Effects Of Duck Circovirus On Immune Function And Secondary Infection Of Escherichia Coli

Duck circovirus and porcine circovirus can both infect the corresponding immune organs and induce immunosuppression.Thus,the immune response of the vaccine is reduced,leading to a significant increase in the probability of infection by other pathogenic microorganisms,and the occurrence of secondary infection or exacerbating the clinical manifestations of other diseases.Duck Escherichia coli disease is also a secondary infectious disease,the spread of the disease faster,greater harm.According to the epidemiological investigation results of duck circovirus in recent years,the secondary infection and mixed infection caused by duck circovirus increased,especially the mixed infection of duck circovirus and Escherichia coli was serious,which caused huge economic losses to the global duck industry.Based on the analysis of 100 samples collected from poultry farms in Shandong Province,it is known that the mixed infection of duck circovirus is serious,especially the mixed infection with bacteria.Therefore,we speculate that the infection of duovirus is more conducive to the secondary infection of pathogenic microorganisms such as bacteria,thus aggravating the mixed infection of duovirus.The mechanism may be that the duovirus invasion into the body causes immune suppression,lowers the immunity,and is prone to secondary infections such as E.coli.Coupled with limited research on duck circovirus due to its recent discovery,Therefore,this study starts from two aspects,one is to explore the influence of duck circovirus on the immune function of the body,the other is to explore the influence of duck circovirus on secondary infection of Escherichia coli.These two experiments revealed two things,First,duck circovirus inhibits normal activity and function by affecting related cytokines in the body,disrupts the balance of the body's immune system,and leads to immunosuppression.The immunosuppression effect is most obvious about 24 days after infection,and then weakens.Second,duck circovirus secondary Escherichia coli infection,can accelerate bacterial colonization,promote disease development,lead to increased mortality,and is related to the immunosuppressive effect.The results lay a foundation for further study on the immunosuppressive mechanism of duck circovirus,and also provide ideas for prevention and treatment of the mixed infection of duck circovirus and bacteria.This study is divided into the following two parts:(1)The effect of duck circovirus on immune functionEighty one-day-old ducklings were randomly divided into two groups:DuCV and PBS control group to explore the effect of duck circovirus on the immune function of the body.Each one-day-old duckling in DuCV group was inoculated with 0.2 m L of10^3.69ID₅₀ virus suspension,The control group was not treated.Then,8 days after DuCV infection,weight was recorded every 4 days.Every eight days randomly selected 10 goose of cheeper duck are partitioned,take its organ,tissue,blood,serum,etc.,to detect changes related viral load in the body and the immune indexes.The results showed that the body weight of ducks in the circovirus group was significantly lower than that of ducks in the control group on days 8-32.Continuous follow-up immune organ index in DuCV group was significantly lower than that in PBS group,CD4+,CD8+T lymphocytes,peripheral blood lymphocyte conversion rate and other related immune indexes were measured.The DuCV group was significantly lower than the PBS group,and the DuCV group showed a downward trend during 8-24 days and an upward trend during 24-32days,indicating that the number of lymphocytes in the body was reduced and the proliferation and differentiation of lymphocytes were inhibited after DuCV infection.Because IL-10 is an important immunosuppressor,IL-10,IL-12,IFN-?were measured.The results showed that after DuCV infection,IL-10 content increased,which affected the secretion of IL-12 and IFN-?and reduced their content,causing immunosuppression.In the detection of viral load in the heart,liver,spleen,thymus and bursa of Fabricius,it was found that viral load in the liver and heart gradually increased on 8.16.24.32 days,but the increase was not significant.Viral load in spleen,thymus and bursa of Fabricius reached the highest on 24 days,and viral load slightly decreased on 32 days.The results of the viral load test were consistent with the previous results of the cytokine test.Based on these results,we can conclude that duck circovirus can cause growth retardation and stunted growth of young ducks.The results showed that the normal activity and function were inhibited and the balance of the body's immune system was destroyed after duck circovirus infection,thus leading to immune suppression.In addition,the immunosuppression effect is obvious at about 24 days,the immunity level is the lowest,and the damage to the immune system is more serious.(2)Effects of duck circovirus on secondary infection of duck Escherichia coli175 one-day-old rats were divided into three groups:DuCV+APEC group,APEC group and PBS group.The DuCV+APEC group was injected with10^3.69ID₅₀ DuCV at 1 day of age,followed by 1ml 10⁸CFU of Escherichia coli at 14 and 24 days of age,respectively,except for the PBS group.The incidence was observed and the colonization of bacteria in the body for 12and 24 hours was calculated and measured.In addition,DuCV infected ducks were randomly selected for the measurement of viral load in each organ at 8,14 and 24 days before E.coli injection.The results showed that the virus was in a state of growth and reproduction in the ducklings before inoculation,and the subsequent experiment of secondary infection provided evidence.According to the statistical morbidity,the 24-hour morbidity rate of DuCV+APEC group at the age of 14 days was 80%,the 48-hour morbidity rate was 100%,and the 24-hour morbidity rate of APEC group was 50%,while the 36-hour morbidity rate of the 24-day mixed infection group had reached 100%,and the statistical mortality rate also showed a consistent rule.Statistics of bacterial colonization showed that DuCV infection significantly enhanced the pathogenicity of Escherichia coli,and 24-day-old inoculation was more serious than 14-day-old inoculation.These results indicate that duck circovirus can significantly enhance the colonization ability of E.coli in the body,leading to the rapid development of the disease and the increase of mortality.At the same time,it is related to the immunosuppressive effect caused by duck circovirus.When the immunosuppressive effect of duck circovirus is strongest,it also has the greatest influence on the secondary infection of Escherichia coli and causes more serious losses.