Abstract：
Porcine reproductive and respiratory syndrome virus (PRRSV) infections greatly impact the health and productivity of growing pigs. The introduction and persistence of wild-type PRRSV (WT-PRRSV) strains in growing pig populations is poorly understood. In an observational prospective cohort study, we monitored and surveyed 63 wean-to-finish (WTF) herds across 10 companies located in medium to high pig dense areas in the U.S. Midwest. All herds received weaned pigs from PRRSV-negative or positive-stable breeding herds. Herds were monitored monthly using oral fluids collected following a fixed spatial sampling regime and samples were tested by PRRSV ELISA, RT-PCR and ORF5 sequencing. In most (90%) of the herds, pigs were vaccinated with PRRSV modified-live vaccines either at processing, weaning or shortly after weaning. Wild type PRRSV (WT-PRRSV) infections were defined by the criterion of having more than 2% nucleotide differences in the ORF-5 region compared with reference vaccine strain sequences. Wild type PRRSV was detected in 42% of the herds with infections being more prevalent in the mid to late growing period, with a mean of 20 weeks post placement. Nineteen distinct WT-PRRSV were identified in seven out of 10 production companies with an average of 3 distinct WT-PRRSV strains per company. Vaccinated WTF herds with and without WT-PRRSV detection were compared to each other showing different PCR and ELISA infection patterns. Close-out mortality in vaccinated herds with WT-PRRSV was numerically higher (6.5%) than mortality in those sites where WT-PRRSV was not detected (5.0%) (p = 0.07). Mortality was also higher (10.5%) when WT-PRRSV was detected earlier at eight weeks post-placement compared to late finishing at 20 and 25 weeks post-placement, 2.9% and 4.5% respectively (p = 0.017). Overall, this study sheds light on WT-PRRSV infection dynamics in vaccinated populations of growing pigs, reinforces the importance of biosecurity practices in this phase of production and calls for better understanding of risk factors associated with PRRSV introductions in growing pig sites.
摘要：
猪繁殖与呼吸综合征病毒(PRRSV)感染极大地影响生长猪的健康和生产力。野生型PRRSV(WT-PRRSV)毒株在生长的猪群体中的引入和持久性知之甚少。在一项观察性前瞻性队列研究中，我们监测和调查了位于美国中西部中高猪密集地区的10家公司的63只断奶(WTF)牛群。所有牛群都接受了来自PRRSV阴性或阳性稳定的繁殖牛群的断奶猪。每月使用固定空间采样方案收集的口腔液监测牛群，并通过PRRSVELISA测试样品。RT-PCR和ORF5测序。在大多数(90%)的牛群中，猪在加工时接种PRRSV修饰的活疫苗，断奶或断奶后不久。野生型PRRSV(WT-PRRSV)感染通过与参考疫苗株序列相比在ORF-5区中具有超过2%的核苷酸差异的标准来定义。在42％的牛群中检测到野生型PRRSV，在生长中后期感染更为普遍，安置后平均20周。在10个生产公司中的7个中鉴定了19个不同的WT-PRRSV,每个公司平均3个不同的WT-PRRSV毒株。将有和没有WT-PRRSV检测的接种的WTF牛群进行比较，显示出不同的PCR和ELISA感染模式。用WT-PRRSV接种的群的接近死亡率在数值上(6.5%)高于未检测到WT-PRRSV的那些位点的死亡率(5.0%)(p=0.07)。当WT-PRRSV在放置后8周早期检测到时，死亡率也更高（10.5％），而在放置后20周和25周后期完成时，分别为2.9%和4.5%(p=0.017)。总的来说,这项研究揭示了WT-PRRSV在接种疫苗的生长猪群体中的感染动态，加强了生物安全实践在这一生产阶段的重要性，并呼吁更好地了解与在养猪场引入PRRSV相关的风险因素。
