were milder than those in the 10-day-old and 15-day-old anti-virus groups.Pathological tissue sections showed that the liver,lungs,kidneys and other organs of ducks in each attack group had different degrees of pathological damage,and a large number of lymphocytes of the spleen and bursa were lost,among which the liver pathological changes in the muscle group were the most obvious,hepatocyte hemorrhage accompanied by cell structure disorders,and diffuse steatosis between liver cells.Extensive congestion of the kidneys,glomerular swelling and bleeding,tubulointerstitial hemorrhage.After quantitative analysis by fluorescence,the copy number of the virus can be measured in the heart,liver,spleen,lung,kidney,thymus and other organs of 10-day-old ducks,among which the liver,kidney,spleen,bursa have the highest virus content,which is consistent with the degree of histopathological lesions,while the ducks in the 15-day-old and 20-day-old attack groups,although the virus can be detected,but the content of the virus has been significantly reduced.After testing,it was found that the content of IL-4 and IL-6 in the attack group was significantly higher than that in the control group.The antibody titer results showed that the older the day,the higher the antibody titer,the stronger the animal resistance,the 20-day-old antibody titer was higher than that of the 10-day-old and 15-day-old attack groups,and the muscle group had a higher antibody titer than the oral group.In summary,the isolate has strong pathogenicity,FAd V-8b is pathogenic to 10-day-old,15-day-old and 20-day-old meat ducks,and the susceptibility is different at different ages,the younger the age,the weaker the resistance,and the ducks are more susceptible to poison.This isolate can cause severe damage to the duck’s immune system,resulting in reduced resistance,and after infection with avian adenovirus,it will be mixed with other viruses at the same time,thereby increasing the mortality of ducks.Through this study,we provide important theoretical support for the pathogenicity of FAd V-8b in meat ducks,and provide strong support for the sustainable development of poultry industry.