- 學位論文
豬鐵士古病毒致病機轉之再探討
A Revisit of Porcine Teschovirus Pathogenesis in Pigs
摘要
豬鐵士古病毒 (Porcine teschovirus, PTV) 在初始疫情 (virgin epidemic) 造成豬隻之腦脊髓炎、腸炎、肺炎、繁殖障礙及其他廣泛症狀。近年來在台灣有兩波PTV-1疫情分別爆發於2000年及2004年，隨後於2008年其感染到達高峰(由家衛所由送驗病材之分離率判定)，即所謂地方性感染 (endemic)。前兩波疫情的特色之一為與其他常見豬病毒如CSFV、PRRSV、PCV2及PrV等共同感染。在無抗體情況下，PTV通常由糞口傳染。但在地方性感染的情況下，因為大多數豬隻已有抗體，其致病機轉變成比較多樣。目前有關PTV致病性研究，大多發表於1960年代，此期間病毒經過演化，同時生物科技有許多技術上的創新，種種理由使我們認為有必要針對台灣PTV分離株的致病性再探討。本研究共分三組鼻腔內接種試驗，第一組將病毒接種至12週齡，PTV-1 seronegative SPF豬，的確很快速地造成PTV的種種臨床症狀，包括發燒、下痢、中樞神經症狀等，病毒亦可在第2天由器官分離得到，但神經症狀比例不高與及腦脊髓炎不夠嚴重，此與中弱毒力的Talfan strain致病力相仿。比較特別的是所有感染豬都有間質性肺炎且接種後第四天其中和抗體即迅速攀升至256-512倍，令人懷疑實驗的SPF豬可能已被PTV及PRRSV所汙染。本實驗證實鼻內感染亦如糞口感染可在豬隻造成症狀與病變，此為過去較少被考慮的致病方式。第二組為將PTV-1接種至已有抗PTV中和抗體的豬隻，在接種後第21天施打LPC疫苗。此試驗證明雖然已有中和抗體，但PTV-1仍然可進行再感染(superinfection)，且LPC疫苗的施打，可能促使PTV由糞便排毒的機率上升。第三組為將PTV-1接種至PTV-1 seronegative SPF豬隻，長期監測其由糞便與鼻腔排毒的狀況，鼻腔在第17天即未見排毒，而糞便到第33天才未見排毒。而控制組豬隻則有斷續排毒，並有後腿痲痺，顯示控制組雖隔離餵養，但若排水或空氣處於試驗組的下游或經由工作人員的飼養或操作仍有機會被感染，此點在長期實驗尤需注意。本研究增進了我們對PTV致病性的了解，其無所不在的汙染變成一個隱形的致病原，當豬隻有其他常見病毒如CSFV、PRRSV及PCV2 等共同存在時，其扮演的致病角色更不能忽視。
並列摘要
Porcine teschoviruses (PTV) induce, in virgin epidemic, a variety of clinical symptoms and lesions including encephalomyelitis, diarrhea, pneumonia, and reproductive disorders. In the recent past, Taiwan has had two waves of virgin epidemic, the first one in year 2000 and the second in year 2004. The PTV infection in swine herds of Taiwan had since increased and peaked at year 2008 (endemic), judged by the isolation rates of PTV from submitted specimens to Animal Health Research Institute. One feature of the last two waves of epidemics was the multiple infection status. That meant other common swine pathogens such as classical swine fever virus (CSFV), porcine reproductive and respiratory virus (PRRSV), porcine circovirus type 2 (PCV2), and pseudorabies virus (PrV) were frequently co-isolated. When pigs have no anti-PTV antibody, the main route of infection and pathogenesis is believed to be fecal-oral-viremia. However in endemic situation when most pigs have antibody, the models of pathogenesis can be multiple. The earlier studies of PTV pathogenesis were largely published in 1960s. Through these years, virus has evolved and new laboratory techniques have been developed, so that it warranted a revisit of PTV pathogenesis. In this study, three groups of animals were experimentally infected via intranasal route. Group I animals were 12-week-old specific-pathogen-free (SPF) pigs infected with PTV-1 Taiwan isolate. Virus could be isolated from various organs at day postinfection (DPI) 2 and pigs succumbed to clinical signs of fever, diarrhea, and neurological symptoms. Because the frequency of neurological symptoms and severity of polioencephalomyelitis were not high, the virulence of PTV-1 Taiwan isolate was consistent that of moderate virulent Talfan strain. It was interesting to note that all infected animals (14/14) has interstitial pneumonia and the neutralizing antibody rose quickly to 1:256-512 at days postinfection (DPI) 4 suggestive of low level of PTV and PRRSV contamination in SPF pigs. This also showed that, in addition to fecal-oral, intranasal route could establish infection in pigs. Group II were 12-week-old pigs that were PTV-seropositive (neutralization antibody) infected with PTV-1 Taiwan isolate. These infected animals were also vaccinated with CSFV LPC vaccine at DPI 21. It showed that PTV superinfection could occur in the presence of high serum neutralizing antibody titers and the vaccination with LPC seemed to induce a higher rate of PTV shedding via feces. Group III were 12-week-old PTV-seronegative SPF pigs infected with PTV-1 Taiwan isolate. It showed that fecal shedding continued, in infected animals, until DPI 33, while the nasal shedding occurred until DPI 17. Notably PTV could be isolated intermittently from the control pig, which also developed hindlimb paralysis. This indicated that, although the control pig was housed separately from the infected pigs, the control pig could be infected if its housing, sewage or airflow, were located in the downstream of those of the infected pigs. In addition the control pig could be infected by the workers for frequent handling during the study period. This may be particularly important in long-term study and this also implicated the similar situation in the field. This study furthers our understanding on the pathogenesis of PTV. PTV is a widespread and seems unavoidable contaminant in nowadays swine industry. When co-exist with the other common swine pathogens such as PRRSV, PCV2 and CSFV, as very typical in nowadays swine production, PTV’s role in causing disease is not negligible.