| Infectious bronchitis (IB) is an acute, highly contagious disease of chickens caused by infectious bronchitis virus (IBV). IBV have many serotypes, but have little or no cross protection occurs between different serotypes of IBV. The Mass-type vaccine has been widely used for many years in our country, and it is genetically different from the QX type strains. Therefore, it is important to develop a QX type vaccine from poultry industry in China.1. Identification of QX type QXL strainThe biological characteristics of QX type IBV QXL strain were studied. The QXL strain was no bacterial and mycoplasma and other exogenous viral contamination. It exhibited mortality rate of40%in SPF chickens within15days post-challenged, and all chickens showed clinical signs. Gross lesions in the upper respiratory tract and bronchus were performed, with presence of catarrhal exudate and yellow caseous exudates. Renal gross lesions were observed with enlargement and distended ureters with urates. Histopathologic and IHC examination revealed that the severe microscopical lesions were also in the trachea and kidney.In addition, severe gross lesions in hens were observed at fallopian tubes and ovaries of the QXL strain. Compared with the healthy control group, the egg production was droped about50%. At180dpi,27%of the chicken showed peritonitis, intestinal adhesions, yellow-white cheese-like substance. In addition there are about36%of chicken ovarian atrophy, deformation, bleeding and some of the reddish brown eggs. Histopathologic and IHC examination revealed that in ovary main lesions including moderate infiltration with infiltration of neutrophils, follicular cell degeneration and necrosis. Simultaneously, we detected the viral antigen in the fallopian tubes and ovary.2. Attenuation of QX type QXL strainDuring the passage of QXL strain, the virus titer rose gradually and caused more severe pathological changes in SPF embryonated chicken eggs. In order to evaluate the pathogenicity of QXL strain, we selected different passages of strains challenged with1-3-day-old SPF chicks, and monitor their morbidity and mortality, and histopathology were analyzed. The results showed that the pathogenicity of the10-30passages strains showed morbidity and mortality as high as80%and20%, respectively. The pathogenicity of the50passages strains was significantly reduced and showed morbidity was20%. No clinical response was observed when the70passages were inoculated. At the same, no microscopical changes were detected in tissues.We examined the immunogenicity of QXL70,80,85,87,90,100,110,120passages strains. The vaccination-challenge test showed that the attenuated QXL strain could afford above90%protection against challenge with virulent homologous virus. Therefore, after70passages strain showed good immunogenicity could provide a satisfactory immune protection. We selected the87strain for further research and named QXL87.Sequencing results showed that the S1gene have four nucleotide mutant, and two amino acid residue change between5and120passages. Therefore, we guessed that the virulence and immunogenicity changes may also relate to some other genes.3. Preclinical studies of QX type QXL87as an attenuated vaccineThe results of distribution in vivo and detoxification showed that could isolate the virus in organs at5and7days after inoculation. At the15days after inoculation could not isolate the virus in organs organs. The QXL87strains in SPF chickens5generations, then the clinical observations and autopsy pathology analysis showed that the QXL87strains serially passaged in vivo by SPF chickens have no increase in virulence phenomenon. In addition, no lesions were observed at fallopian tubes and ovaries of the QXL87strain.In summary, QXL87strain is a good candidate attenuated vaccine strains of QX type IB. |
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