Pathogenicity Of Fowl Adenovirus E Serotype 8a And 8b Isolates

The fowl adenovirus?FAdV?which causes the disease of chickens includes 5 species?A-E?and 12 serotypes?1-8a,8b-11?.Nowadays,it is known that different serotypes of FAdV can cause hepatitis hydropericardium syndrome?HHS?,inclusion body hepatitis?IBH?and gizzard erosions?GE?in chickens.Since the outbreak of HHS in large area in China in 2015,there are many serotypes of FAdV,among which,FAdV-8a and FAdV-8b are more and more prevalent in China,which makes it difficult to prevent and control the disease.To evaluate the pathogenic characteristics of JL/170408 strain?FAdV-8a?and FAdV-8b HLJ/151129 strain?FAdV-8b?,fourty two 10-day-old SPF white leghorn chickens were randomly divided into 3 groups:the first group consisted of seventeen chickens were challenged with JL/170408strain,the second group consisted of seventeen chickens were challenged with HLJ/151129 strain and the third group consisted of eight control chickens.Three chickens in the control group and five chickens in the challenged group were killed at 5 day post-infection.In this study,the chickens were challenged with virus by intranasal and ocular routes.The challenged dose was 10^5.0 TCID50 with one chicken,and the control group was inoculated with an equal volume of sterile PBS.Symptoms were observed every day after the chanllenge.The clinical symptom were observed ay 3 days after challenge.The challenged chickens showed different signs,including depression,neck narrowing,drowsiness and sleepiness.Occasionally,the chickens had temporary?1-2 days?dyspnea,which lasted about 14 days after challenge.The chickens recovered gradually.The incidence rate was 100%?12/12?,and the mortality rate was 0.In the challenged group,no obvious pathological changes were found at 5 days after the challenge.Occasionally,there were mild pericardial effusion and gizzard hemorrhage?1/5?.No obvious histopathologic changes were found in the liver and gizzard.The virus was excreted through the mouth and the cloaca for more than 60 days in challenged chickens from 3 day post-infection,and the detoxification was repeated that virus nucleic acid could still be detected in blood at 60 days after challenged.Viremia appeared at 3 day post-infection,and continued to be detected in the blood until 60days after challenge.At 15 days after challenge,some chickens began to show antibody positive conversion,and all chickens showed antibody positive conversion at 54 days after challenge.JL/170408strain proliferated in all tissues of challenged chickens in different degrees,and the concentration of virus was relatively high in gizzard,and the virus had a broad tissue tropism.Chickens challenged with HLJ/151129 strain began to show slight clinical symptoms at 3 days after,including mild depression,slight narrowing,some chickens had disordered hair and normal eating and drinking.The chickens began to recover gradually at 6 days after challenged.The incidence rate was 83.3%?10/12?,and the mortality rate was 0.No obvious pathological changes were found at 5 days after challenged,and no obvious histopathologic changes were found in the liver and gizzard.In challenged chickens,the virus was excreted through mouth,pharynx and cloaca for about 60days after challenge.Viremia appeared at 3 days after challenged and lasted until 60 days after challenged.Virus nucleic acid could still be detected in blood at 60 days after challenged.At 12 days after challenged,antibodies of some chickens began to turn positive.At 54 days after challeng,all antibodies of chickens turned positive.HLJ/151129 strain proliferated in different tissues of challenged chickens in different degrees.The results of cross-neutralization test of JL/170408?FAdV-8a?and HLJ/151129?FAdV-8b? showed that antiserum of JL/170408 and HLJ/151129,antiserum HLJ/151129 and JL/170408 had cross-neutralization,but there was antigenicity difference between the two serotypes.