| Porcine reproductive and respiratory syndrome(PRRS),caused by Porcine reproductive and respiratory syndrome virus(PRRSV),is a highly-contacted infectious disease with high mortality.It is characterized by reproductive failure in pregnant sows and respiratory distress in growing pigs,causing a substantial economic loss in the global swine industry.PRRS was first reported in America in1987 and then spread to North America,Asia and Europe.In China,PRRSV was first isolated from infected pig sera in 1996 by Guo Baoqing.Subsequently,the disease spread rapidly across the North and the Northeast of China.In this study,the molecular epidemiology of PRRSV in parts of China during2015～2017,genetic characteristics and antibody level of PRRSV wild strains,and the safety and effectiveness of TJM-F92(highly-pathogenic PRRSV live vaccine strain)were analyzed.The etiological detection of PRRSV in pig farms during2015～2017 indicated that the infection rate of PRRSV was 27.5%(lowest)in 2017 and was36.2%(highest)in 2016.The detection results of PRRSV in different quarters showed the infection rate in the following order(from high to low): the third quarter,the first quarter,the fourth quarter,and the second quarter.The detection of PRRSV in different areas showed that the infection rate in the northwest region remained the lowest for the three consecutive years,and the south and central China were among the top during the three years.From 2015 to 2017,the infection rate of nursery piglets was 64.9%~68.3%,which was the highest in different stages of the pig population.The infection rate of breeding pigs was 12.65%~15.6%,which was the lowest.The investigation on the co-infection of PRRS with other viral diseases such as classical swine fever(CSF),pseudorabies(PR),porcine circovirus type 2(PCV2)and porcine circovirus type 3(PCV3)showed that the co-infection rate with PCV2 was 12.4%~20.7%,which was the highest;and the co-infection with PR was the lowest: 0.60%~1.47%;in 2016,the co-infection with PCV3 was found with the co-infection rate1.89% ~ 2.83%.Although PRRS has been sporadic in recent years,we should maintain cautious for this disease.The nucleotides of 58 PRRSV isolated wild strains in some areas of China were compared with those of standard strains by DNASTAR: there are at least six subgroups of PRRSV in China,and there were six wild strains belonging to the first subgroup,whose homology with the first subgroup was 93.7%~99.3%,and that with other subgroups was 83.1%~91.4%.The nucleotide homology of the 35 wild strains in the same branch of the fourth subgroup with the fourth subgroup was 94.9%~99.7%,and that with other subgroups was 83.3%~94.4%.The nucleotide homology of 11 strains of the fifth subgroup represented by NADC30 was 94.2%~99.7% and82.3%~90.2%,respectively.The homology between the 6 strains belonging to new subgroups was 93.7%~99.2%,and that with other subgroups was 82.1%~85.6%.The analysis of the gene sequence of all the 58 strains showed that the virulence and neutralization protection sites of different subgroups were different.And some of the strains of the same subgroup were also different in virulence and protection sites.Besides,comparing with the strains isolated in 2014,it was found that all the 36 strains tested in 2014 belonged to the fourth subgroup.Since 2015,new subgroups have appeared continuously and showed obvious chemotaxis to the new subgroup,indicating that the variation of PRRSV is continuous.Therefore,when carrying out the prevention and control of PRRS,it’s suggested to detect and analyze the wild strain first,and then to select the corresponding type of vaccine for immunization.From2015～2017,the antibody detection of PRRSV from 2,012 pig farms showed that the average level of PRRSV antibodies was increasing gradually from75.6% in 2015 rose to 81.3% in 2017.According to the detection results of PRRSV antibody from different regions from 2015 to 2017,the average antibody level in east China reached 82.9%(highest),while southwest 74.1%.(lowest).The detection results of swine antibody in different stages showed that the antibody level of breeding pigs was between 83.7% and 87.6%,which was the highest.The proportion of antibodies against PRRSV in different distribution intervals from 2015 to 2017 showed that the S/P value of PRRSV antibody was mainly distributed in the interval of 0.4 to 2.0,followed by the proportion less than 0.4,and the proportion greater than3.5 was the smallest.Therefore,we can speculate that due to the emphasis on PRRSin farms,the overall prevention and control level of the disease is also improving year by year in China.The study on the safety and effectiveness of the highly-pathogenic PRRS attenuated vaccine(TJM-F92 strain)showed that there was no change in feeding intake,body temperature,failure in pregnancy rate,stillbirth and mummified fetus.After immunization,the antibodies in the whole production process were detected,and the antibody in each stage of pig breeding and respiratory syndrome could reach the qualified level.Therefore,it can be speculated that the HP-PRRSV attenuated vaccine(TJM-F92 strain)was safe,and the combination of the virus strain and the CSFV vaccine would not inhibit the antibody of CSFV vaccine,so we can more reasonably optimize the pig farm immune program. |
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