Pathogenicity Of Group ? Fowl Adenovirus Serotype 4 In Cherry Valley Ducks

Hydropericardium hepatitis syndrome(HHS)is a fowl disease caused by group ?fowl adenovirus serotype 4(FAV4),and causes sudden death of 3~5 week meat chicken.The major features are accompanied pericardial effusion and inclusion body hepatitis.The occurrence and death rate are both high.In 1987,the disease was firstly reported in Angola of Pakistan;Later,it was reported in Mexico in 1989 and then in India,Russia,South Korea,Japan and other countries.The disease has been exiting in meat chicken flock in China,but no extensive epidemic has been shown.FAV4 has broken out in several Chinese provinces,and infection host range kept expanding.Especially for commercial meat ducks,it presents strong pathogenicity.Enhancing research on the disease is significant for advancing healthy development of aviculture.A total of 75 healthy Cherry Valley ducks(15 days)were randomly divided into the oral group(n=25),the subcutaneous group(n=25)and the control group(n=25).The ducks were fed under quarantine condition.The ducks(15 days old)were artificially infected via oral and subcutaneous administration of isolated FAV4,and every duck was inoculated with 10-7.569EID50/1ml allantoic fluid.Ducks of the control group were inoculated with 1m L PBS with same routes.After the challenge,the ducks were continuously observed for 15 d and conditions concerning disease onset were recorded.The ducks were weighed at 3,d,9,12 and 15 d after the challenge.Ducks from the challenged groups and the controlled groups(5 each)were randomly killed and dissected;clinical symptoms,dissection change and histopathological changes were recorded;blood was sampled to test blood biochemical indexes and cellular factors;quantitative real-time PCR method was adopted to test viral load in tissue;ELISA was used to determine valence of serum antibody.The challenge dosage and experimental method for the 25d-old ducks were same as that of 15d-old ducks.It is indicated by clinical symptoms that after infection with FAV4,some of the 15d-old ducks presented severe clinical symptoms,such as appetite decrease,white liquid manure and emaciation,while FAV4-infected 25d-old ducks presented milder symptoms than 15d-old infected ducks.It was indicated by dissection that 15d-old infected ducks presented pericardial effusion,yellow staining of coronary fat,liver enlargement with fragile texture and bleeding,pulmonary edema and congestion,kidney enlargement and bleeding,intestinal track bleeding,atrophy of bursa of Fabricius,thymus atrophy with surface hemorrhagic spot,and other lesions.It can be viewed by observing histopathological slide that hepatic cells show diffusive vesicular degeneration and fatty degeneration,necrosis of liver cells,intranuclear inclusion bodies in liver cells,glomerulus enlargement and generation,narrowing of glomerulus cavity,detachment of renal tubular epithelial cells from basilar membrane,kidney tubules mesenchyme bleeding,lymphocyte invasion in kidney tubules mesenchyme,substantial loss and decreased density of white pulp lymphocytes and formulation of a blank zone in bursa of Fabricius,congestion of small intestine lamina propria,mucous epithelial cellular degeneration and necrosis,and congestion,bleeding and lymphocyte invasion-caused inflammatory focus of liver.With the increase of age(in day),the 25d-old ducks showed less severe dissection symptoms and histopathological lesion.It was indicated by blood biochemistry that ALT,AST,ALP,UREA,LDH all showed a rising-to-decreasing trend and were all significantly different with the control group(P<0.05).IFN-? and IL-2 levels in the challenge groups were significantly higher than the control group(P<0.05).According to the detection of serum antibody valence,the serum antibody valence in the 15d-old infection ducks was significantly lower than the 25d-old ducks.It was indicated by fluorescent quantitative test that virus could be detected in liver,kidney,intestinal tract,heart,brain and other tissues and organs in 15d-old ducks,and the virus load amounts were relatively high in liver,kidney and intestinal tract.There results were consistent with the severities of clinical symptoms and histological lesion;however,the virus load volumes were significantly lower in tissues and organs in the 25d-old group.It is suggested that,with the increase of day age,an engine body will has higher resistance to FAV4.In conclusion,it is demonstrated by the artificial infection experiment that FAV4 shows strong pathogenicity for 15d-old and 25d-old cherry valley ducks.The susceptibility toward FAV4 shows certain correlation with day age suffering affection;also,a duck with younger day age is more vulnerable to infection.The experiment result provides certain reference for pathogenicity and pathogenesis of FAV4 in commercial meat ducks.