Isolation,Evolutionary Analysis And Prevention And Control Measures Of PRRSV In Some Areas Of South China

Porcine reproductive and respiratory syndrome(PRRS),characterized by respiratory disease and reproductive failure,is a viral infectious disease caused by porcine reproductive and respiratory syndrome virus(PRRSV).PRRS was first reported in the United States in the 1980 s,then quickly spread to Europe and Asia,becoming a major threat to global swine industry.Since its first emergence in China in 1996,RRS has been widely spreading and continuing to evolve rapidly in China,causing significant economic losses in the world’s largest pig-raising and pork-consuming country.In 2006,a novel PRRSV strain with high virulence was detected in Jiangxi province,caused a disease characterized by high fever,high mortality and high morbidity.Since 2008,highly-pathogenic PRRS was listed as one of Class A diseases in China.In 2013,NADC30-like strain was first detected in pig herds in North China.NADC30-like strains show more recombination possibilities,thus produced lots of novel PRRSV strains,which have significantly increased the difficulties in PRRS control.PRRSV is evolving fast in the field.Mutation and recombination are the two main mechanisms which lead the diversity and complexity of the epidemic strain in the field.PRRS has been prevalent more than 30 years,but no ideal vaccine has been developed.Attenuated vaccine can provide only partial protection to homologous strain,not to mention its safety concerns.Inactivated vaccine was proven not be able to induce effective humoral and cellular immune response in PRRSV negative pigs.In view of this,we collected samples from pigs suspected of PRRS infection in some areas of South China(mainly Guangxi and Guangdong province)between 2017 and 2021 to conduct an epidemiological investigation.We successfully isolated 11 PRRSV strains.We analyzed the whole genome of the 11 isolates and 479 ORF5 gene sequences from PRRSV-positive samples.We further analyzed the impact of two isolates on the productivity of sow farm.Then,we carried out trials using attenuated and inactivated vaccine,try to find ways of better use of PRRSV vaccines for prevention and control of PRRS.1.Epidemiological investigation,isolation and evolutionary analysis of PRRSV in some areas of South ChinaWe collected a total of 6795 samples,of which 1279 were detected positive of ORF5 gene.The detection rate was 18.82%.We obtained 479 ORF gene sequences from those positive samples.All the 479 strains were PRRSV 2,of which 85.39% were highly-pathogenic strains,13.78% were NADC30-like strains and 0.84% were QYYZ-like strains.Notably,93.15% of the highly-pathogenic strains detected in this study were vaccine-derived strains,indicating that the wide use of PRRSV attenuated vaccines largely lead to the high prevalence of highly-pathogenic PRRSV.Whole genomes of the 11 isolates were compared and analyzed,of which 7 isolates were vaccine-derived strains,6 isolates were identified with recombination events.Breakpoints mainly located in Nsp2 region.Notably,2 strains were found to be recombined from vaccine-derived strains.These findings indicated that PRRSVs circulating in the field is undergoing rapid mutation and recombination,which increased the complexity and diversity of the strains in the field and undoubtedly poses great challenge for controlling the disease.2.Assessment of economic impact of PRRS infection on sow farmsThe impact of two NADC30-like isolates GX505 and GX1858 on the sow farms were evaluated.The duration of the impact was approximately 4-5 months.Almost all the productive indexes were affected.For farm A and B,farrowing rate decreased by5.96% and 5.99%,sow mortality increased by 30.51% and 68.97%,empty sow rate increased by 53.11% and 58.75%,stillbirth per litter increased by 36.05% and 67.86%,mummified per litter increased by 48.28% and 69.23%,pre-weaning mortality increased by 20.48% and 182.08%,and 28-day weaning weight decreased by 10% and 5.31%,respectively.We further speculated the economic impacts due to the loss of weaning piglets.Farm A and B suffered a loss of 1.22 and 1.51 weaning piglets per sow per month,respectively.And the loss of total weaning weight were 14.45 kg and 15.31 kg per sow per month,respectively.Thus,we calculated the economic losses of farm A and B were1637.16 yuan and 1734.92 yuan per sow per month,respectively.3.Using PRRSV MLV vaccine for gilts acclimationExperiments were carried out to evaluate the optimal dose of PRRSV MLV vaccine(TJM-F92 strain)and drugs combination by monitoring the duration of viremia and PRRSV antibody post PRRSV MLV vaccination.We found that administration of pachyman and Tylosin 14 days post-MLV-vaccination can shorten the PRRSV viremia to6 weeks.In addition,0.5 dose of PRRSV MLV had one-week shorter PRRSV viremia than 1 dose.The acclimation program was carried with 1200 PRRSV-negative young gilts.Those gilts remained PRRSV negative when introducing to the sow farm,the weaning piglets were also PRRSV negative.A total of 542 acclimated gilts were introduced to sow farm.The main productive indexes were monitored and analyzed.Farrowing rate(83.76%),sow mortality(4.61%)and pre-weaning survival(91.90%)failed to meet the standard due to some management reason,the other indexes met the standard,alive piglets per litter was 13.09,stillbirth per litter was 0.21.mummified per litter was 0.24,average birth weight was 1.3kg,28-day weaning weight was 7.29 kg.These findings indicated that our acclimation program was effective and can be worthy of promotion and application.4.Using PRRSV inactivated vaccine for control of PRRSV positive unstable sow herdThe sow herd was diagnosed as infection of highly-pathogenic PRRSV in laboratory.We used a commercial PRRSV inactivated vaccine(CH-1a strain)to vaccinate the sow herd,combine with drug administration and complete herd closure.The herd returned to PRRSV positive stable in 4 months,as weaning piglets per sow rose to 11.12 which is comparable to that before the outbreak of PRRS,and PRRSV negative weaning piglets were produced in 21 weeks.These findings suggested that inactivated vaccine can help improving the performance of PRRSV positive herds.