Study On The Pathogenicity Of Novel Goose Parvovirus And Duck Circovirus Co-infection In Cherry Valley Meat Ducks

Beak atrophy and dwarfism syndrome(SBDS)has been constantly breaking out in multiple duck farming areas in China.The disease mainly causes duckling growth retardation,beak atrophy,tongue swelling,fracture of tibia and wing bone,diarrhea,paralysis,and low body weight,leading to a decline in the qualification rate,resulting in a lower pass rate.It causes great economic losses for the waterfowl breeding industry in China.The disease was caused by a novel goose parvovirus(NGPV)by virus isolation and identification.However,it was difficult to replicate SBDS in ducklings by a natural infection with NGPV.Epidemiological investigation later showed that there was high co-infection rate with duck circovirus(DuCV)and NGPV ducks in the ducks with SBDS.Since the clinical symptoms of other circovirus infections in poultry were similar to SBDS,DuCV was suspected to be one of the pathogens causing SBDS.In this study,we established an animal model of NGPV and DuCV co-infection to investigate whether co-infection lead to SBDS and the interaction between DuCV and NGPV in ducks.A total of 200 healthy 1-day-old Cherry Valley meat ducks were randomly divided into four groups,including three experimental groups(NGPV single infection group,DuCV single infection group,NGPV and DuCV co-infection group)and control group.The ducks in experimental groups were artificially infected orally by DuCV YB03 strain and NGPV JS1123 strain and the ducks in control group was orally inoculated with normal saline.Six experimental ducks were randomly selected to be killed at 1,3,7,14,21,28 and 35 DPI(Day post infection),and some experimental indicators were monitored,such as weight,beak length,tibia length,clinical symptoms,tissue lesions,serological changes,and the viral load of swabs and various organs,etc.NGPV and DuCV were detected during the experiment in the throat swab,anal swab,and serum of the experimental ducks.The ducks of all the virus inoculation groups exhibited feathering disorder,depressed,growth retardation and lying frequently.The most severegrowth retardation was observed in the co-infection group,followed by the NGPV infection group and the DuCV infection group.In addition,drooling and diarrhea were observed in the NGPV group,and some ducks in the co-infection group also showed beak atrophy,tongue swelling and paralysis.Organ pathological changes was observed to be more severe in the co-infection group than other two single infection groups,with liver being brittle and yellowish with hemorrhages,splenomegaly,thymus being enlargement and hemorrhages,and bleeding points being in kidney and duodenum.In the DuCV single infection group,the thymus of ducks had symptoms of atrophy,and the immune organ index was significantly lower than that of the control group(P < 0.05),while the immune organ index of the other two infection groups was significantly higher than the control group(P < 0.05).Histopathological lesions in the co-infection group showed the lesions in NGPV infection group and DuCV infection group at the same time,showing lymphocyte apoptosis in the immune organs(Splenic sinus ectasia was also found),epithelial cell shedding in renal tubules,balloon degeneration of hepatocytes and a small amount of lymphocyte infiltration in the liver,local eosinophil infiltration around the bile duct and duodenal epithelium.Serum biochemical tests showed that compared with the control group,the phosphorus and calcium levels of the infection groups were reduced,while the levels of alanine aminotransferase and total bilirubin were increased.In addition,the creatine kinase levels in the ducks of the co-infection group and NGPV single group were increased,too.Examination of the viral load in different organs revealed that DuCV and NGPV had extensive tissue tropism.In general,the copy number of NGPV in sick ducks was greater than DuCV,and the DuCV viral load in most organs in the co-infection group was higher in the early stage and lower in the later stage than that of the DuCV single group.In this thesis,the pathogenicity of NGPV and DuCV co-infection on Cherry Valley meat ducks was studied.It was found that NGPV and DuCV co-infection could lead to more severe SBDS,which provided important information for the further research and prevention of SBDS.