The Pathogenicity Of Fowl Adenovirus Serotype 4 To Ducks

Ankara disease,also known as hepatitis-hydropericardium syndrome(HHS),is caused by fowl adenovirus serotype 4(FAdV-4).In China,the disease sporadically occurred before2014,however,it had spread widely since June 2015.In 2017,a strain of FAdV-4 was successfully isolated by our lab which was named as FAdV-4 JS strain.Previous studies revealed that the virus was the main epidemic strain in China,and its pathogenicity to SPF chickens had been evaluated.We found that artificial-intramuscular injection could successfully replicate the disease in SPF chickens,with 90% mortality rate.Virus DNA could be detected in a variety of organs,throat swabs and cloacal swabs,which proved that FAdV-4 JS strain was hypervirulent to chickens.FAdV-4 can also infect ducks,geese,ostriches,mandarin ducks and other animals,which means that FAdV-4 is prone to harm duck breeding.Furthermore,there are several reports about successful isolation of duck-originated FAdV-4in China recently,in which the virus has been identified as the main epidemic FAdV-4 strain.However,compared to the study on the pathogenicity of different FAdV-4 strains to chicken,few studies were performed on other economical animals such as ducks and geese.Therefore,in order to estimate the pathogenicity of FAdV-4 JS strain to other kinds of poultry,and enrich the relevant research on the pathogenicity of FAdV-4 in China,provide more data for the prevention and control of FAdV-4 epidemic,the study was carried out to assess the pathogenicity of FAdV-4 JS strain to ducks of different ages.In this study,7-and 21-day-old experimental ducks were infected with the FAdV-4 JS strain by intramuscular injection into the legs,and the pathogenicity of the FAdV-4 JS strain in ducks of different ages and the response of the host organism to viral invasion were studied.Results showed that only mild clinical symptoms occurred after ducks challenging with the FAdV-4 JS strain during the whole study,but the weekly body weight gain of 7-day-old and21-day-old ducks both decreased after challenge with FAdV-4 JS strain.Within six weeks of continuous observation,the weight difference between the experimental group and control group gradually increased.After challenge,regular dissection(3 dpi,6 dpi,9 dpi,12 dpi,15dpi)was performed to observe the organ pathological changes.It was found that the visceral pathological changes became serious at 6 dpi.In detail,the liver was enlarged and discolored.The spleen was congested.The myocardium was pale and soft.And the kidney was swollen.In some cases,edema and bleeding of bursa of Fabricius,punctate hemorrhage of thymus and diffuse hemorrhage of leg muscles could also be seen.The increase of serum biochemical indexes and the fluctuation of m RNA transcription level of inflammatory factors in organs proved that there was inflammatory injury in the tissue.Histopathological examination and immunohistochemical staining of different organs at 6 dpi showed that the structure of hepatic cord disappeared,and hepatocytes vacuolated with basophilic inclusion bodies in cytoplasm.Many lymphocytes proliferated in the spleen,and the boundary of red pulp and white pulp was blurred.The lungs were congested with wider pulmonary interstitium.There was degeneration and necrosis of renal tubular epithelial cells with protein cast in tube and renal interstitium congestion.There was lymphocyte necrosis and density decreased in the medulla of bursa of Fabricius.After immunohistochemical staining,FAdV-4 specific reaction could be observed in different tissues.The results of virus detection in organs,throat swabs and cloacal swabs showed that the virus can infect different organs,and the viral load in kidney and spleen was relatively high.The virus could be detected in throat swabs and cloacal swabs at 1dpi,indicating that the virus could be transmitted by fecal-oral route.The serum FAdV-4 specific antibody level in challenged ducks increased in the first week after infection and reached the peak at the 3rd week(21-day-old ducks)and 4th week(7-day-old ducks),respectively.Some 7-day-old ducks were immunized with inactivated vaccine of avian influenza serotype H9 two weeks after challenge.HI titer changes of serum specific antibody against H9 in experimental ducks within 3 weeks after immunization showed that the vaccine could lead to similar level in immune response of ducks both in challenge group and control group,and there was no difference in serum HI titer between these two groups.At the end of the experiment,bursa of Fabricius of 7-day-old ducks was collected.It was found that the organ index and length of the challenge group were significantly lower than those of the control group.In summary,the FAdV-4 JS strain has certain pathogenicity to ducks,especially to the ducklings.Virus infection in ducks can cause mild clinical symptoms of the host,with massive organ damage,inhibition of immune organs development and production performance of the host.At the same time,there is the risk of virus transmission by fecaloral route,which poses a threat to infect other healthy animals in the same flock.