Research Of Pathogenic Characteristics And Immune Protection Of HP-RRSV Pressure Passage-attenuated Strain

Porcine reproductive and respiratory syndrome (Porcine Reproductive and Respiratory Syndrome, PRRS) is a highly contagious disease in pigs caused by porcine reproductive and respiratory syndrome virus (PRRSV). Since the first outbreak in the United States1987, it has been spread to the the major pig-raising countries around the world. In the summer of2006, pigs "high fever" disease characterized by high fever took place in south of China in hot and humid season. The study showes it was caused mainly by porcine reproductive and respiratory syndrome virus as the main pathogen associated with a variety of pathogenic infection and secondary infection of other pathogen at the same time. Pig "high fever" epidemic in Shandong Province and across all the country, causing unprecedented damage to the pig industry. In this study, it shows infection, pathology, immune response and artificial infection of virulent PRRSV protective effect of HPRRSV Shandong isolate SD0612antibody selection pressure passage-attenuated strain returning to swine. To compare the dynamic of response in pigs between SD0612original strain and it’s mutants in the selective pressure, and study its immunization safety and virulent resistance after the inoculation to swine as a vaccine candidate strain,two animal experiments were conducted in this study. In the first experiment,15healthy pigs were divided into five groups as the SD0612original strain infection group, the first passage mutant F40infection group and the second passage mutant e40infection group, the second passage mutant e40infection group without antibody and negative control group, antibody was determined by ELISA kit in1w,2w,3w,4w,5w after challenge. Then dead pigs were dissected, and specimens were immediately fixed by formalin and imbedded by paraffin. Microscopic section was examined by microscopy after HE staining. The results showed that:Antibodyof the F40infection group is one week later than the original strain group, and it has no significant difference with the e40and a40group. It is found after the comparison of four strains that the antibody of SD0612original strain infection group is always the highest, indicating its highest graphetic level and the best antigenicity.SD0612original strain infection group quickly appeared the typical symptoms of PRRS, then died after vaccination10-13days. Dissected checks and pathological changes also fully comply with the typical symptoms. And except for one reason mutations of the fever, the other three mutant strains did not see other symptoms and obvious pathological changes, but Alveolar walls mild thickening, a few lymphocytic infiltratesand liver cells mild granulovacuolar degeneration and vacuslation were found by microscopy in one-third of the pigs.Each strain has not the obvious difference, suggesting the pathogenicity of mutant strains decreased significantly,and it has become a "nonpathogenic " weak strain, but there may be still some problems in security.To compare the immune protective effect after challenge among e40, F40and a40,20pigs were divided into four groups in the second experiment as the F40vaccinated group, e40vaccinated group, the a40vaccinated group and control group. At the40th day all groups were inoculated with virulent PRRSV,antibody was determined by ELISA kit in1w,2w,3w,4w,5w after challenge.Temperature was measured and incidence of performance was observed daily.Then pigs were dissected regularly,and specimens were immediately fixed by formalin and imbedded by paraffin. Pathological changes of different organs especially lung, lymphoid tissue and the brain were examined by microscopy after HE staining. The results showed that: Antibody rise of the F40vaccinated group is slower than the control group, and there is no significant difference. The immunity F40induced could still be able to protect the immune pig from strong virus attack, indicating some amino acids of F40has changed in some sites, but its antigenicity still in the near range of the original strain. Antibody changes of the vaccinated group have no significant difference with the F40vaccinated group.,and their symptoms and pathological changes were also similiar with the F40vaccinated group,showing good immune protective effect and antigenicity not changing greatly.But antibody of the e40vaccinated group was obviously higer than the F40and a40vaccinated group’s.The immunity e40induced could not be able to protect the immune pig from strong virus attack,it shows from one side the antigenicity of e40has changed greater than F40especially the original strain.After comparing three strains, F40and a40with high safety will be able to protect the immune pig from strong virus attack, and is the candidate vaccine strain with high development value.