| Mycoplasma Gallisepticum(MG),one of the major pathogenic mycoplasmas in poultry,can cause Chronic Respiratory Disease(CRD)in chickens and infect other birds such as turkeys,quails,geese,and house finches,sweeping across the world.When chickens are infected solely with MG,inapparent infection occurs.However,if stress or mixed infection occurs,it will cause severe pathological damage and obvious clinical symptoms,thus severely hurting the poultry industry.MG is widely prevalent in Chinese poultry farms.Domestic research on this pathogen had a later start,and the currently-available measures against MG are less effective,which has posed tremendous pressure on its prevention and control in the Chinese poultry industry.In this study,the pipped(eggshell-rapturing)embryo samples were collected from different provinces for Quantitative Real-time PCR(q PCR)detection of MG and isolation of the field strains.After that,the mgc2 gene and six housekeeping genes of the isolates were sequenced and analyzed.And the microdilution method was applied to measure the minimum effective inhibitory concentrations of 11 antimicrobial drugs on the isolates.Furthermore,this study conducted a 7-day-old SPF chicken yolk sac inoculation test and a 15-day-old SPF chicken intratracheal injection test to assess the pathogenicity of isolates and performed an immunoprotective test to assess the immunogenicity of the isolates.The following findings were gained:1.Epidemiological Survey and Drug Susceptibility Tests on MG.In this study,a total of 1108 pipped embryo samples were collected from the hatcheries of ten provinces(Guangdong,Guangxi,Hunan,Hubei,Sichuan,Yunnan,Shandong,Henan,Anhui,and Jiangsu)between May 2021 and August 2022.According to the results,the average positive MG rate of the samples was 75.18%,and the rate of the samples from Sichuan and Hunan was as high as 100.00%.41 MG strains were isolated from the pipped embryo samples,and30 isolates of different hatcheries were selected from the 41 MG strains.On this basis,the study sequenced and analyzed the mgc2 gene and housekeeping genes of the 30 isolates,constructed the phylogenetic tree,and found that the isolates had a long genetic distance from the attenuated vaccine strains and standard strains and the prevalent MG strains in China had special and diverse genotypes.Lastly,according to the vitro drug sensitivity tests on 15 isolates,Tiamulin,Tylvalosin,and Doxycycline had an excellent antibacterial effect on MG,and Tilmicosin and Kitasamycin had a poor inhibiting effect and strong drug resistance to MG.2.Pathogenicity Tests on MG Isolates.According to the pathogenicity test results of MG isolates in SPF chicken embryos,the isolates SDTA20220301 and GDCG20210701 showed strong pathogenicity of mortality and air sacculitis,the two caused mortality of58.62% and 46.67%,respectively,and air sacculitis of 41.67% and 25.00%,respectively.According to the result of inoculation with SPF chickens,the strain AHBZ20220801 caused overt air sacculitis in chickens and had an average score of 3.4 in air sac lesions,significantly higher than other groups.Moreover,the strain AHBZ20220801 greatly affected the chicken’s weight,causing an average weight gain of 40.58 g lower than the control group.3.Immunogenicity Screening and Immue Effect Evaluation of MG Isolates.Nine MG isolates were selected and prepared into inactivated vaccines for immunoprotective tests.By detecting the post-immunization antibody level and analyzing challenge protection,respiratory detoxification,and bacterial load in organs,our study found that the isolate SDTA20220301 was highly immunogenic and had good immune protection;the isolate could produce low levels of antibodies in chickens and largely alleviated the invasive effect of MG on air sacs;this isolate provided 61.54% protection against air sacs in chickens so it can be taken as a potential vaccine candidate;this isolate could not significantly affect the detoxification level of the respiratory tract,but could drastically lower the MG load in the lungs of chicken.Focusing on the effect of MG on the poultry industry,our study conducted epidemiological surveys and drug sensitivity tests and found that the Chinese poultry industry is currently suffering high pressure and widespread prevalence of MG infection.Due to the overuse and long-term use of antibiotics,some isolates have experienced a sharp rise in drug resistance and caused pipped embryos and air sacculitis,which will undoubtedly pose a serious threat to the production chain.The strain SDTA20220301 showed immunogenicity in the immunoprotective test,so it can become an inactivated vaccine candidate.This study investigated the epidemiology and pathogenic characteristics,tested the MG pathogenicity,selected vaccine strains,and assess their immune effect,providing theoretical reference and technical support for the fundamental research on MG pathogeny and the formulation of preventive and control measures against MG. |
