臺灣2014-2019年豬生殖與呼吸道綜合症病毒之ORF5序列親緣分析與新商品化活毒疫苗之田間效力評估

Abstract

豬生殖與呼吸道綜合症（PRRS）對全球養豬產業具有重大經濟影響，主要造成母豬流產、死產、產下木乃伊胎及弱仔等繁殖障礙，以及對生長期豬隻造成呼吸道症狀及生長性能低下。豬生殖與呼吸道綜合症病毒（PRRSV）是具有封套的正鏈單股RNA病毒，分類上屬於動脈病毒科（Arteriviridae）和Porartevirus病毒屬。PRRSV的基因體全長約15kb，並由10個開放閱讀區（open reading frames）所組成，其中開放閱讀區5（ORF5）及其所編碼之封套蛋白GP5是具高度變異性的區域。本研究之實驗目的為深入了解臺灣2014至2019年於田間流行之PRRSV其OFR5序列的基因變異。親緣分析結果顯示，此時期所收集的臺灣PRRSV（n = 73）皆屬於北美型病毒，這些病毒株具有顯著的基因特異性，屬於第三譜系（lineage 3）的北美型PRRSV，並可進一步將所有第三譜系的台灣PRRSV區分為7個不同的亞譜系（sublineage A-G）。台灣PRRSV與其他國家的病毒株和疫苗毒株具有高度基因序列差異，尤其在免疫相關之抗原決定位和N-醣基化位點具有顯著變異。在本研究中所分析的SW2018001-YL病毒株為來自雲林縣的臺灣PRRSV分離株，以次世代定序分析獲得全長基因體，並以RDP重組分析軟體測得在第1-538及14367-末端核苷酸位點發生序列重組，而其親代病毒株可能為HC120904-CHYL病毒株。 臺灣田間豬場自1990年代末期開始使用PRRS商品化活毒減毒疫苗，然而，至今仍不斷有偶發的PRRS疫情傳出。本研究擇一位於新北市之PRRS陽性豬場，進行活毒減毒疫苗之免疫效力評估。該豬場於2017年7月中由A品牌活毒疫苗更換為B品牌活毒疫苗，評估參數包含疫苗毒ORF5序列和田間病毒之相似性、母豬之繁殖性能、血清中之PRRSV含量及ELISA抗體力價等。結果顯示在資料收集期間（2016年9月至2019年2月），同時於分娩後母豬及一週齡仔豬使用B品牌活毒疫苗，能夠有效降低死產率、減少保育豬之病毒血症及增加ELISA血清抗體力價。此外，相較於A品牌活毒疫苗，B品牌活毒疫苗與野外毒株（SW2017001-TP病毒株）在ORF5（GP5）序列及T1抗原決定位具有較高之基因同質性。

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically significant diseases of swine industry worldwide. It causes reproductive syndrome in sows and respiratory syndrome with poor growth performance in growing pigs. PRRS virus is an enveloped, single-stranded positive-sense RNA virus belonging to the family of Arteriviridae, and the genus of Porartevirus. Within the genome of PRRSV, the ORF5 gene, encoding the major envelope protein, GP5, is the highly variable region. The aim of this study was to gain insight into the genetic variation of the ORF5 gene of PRRSVs circulating in Taiwan from 2014 to 2019. The phylogenetic analysis results showed that all of the field PRRSVs (n = 73) belonged to the North American genotype and have established significant genetic characteristics. These Taiwanese field PRRSVs belong to lineage 3, and they can be further classified into 7 sublineages (sublineage A to G). The Taiwanese PRRSVs show genetically distinct from other global PRRSVs or vaccine viruses, especially in the immunodominant epitopes and the N-glycosylation sites. A Taiwanese PRRSV isolate from Yunlin County, SW2018001-YL strain, was submitted for the next-generation sequencing to obtain the full-length PRRSV genome. Recombinant events were detected by RDP program at the nucleotide positions, 1-538 and 14367-end, and its putative parental strain was HC120904-CHYL. Although the modified-live virus (MLV) vaccine of PRRSV has been used in Taiwan since the late 1990s, sporadic outbreaks of PRRS still continue occurring. In the present study, a PRRS-endemic pig farm located in New Taipei City was selected for evaluating the efficacy of commercial PRRS MLV vaccines. This pig farm had switched from the brand A PRRS MLV vaccine to the brand B PRRS MLV vaccine since mid-July of 2017. Four parameters, including ORF5 sequence similarity among field and vaccine viruses, reproductive performance, PRRSV viral load in serum, and ELISA anti-PRRSV serum antibody titers, were used to evaluate the efficacy of these two vaccines. The results showed that vaccination of postpartum sows and one-week-old piglets with the brand B PRRS MLV vaccine effectively reduced the stillbirth rate and PRRSV viremia of nursery-growing pigs, and increased the ELISA anti-PRRSV serum antibody titers during the period of September of 2016 to February of 2019. Besides, comparing to the brand A PRRS MLV vaccine strain, the brand B PRRS MLV vaccine strain was more homologous to the field virus (SW2017001-TP strain) in the ORF5 (GP5) and T1 epitope.