Study On The Intervention Effect Of Cecropin On Co-infection H9N2 Subtype Avian Influenza Virus And Escherichia Coli In Broilers

The co-infection of H9N2 subtype avian influenza virus（H9N2AIV）with avian pathogenic Escherichia coli（APEC）is the main cause of respiratory diseases in broilers.At present,the prevention and control of this disease is mainly based on vaccine immunization combined with antibiotic intervention,However,due to the continuous mutation of H9N2 virus and antibiotic resistance,the prevention and control effect of this disease is not ideal.In addition,due to drug residues and product quality and safety problems in the use of antibiotics to some extent,finding an effective substitute for antibiotics for prevention and control has become an important issue to be urgently solved.Research shows that as a natural small peptide,cecropin has a wide range of antibacterial and antiviral activities and has been widely used.Therefore,based on the establishment of the co-infection model of chicken H9N2AIV and APEC in the early stage,this study carried out the research on the intervention effect of cecropin on the respiratory tract inflammatory exudation caused by the co-infection of chicken H9N2AIV and APEC,revealed the intervention effect of cecropin on the inflammatory exudation caused by the co-infection of chicken H9N2AIV and APEC,and provided a reference for the effective prevention and treatment of the co-infection of clinical H9N2AIV and APEC.In this study,240 one-day-old 308 broilers were randomly divided into four groups to establish the infection model after being fed to the age of 14 days:（1）negative control group,normal saline was added into the nasal cavity;（2）H9N2 infected group,0.2 ml of chicken embryo allantoic fluid containing virus(about 10⁵EID₅₀H9N2 virus)was inoculated via nasal route;（3）APEC infection group,nasal cavity inoculation 0.2 ml contains 2.09×10⁹CFU/ml O2 strain broth;（4）The H9N2+APEC infection group was infected with H9N2 and APEC according to（2）and（3）.After the establishment of infection model,each group was subdivided into two treatments:（1）Intervention group:Basal diet+normal water intake;（2）Unintervented group:Basal diet+cecropin drinking water（300mg/kg）.Subsequently,clinical symptoms and mortality were observed closely.At the 3rd,7th,14th and 21st days after intervention,3 animals were randomly selected from each group for weighing.Blood was collected from subwing vein,serum was separated,and tissues such as lung,trachea,thymus,spleen and bursa of Farnsworth were collected to observe histopathological changes,count immune organ index,virus or bacteria content in tissues,and analyze the changes of IL-6,TNF-α,free radicals and other related components were analyzed.The results showed:（1）Cecropin can significantly relieve the clinical symptoms such as dyspnea and reduce the mortality of broilers.Broilers in each infected group showed different degrees of mental depression,decreased feeding,diarrhea,dyspnea,open mouth breathing and other clinical symptoms.In particular,the above symptoms were more obvious after infection with H9N2+APEC,some infected chickens showed symptoms such as neck stretching and breathing,and died on the third day after infection.By 21days after infection,the mortality rate was 60%.H9N2 and APEC infection groups also had a certain degree of death,but the mortality of10%and 30%,respectively,were significantly lower than that of co-infection group（P<0.05）.Although the Cecropin clinical symptoms in the intervention group were similar to those in the non-intervention group,the clinical symptoms were relatively mild,and the mortality rate was 36%,significantly lower than that in the non-intervention group（P<0.05）.（2）The intervention of cecropin can significantly alleviate the degree of organ damage.The autopsy showed that after H9N2+APEC infection,the trachea and bronchus were congested and bled to varying degrees,accompanied by a large amount of mucus.Some severely infected chickens would have cheese like emboli in the lower trachea and bronchus that blocked the airway;The lungs also have different degrees of bleeding,and there are also cellulose exudates or columnar cheese like substance in the internal small bronchial lumen.The autopsy of the dead cases also showed varying degrees of fibrinous pericarditis,hepatic capsulitis,and air sac inflammation.Histological observation showed that a large number of epithelial cells were necrotic and exfoliated,respiratory cilia disappeared,inflammatory cells infiltrated between tissues,many organelles were damaged to varying degrees,and nuclear pyknosis was observed.In each intervention group,cecropin significantly improved the trachea and lung damage caused by infection in broilers.Histological observation showed that the intervention of cecropin significantly reduced tracheal mucosal exfoliation,collagen fiber swelling,and endoplasmic reticulum swelling.（3）Cecropin can Significantly lower the damage of immune organs caused by infection.The results showed that the immune organs of infected broilers were significantly damaged,and the organ index was significantly lower than that of non infected broilers（P<0.05）;Although there was still a significant difference between the intervention group and the negative control group（P<0.05）,it was significantly higher than the non-intervention group,indicating that the intervention group could slow down the damage of immune organs caused by infection.（4）Cecropin can significantly reduce the copy number and APEC load of H9N2 virus in lung tissue.In order to preliminarily understand the protective effect of cecropin on H9N2 and APEC infection,This experiment detected the copy number of H9N2 virus and the load of Escherichia coli in lung tissue.The results showed that although there was still a certain gap between the intervention group and the negative control group,the copy number and APEC load of H9N2 virus in the intervention group were significantly lower than those in the non intervention group（P<0.05）.（5）Cecropin could significantly reduce the contents of IL-6,TNF-α,TGF-β1,i NOS and MDA in serum of infected broilers,and reduce the consumption of SOD（P<0.05）.