Isolation Of Prevalent Strains And Evaluation Of Vaccination Procedure Of Porcine Reproductive And Respiratory Syndrome

Porcine Reproductive and Respiratory Syndrome Virus(PRRSV),which is widely prevalent in pig farms across the world and causes long disease duration and immunosuppression,resulting in enormous economic losses to pig farms,primarily causes disorders of the reproductive system of sows and the respiratory system of nursery pigs.The clinical use of PRRS vaccine and the clinical protective effect of the immunization program were generally confusing.Therefore,in this study,we first isolated the predominant strains of PRRSV from clinical samples.On this basis,to evaluate and screen the optimal immunization program,different immunization programs and the crossprotective effect of their immunization programs were evaluated using prevalent PRRSV strains isolated from clinical material.The specific results are as follows:1.Isolation and identification of two recombinant PRRSV strainsTo characterize strains from PRRSV-positive pig farm,two PRRSV strains(HN-1 and HB-8 strain)were isolated from samples using Marc-145 cells and purified by identifying cytopathic effects and performing quantitative PCR,plaque assays,and the indirect fluorescent antibody(IFA)assay.Sequence alignment,homology,evolution,and recombination of PRRSV strains were analyzed using biological software for ORF5,NSP2,and the whole genome.The results showed that two PRRSV strains were successfully isolated from the samples.The whole-genome sequence analysis showed that both PRRSV isolates were recombinant strains.The HN-1 strain was recombinant from NADC30 and VR2332 strain(7348-7669 bp and 12311-12779 bp).The HB-8 strain was recombinant from NADC30 and JXA1 strain(1370-2009 bp and 5220-6282 bp,5220-6282 bp,and 6361bp-7982 bp)and TJ strain(6283-6360 bp).In addition,The ORF1 b,ORF2,and ORF4-7sequences of the two strains(HN-1 and HB-8)showed the highest similarity to the NADC30 strain,and the similarity between these six open reading frames of the two strains was more than 90%.Therefore,biosecurity measures should be strengthened in the clinic to prevent the transmission of PRRSV.2.Evaluation of the immunization effect of different vaccination programsTo evaluate the immunization effect of the strain of the classical PRRS vaccine under different immunization protocols.In this study,180 PRRSV antigen-and antibody-negative pigs were selected from the pig farm.And modified live virus vaccine(CH-1R strain)and inactivated vaccine(CH-1a strain)were selected.Three immunization protocols were set up.the first group was the first immunization with the modified live virus vaccine plus the inactivated PRRSV vaccine(Group MLV+INV),the second group was two immunizations with the modified live virus vaccine(Group MLV+MLV),and the third group was two immunizations with the inactivated vaccine(Group INV+INV).And sera were collected at14,28,42,and 56 d after the first vaccination for PRRSV ELISA(N protein)and neutralization tests(HP-PRRSV,C-PRRSV,NADC30-Like PRRSV).The clinical evaluation data were collected and analyzed.The S/P value of ELISA 56 days after immunization in the group MLV+INV was highly significantly higher than group MLV+MLV and INV + INV(P<0.001).The neutralizing antibody titers against the recombinant strains were lower than 1:4 in all vaccination programs.The different vaccination programs did not provide protection against the recombinant strains.However,the results of cross-protection against C-PRRSV and HP-PRRSV strains showed that neutralizing antibody titers significantly increased in the MLV+INV group 56 days after immunization compared with the other groups(P<0.001).For the C-PRRSV strains,the percentage of samples with neutralizing antibody titers up to 1:4 was 100%.And for titers up to 1:8,it was more than 70%.For the HP-PRRSV strains,the percentage of samples with neutralizing antibody titers up to 1:4 was over 95% and for titers up to 1:8 it was over40%.In other groups,the proportion of samples with neutralizing antibody titers up to1:4(against C-PRRSV and HP-PRRSV strains)was less than 20%.The titers of 1:8 were not observed in any of the samples.the differences between the number of live litters,the number of live litters,weaned piglets and total litter size after delivery were no significant difference in all group(P>0.05).In conclusion,group MLV+INV was significantly more effective than group MLV+MLV and INV+INV.This study provides reliable scientific data for the correct selection of the PRRS vaccination programs and lays the foundation for prevention and control of PRRSV infection.