Application And Effect Analysis Of Prevention And Control Strategies Of Porcine Reproductive And Respiratory Syndrome In Large Scale Swine Production Company

Porcine reproductive and respiratory syndrome(PRRS)is an important viral disease of pigs and a growing threat to the swine industry,the etiology of which is PRRS virus(PRRSV).Despite the implementation of strategies such as biosecurity,vaccination,herd closure and herd acclimatization,PRRS still has significant negative influence on pig production globally.So it is important to study the efficiencies of different control and prevention strategies in the field.236 samples of PRRS-suspected pigs from 96 large-scale pig farms nearby the farms of one large scale pig production company were tested by RT-PCR.ORF5 genes of positive samples were amplified and sequenced and then phylogenetically analyzed.Phylogenetic analysis showed 4 different lineages of PRRSV strains existed in these fanns,including lineage 1,3,5 and 8.The detection rate of lineage 1 was the highest and the PRRSV strains from Lineage 1 became the most prevalant.Full genomes of seven PRRSV isolates from one swine production company were sequenced and analyzed.Phylogenetic analysis indicated that the seven isolates belonged to the North American PRRSV genotype.The homology of whole genomes among these 7 isolates was 81.7%-99.1%.Phylogenetic analyses based on the sequence of the ORF5 showed that the 7 isolates belonged to 4 different lineages,including lineage 1,3,5 and 8.Nsp2 of these isolates exhibited different models of amino acids deletion with the exception of SDwh1701,while Nsp2 of SDqd1501 existed 35 amino acids insertion.These 7 isolates originated from TJM,JXA1-P80,NADC30,QYYZ with different models of recombination.PRRSV transmission routes of 4 PRRSV infected farms were surveyed and analyzed.The results showed that replacement gilts introduction from other farms and the PRRSV infection status of nearby swine farms were the most important risk factors of PRRSV transmission.After PRRSV outbreaks,replacement gilts utilization rate decreased by 8.56%on average,farrowing rate decreased by 8.57%,total born decreased by 0.73 pig/litter,pre-weaning mortality increased by 2.74%,post-weaning mortality increased by 4.97%and animal healthcare cost per weaner increased by 10.40 RMB.The average total economic loss including sow herds and fattening herds due to PRRSV infection was 1493.81 RMB per sow.Lungs lesions of 672 marketing pigs before and after PRRSV outbreaks were evaluated at abattoir.The results showed that before and after PRRSV infection,the prevalence of gross lesions consistent with interstitial pneumonia was 1.22 and 10.42%(p<0.01),respectively.Prevalence of lesions consistent with enzootic pneumonia was 26.83%and 55.00%(p<0.01),respectively.Prevalence of pleuritis was 1.22%and 7.94%(p<0.01),respectively.The results suggested that after PRRSV infection,growers and finishers were significantly more susceptible to Mycoplasma hyopneumoniae and APP.The efficiency of different PRRSV acclimatization protocols was evaluated.The results showed that the safety of PRRSV MLV for gilts acclimatization was much higher than PRRSV FVI,especially for piglets younger than 80 day-old.It is not recommended to use FVI to acclimatize nursery piglets.The recommended age to acclimatize gilts using FVI was 80-120 day old.Antibody dynamic and PRRSV infection status of weaning piglets from PRRSV stable sow herd were investigated.The results indicated that if the sow herd was PRRSV stable,the PRRSV infection risk of grower herds was low,even at the absence of vaccination in weaning piglets.The efficacy of RRRSV MLV vaccination on weaning piglets from PRRSV infected sow herds was evaluated.The results showed that when smaller percentage of weaning piglets were infected by PRRSV wild strains,the mortalities of PRRSV MLV vaccinated group and nonvaccinated group were 5.35±0.66%and 8.28±0.66%(p<0.01),respectively.When majority of weaning piglets had been infected by PRRSV wild strain,the mortality of PRRSV MLV vaccinated group and nonvaccinated group was 13.24±0.96%vs 9.91 ± 1.09%(p<0.05)These results indicated that it was not desirable to vaccinate PRRSV infected weaning piglets.Efficacy of herd closure management in 4 PRRSV infected farms of one swine production company was analyzed.The results showed that time to stability(TTS)was 26.92±10.25 week,time to baseline production(TTBP)was 17.00±2.94 week.450 imported replacement gilts with high health status were vaccinated with PRRSV MLV twice with 30 days interval.LCE protocol was executed and PRRSV antibody were monitored.The results showed that 13.04%replacement gilts were still ELISA Ab positive and culled in the 58th weeks post vaccination.PRRSV elimination was achieved.