蔣昕彧 王嘯宇 裴超 周勇 李鵬飛 孔祥會
摘要:鯉(Cyprinus carpio L.)是我國重要的水產(chǎn)養(yǎng)殖品種之一,但隨著養(yǎng)殖密度和養(yǎng)殖規(guī)模的不斷擴(kuò)大,其養(yǎng)殖生態(tài)系統(tǒng)受到不同程度破壞,各類疾病頻繁暴發(fā),其中又以病毒性疾病的影響范圍最廣、死亡率最高,已成為制約鯉養(yǎng)殖產(chǎn)業(yè)可持續(xù)健康發(fā)展的瓶頸問題。文章通過綜述鯉春病毒血癥(Spring viraemia of carp,SVC)、錦鯉皰疹病毒?。↘oi herpesvirus disease,KHVD)、鯉病毒性浮腫?。╒iral edema of carp disease,VEC)和鯉痘瘡病毒?。–arp pox disease,CPD)等病毒性疾病的病原生物學(xué)、發(fā)生機(jī)理、流行特點及其防控策略,發(fā)現(xiàn)鯉病毒性疾病仍存在諸多問題亟待深入研究,包括KHV可感染多種鯉科近緣魚類,但為何只發(fā)展成為病毒攜帶者而不發(fā)病;養(yǎng)殖水溫是否是決定病毒感染的關(guān)鍵因素;病毒可通過哪些途徑逃避免疫監(jiān)視而進(jìn)入魚體;在不同感染階段病毒的傳播方式是否存在差異等。此外,由于目前尚缺乏高效的防控方法,加強(qiáng)檢疫和免疫防控仍是防控鯉病毒性疾病的主要手段。因此,今后要繼續(xù)加強(qiáng)對病毒致病機(jī)理和傳染途徑的研究,研發(fā)新型口服疫苗載體及傳送系統(tǒng),并基于RNAi技術(shù)研制能有效防治病毒性疾病的藥物,以確保我國鯉養(yǎng)殖產(chǎn)業(yè)的健康可持續(xù)發(fā)展。
關(guān)鍵詞: 鯉;病毒性疾病;流行特點;防控策略;SVC;KHVD;VEC;CPD
中圖分類號: S941.41? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?文獻(xiàn)標(biāo)志碼: A 文章編號:2095-1191(2021)02-0509-09
Abstract:Common carp(Cyprinus carpio L.) is an important economic fish species in China.? With the gradual increase of breeding range and density in aquaculture, the aquaculture ecosystems are being damaged to varying degrees, the diseases frequently break out in common carp. Among them, the influence range caused by viral diseases is the most extensive,? and the mortality is the highest. It has become a bottleneck problem restricting the sustainable and healthy development of carp breeding industry. In this paper, the pathogen biology, occurrence,? epidemic characters and prevention and control strategies of viral diseases in common carp such as spring viraemia of carp(SVC), koi herpesvirus disease(KHVD), viral edema of carp disease(VEC) and carp pox disease(CPD) were summarized, and the future research trends of viral diseases were prospected. There are still many problems to be further studied, such as why KHV could infect a variety of Cyprinidae related fishes, but the fish only carried virus not becoming a disease-causing? Whether the water temperature is a key factor for virus infection? How can viruses escape immune surveillance and infect fish? Whether there are differences in the transmission of viruses at different stages of infection? Since lack of effective methods to control the carp viral diseases, strengthening quarantine and immune control remains the main means of preventing and controlling viral diseases of carp. Therefore, in the future, the researchers should continue to strengthen the research on the pathogenic mechanism and transmission pathway of the virus, develop a new oral vaccine carrier and transmission system, and develop drugs that can effectively treat viral diseases based on RNAi technology, in order to ensure the healthy and sustainable development of carp breeding industry in China.
Key words: common carp; virus disease; epidemic characteristics; control and prevention strategies; SVC; KHVD;VEC; CPD
Foundation item: National Key Research and Development Program of China(2019YFD0900105); Scientific and Technological Project in Henan(202102110260); Construction Project of Innovative Scientific and Technological Team of Aquatic Animal Immunity and Disease Prevention and Control in Henan(201706081)
0 引言
鯉(Cyprinus carpio L.)是我國重要水產(chǎn)的養(yǎng)殖品種之一,近年來我國的鯉年均產(chǎn)量維持在296萬t左右,其養(yǎng)殖產(chǎn)業(yè)形成了巨大的社會效益和經(jīng)濟(jì)效益。我國鯉養(yǎng)殖歷史悠久,據(jù)河南省賈湖遺址考古數(shù)據(jù)顯示,早在8000年前的新石器時代就開始馴養(yǎng)鯉(Nakajima et al.,2019)。但隨著鯉養(yǎng)殖密度和養(yǎng)殖規(guī)模的不斷擴(kuò)大,其養(yǎng)殖生態(tài)系統(tǒng)受到不同程度破壞,導(dǎo)致疫病頻繁暴發(fā)而造成嚴(yán)重的經(jīng)濟(jì)損失。至今,已發(fā)現(xiàn)多種病原微生物能侵染鯉,并造成暴發(fā)流行,其中又以病毒性疾病的影響范圍最廣、死亡率最高(朱霞等,2011),已成為制約鯉養(yǎng)殖產(chǎn)業(yè)可持續(xù)健康發(fā)展的瓶頸問題。鯉易感的病毒性疾病有鯉春病毒血癥(Spring viraemia of carp,SVC)、錦鯉皰疹病毒?。↘oi herpesvirus disease,KHVD)及鯉病毒性浮腫?。╒iral edema of carp disease,VEC)等,且這些病毒性疾病的死亡率均在50%以上,一旦大規(guī)模暴發(fā)流行,其病情很難控制,給鯉養(yǎng)殖業(yè)帶來重大經(jīng)濟(jì)損失(高隆英等,2002;朱霞等,2011;陳昌福,2017)。為了更好地防控鯉病毒性疾病的暴發(fā)流行,本文通過綜述鯉病毒性疾病的病原生物學(xué)、發(fā)生機(jī)理、流行特點及其防控策略,以期為實際生產(chǎn)中鯉病毒性疾病的科學(xué)防控提供參考依據(jù)。
1 鯉春病毒血癥
SVC是由鯉春病毒血癥病毒(Spring viraemia of carp virus,SVCV)引起的一種急性高致死率傳染病,已被世界動物衛(wèi)生組織(World Organization for Animal Health,OIE)列為必須申報的疫病之一,也是我國農(nóng)業(yè)農(nóng)村部規(guī)定的二類動物疫病。SVCV是一類高致病性病毒,屬于彈狀病毒科(Rhabdoviridae),其病毒粒子長80~180 nm,直徑60~90 nm,形態(tài)與其他彈狀病毒相似,呈一面凸出而另一面扁平的子彈狀(Ahne et al.,2002)。SVCV遺傳物質(zhì)為11 kb的反義單鏈RNA,包含5個開放閱讀框(ORF),分別編碼核蛋白(Nucleoprotein,N)、磷蛋白(Phosphoprotein,P)、基質(zhì)蛋白(Matrix protein,M)、糖蛋白(Glycoprotein,G)及RNA依賴的RNA聚合酶(RNA-dependent RNA polymerase,L)(圖1)。上述5個病毒基因的排列順序為3'-N-P-M-G-L-5'(Teng et al.,2007;Stone et al.,2013;Ashraf et al.,2015;Zhang and Gui,2015),是彈狀病毒的典型特征。
1. 1 鯉春病毒血癥的發(fā)生及流行特點
SVCV于1971年在南斯拉夫首次被發(fā)現(xiàn),隨后在美洲、亞洲和一些歐洲國家(英國、法國、德國、西班牙、丹麥、荷蘭和俄羅斯等)相繼報道有魚體感染SVCV(Hoffmann et al.,2005;Miller et al.,2007;Warg et al.,2007;Basic et al.,2009;Stone et al.,2013)。SVCV能感染多種鯉科魚類,其中鯉是主要易感宿主,不同年齡的鯉均可感染。仔魚和幼魚感染SVCV后,其死亡率相對于其他年齡階段較高,1齡仔魚的感染死亡率達(dá)70%,而成魚的感染死亡率相對較低。SVC主要在春季流行,該病的暴發(fā)與水溫密切相關(guān),當(dāng)水溫處于10~17 ℃時,SVCV最易感染鯉并造成高死亡率,進(jìn)而導(dǎo)致SVC的暴發(fā)流行。已有研究表明,當(dāng)水溫高于17 ℃時,SVCV可感染成年鯉;水溫處于22~23 ℃時,SVCV雖然可感染仔魚,但不會造成大規(guī)模發(fā)?。ˋhne,1986)。這可能是由于水溫較高時,機(jī)體代謝旺盛,免疫系統(tǒng)處于活躍階段,由體液免疫產(chǎn)生的抗病毒類物質(zhì)如γ干擾素(IFNγ),能有效抑制SVCV在魚體內(nèi)復(fù)制;以及由細(xì)胞免疫產(chǎn)生的特異性抗體,促使被感染而不致死的鯉可抵御SVCV的二次感染。SVCV可通過魚類的排泄物和體表黏液進(jìn)行傳播,同時可利用某些寄生蟲作為媒介進(jìn)行傳播,排出體外的病毒顆粒仍具有較高感染活性。在4~10 ℃的自然水體中,SVCV感染活性保持周期長達(dá)4周,而在底泥中可維持6周。因此,在養(yǎng)殖過程中暴發(fā)SVC時需對養(yǎng)殖環(huán)境進(jìn)行徹底消殺,才能有效消滅病原體(Ahne et al.,2002)。
1. 2 鯉春病毒血癥的典型癥狀及組織病理學(xué)變化
鯉感染SVCV后其體色變暗發(fā)黑,腹部腫大,鰓絲無血色,眼球突出,體表出現(xiàn)血色斑點,肌肉由于出血而呈鮮紅色;體內(nèi)滲透壓遭到破壞,造成臟器出血、腹腔積水,進(jìn)而引發(fā)腹膜炎癥和腸道炎癥,鰾出現(xiàn)血色斑點,部分魚體脾臟腫大(Ghasemi et al.,2014;Misk et al.,2016)。SVCV可在鯉的體內(nèi)大量增殖,破壞魚體穩(wěn)態(tài),致使免疫力下降,臨床上常伴隨其他細(xì)菌或寄生蟲的繼發(fā)感染,且呈大規(guī)模暴發(fā)流行。SVC的組織病理學(xué)變化主要表現(xiàn)為:肝臟血管壁水腫,部分血管壁結(jié)構(gòu)消失,肝實質(zhì)充血,多灶性壞死及脂肪變性。脾臟水腫,網(wǎng)狀內(nèi)皮細(xì)胞大量增生(Ghasemi et al.,2014);高鐵紅細(xì)胞內(nèi)脂褐質(zhì)儲存量增加,淋巴管明顯擴(kuò)張,巨噬細(xì)胞和淋巴細(xì)胞腫大(圖2),部分細(xì)胞破碎(Ahne et al.,2002;Misk et al.,2016)。肝胰腺出現(xiàn)多灶性壞死和非化膿性炎癥,心臟呈心包炎及間斷性肌變性。在腸道中可見血管周炎和絨毛萎縮。體腎出現(xiàn)空泡,且有玻璃樣病變,腎小管堵塞(Misk et al.,2016)。鰾上皮細(xì)胞層形成不連續(xù)的多層結(jié)構(gòu),黏膜下層有出血現(xiàn)象(Ahne et al.,2002)。
1. 3 鯉春病毒血癥的診斷與防控
SVC可對鯉養(yǎng)殖業(yè)造成重大影響,因此快速檢測鑒定SVCV是控制SVC暴發(fā)的關(guān)鍵。我國針對SVC的診斷標(biāo)準(zhǔn)為:將病魚組織勻漿10倍梯度稀釋后,接種至生長約24 h的鯉上皮瘤細(xì)胞(EPC)、草魚性腺細(xì)胞(CO)或胖頭鱥肌肉細(xì)胞(FHM)中,接種的細(xì)胞板置于(20±2)℃培養(yǎng)箱中培養(yǎng)7 d。若接種勻漿稀釋液的細(xì)胞在培養(yǎng)過程中出現(xiàn)細(xì)胞病變效應(yīng)(CPE),則立即采用實時熒光定量PCR、酶聯(lián)免疫吸附測定(ELISA)或間接免疫熒光法(IFAT)進(jìn)行SVCV鑒定。但該方法操作繁瑣,試驗周期較長,難以實現(xiàn)快速檢測;且采用ELISA和IFAT時,SVCV易與其他彈狀病毒發(fā)生交叉反應(yīng),而導(dǎo)致假陽性結(jié)果(Way,1991;Rodák et al.,1993)。單克隆抗體是檢測SVCV的重要工具,但傳統(tǒng)的單克隆抗體制備方法繁雜,且對專業(yè)技術(shù)水平要求較高(Chen et al.,2008;Luo et al.,2014;Li et al.,2015)。因此,亟待研發(fā)新的檢測方法,替代傳統(tǒng)單克隆抗體檢測,實現(xiàn)簡便、快捷檢測鑒定SVCV。Liu等(2013)研發(fā)出一種可快速檢測SVCV的新型抗體,其原理是將SVCV特異性抗體可變區(qū)的單鏈片段,通過噬菌體表面展示的方法表達(dá)于噬菌體表面,而實現(xiàn)對SVCV的快速檢測。
鑒于PCR檢測方法的高靈敏性,已廣泛應(yīng)用于SVCV檢測。高隆英等(2002)首次報道利用RT-PCR和半嵌套PCR擴(kuò)增SVCV的糖蛋白基因序列,結(jié)果分別擴(kuò)增獲得長度為714和606 bp的特異核酸序列片段,說明2種方法均可用于SVCV檢測。Koutná等(2003)研究表明,利用RT-PCR與巢式PCR相結(jié)合的方法能快速從細(xì)胞培養(yǎng)物和魚組織中檢測出SVCV,其靈敏度達(dá)10-1 TCID50/mL。傳統(tǒng)的PCR檢測需配備專門的儀器設(shè)備及具備熟練的RNA提取操作技術(shù),不利于現(xiàn)場快速診斷。為此,Shimahara等(2016)依據(jù)編碼G蛋白基因序列設(shè)計一對特異性反轉(zhuǎn)錄引物,成功建立從魚體組織中精確檢出SVCV的方法,且適用于非實驗室條件下的常規(guī)診斷。
目前,SVCV防控的主要策略是嚴(yán)格控制疾病暴發(fā)期水體環(huán)境的穩(wěn)定性,并及時清除和處理已被感染的病魚。由于病毒寄生于細(xì)胞內(nèi),至今尚無針對SVC的特效治療藥物,SVCV疫苗研發(fā)也還處于實驗室試驗階段。SVCV的G基因能編碼其衣殼糖蛋白,誘導(dǎo)宿主產(chǎn)生免疫應(yīng)答反應(yīng),因此包含G基因的DNA疫苗得到廣泛關(guān)注(Kanellos et al.,2006;Emmenegger and Kurath,2008)。Kanellos等(2006)通過檢測10種表達(dá)G基因的DNA疫苗,結(jié)果發(fā)現(xiàn)這些DNA疫苗的免疫保護(hù)效果較弱,多數(shù)DNA疫苗的相對免疫保護(hù)率只有11%~48%。Emmenegger和Kurath(2008)研究證實,基于SVCV南美株G基因設(shè)計的DNA疫苗對錦鯉(Cyprinus carpio haematopterus)和金魚(Carassius auratus Linnaeus)具有較好的免疫保護(hù)力,其相對免疫保護(hù)率均在50%以上??梢?,利用G基因設(shè)計的DNA疫苗可作為SVCV預(yù)防性治療的候選疫苗。
2 錦鯉皰疹病毒病
錦鯉皰疹病毒(Koi herpesvirus,KHV)又稱3型鯉皰疹病毒(Cyprinid herpesvirus 3,CyHV-3)(圖3),屬于異皰疹病毒科(Alloherpesviridae)鯉皰疹病毒屬(Cyprinivirus)(Waltzek et al.,2005;Hedrick et al.,2006)。目前,已分離鑒定出4種鯉皰疹病毒,分別為CyHV-1、CyHV-2、CyHV-3(KHV)及感染淡水鰻鱺(Anguilla japonica)的鰻鱺皰疹病毒(Anguillid herpesvirus 1,AngHV-1)。其中,CyHV-1常被稱為鯉痘瘡病毒,CyHV-2是引起皰疹病毒性造血器官壞死病的病原體。KHV是一種高致病性和高傳染性的病原微生物,通常存在7~15 d的感染潛伏期,鯉感染KHV后7 d內(nèi)的累計死亡率可達(dá)80%以上(劉宗曉等,2006;羅璋等,2018;周瑤佳等,2018)。因其傳播特性和高致病性,OIE已將KHV引發(fā)的KHVD列為必須申報的動物疫病之一,也是全球進(jìn)口檢驗檢疫必檢的疫病;在我國KHV是農(nóng)業(yè)農(nóng)村部規(guī)定的二類動物疫病病原之一。KHV對常見消毒試劑及紫外線的抵抗力較差,經(jīng)35 ℃作用48 h或60 ℃作用30 min均可使其喪失感染能力。KHV在23 ℃的自然水體中存活不超過21 h,但在魚體分泌物和池塘底泥中存活時間較長。
2. 1 錦鯉皰疹病毒病的發(fā)生及流行特點
1998年,美國科學(xué)家分別從鯉和錦鯉中首次分離出KHV,之后在歐洲、美洲、非洲和亞洲相繼報道KHVD大規(guī)模暴發(fā)流行(Haenen et al.,2004;Hedrick et al.,2006)。KHV傳染性強(qiáng)、致死率高,但其宿主范圍單一,僅感染包括鯉在內(nèi)的少數(shù)鯉科魚類,而不感染其他魚類,包括與鯉親緣關(guān)系較近的金魚、鰱(Hypophthalmichthys molitrix)、鯽(Carassius caras-sius)、草魚(Ctenopharyngodon idella)及羅非魚(Oreo-chroms mossambcus)等(Bergmann et al.,2010;Michel et al.,2010;Fabian et al.,2013)。KHV可在上述魚體內(nèi)復(fù)制增殖,并將具有感染活力的病毒顆粒釋放到水體環(huán)境中,但不會表現(xiàn)出KHV感染的臨床癥狀。鯉對KHV尤為敏感,不同年齡段的鯉均可感染,且成魚較幼魚更易感(Bergmann et al.,2010;Michel et al.,2010)。KHV主要流行于秋季,當(dāng)養(yǎng)殖水溫維持在18~28 ℃時易發(fā)病,在23~28 ℃時極易暴發(fā)流行;養(yǎng)殖水溫低于18 ℃時,KHV雖然也感染鯉,但無臨床癥狀,一旦溫度適宜,病毒暴發(fā),帶毒魚體即表現(xiàn)出KHVD的典型臨床癥狀,可造成80%以上的死亡率。
KHV主要依賴于養(yǎng)殖水體進(jìn)行傳染,水體中的病毒主要通過鰓組織進(jìn)入魚體,也可通過后腸進(jìn)入魚體(Gilad et al.,2004;Haenen et al.,2004;Pikarsky et al.,2004)。Costes等(2009)研究認(rèn)為,KHV主要依賴皮膚進(jìn)入魚體。但KHV感染48 h內(nèi),在無損傷且黏膜覆蓋完好的皮膚中檢測不到KHV;而通過浸泡感染鯉2~5 h,在被感染魚體的鰓組織和腸道中均能檢測到KHV,感染4~6 h后在鰓組織的巨噬細(xì)胞和腸道組織的淋巴細(xì)胞中也能檢測到KHV(Monaghan et al.,2015)。此外,KHV通過血液循環(huán)系統(tǒng)在感染后5~7 d可將病毒粒子運(yùn)送到機(jī)體各組織和器官(Haenen et al.,2004)。
2. 2 錦鯉皰疹病毒病的典型癥狀及組織病理學(xué)變化
鯉感染KHV后的典型癥狀為:反應(yīng)遲鈍,無法正常進(jìn)食,在水體表面離群散游,或頭朝下懸垂于水面;體表黏液分泌異常,局部發(fā)白,部分表皮不分泌黏液,且表現(xiàn)出砂紙樣紋理結(jié)構(gòu);眼球凹陷,魚鰭出血甚至腐爛;病魚鰓絲末端潰爛(圖4),喪失生理功能,導(dǎo)致呼吸困難;剖開腹腔可觀察到體腎腫大,且伴有腹水(Oh et al.,2001;Gray et al.,2002)。組織病理學(xué)研究顯示,鯉感染KHV后其鰓上皮細(xì)胞發(fā)生退行性病變,在感染細(xì)胞中發(fā)現(xiàn)核內(nèi)包涵體。肝臟、脾臟、腎臟和腸道薄壁細(xì)胞壞死,且在單核細(xì)胞/巨噬細(xì)胞內(nèi)部發(fā)現(xiàn)被吞噬的細(xì)胞碎片。Hedrick等(2000)研究顯示,鯉感染KHV后其神經(jīng)元細(xì)胞中出現(xiàn)核內(nèi)包涵體,但在KHV感染過程中神經(jīng)系統(tǒng)并未參與免疫應(yīng)答反應(yīng)。
2. 3 錦鯉皰疹病毒病的診斷與防控
感染KHV后患病鯉臨床癥狀明顯,首先可通過臨床病癥和流行病學(xué)進(jìn)行初步診斷;其次可通過細(xì)胞培養(yǎng)技術(shù)進(jìn)行診斷,感染KHV的鯉腦組織細(xì)胞系(CCB)經(jīng)20 ℃培養(yǎng)5 d后可產(chǎn)生特異性細(xì)胞病變,具體表現(xiàn)為細(xì)胞體積變大,部分細(xì)胞發(fā)生融合及出現(xiàn)明顯的細(xì)胞質(zhì)空泡化;最后可通過分子生物學(xué)技術(shù)進(jìn)行確診,利用巢式PCR或環(huán)介導(dǎo)等溫擴(kuò)增法(Loop-mediated isothernal amplification,LAMP)等分子檢測方法確定鯉是否感染KHV(Gunimaladevi et al.,2004)。我國防控KHV主要采取以預(yù)防為主的策略。KHV大規(guī)模暴發(fā)時應(yīng)保持水質(zhì)穩(wěn)定,養(yǎng)殖水體的溶解氧含量需保持在5 mg/L以上,同時避免拉網(wǎng)及其他作業(yè)對魚體產(chǎn)生應(yīng)激反應(yīng)。目前,市場僅有一種獲批銷售的KHV商業(yè)化疫苗,即以色列生產(chǎn)的弱毒疫苗(KoVax Ltd./Phibro Animal Health Corp.)(Ronen et al.,2003;Perelberg et al.,2005)。此外,一些傳統(tǒng)疫苗和DNA疫苗也正在研發(fā)之中,并證實對KHVD有一定預(yù)防效果(Rosenkranz et al.,2008;Zhou et al.,2014;Boutier et al.,2015;Klafack et al.,2019;Schroder et al.,2019)。Matras等(2017)研究表明,螺旋藻胞外多糖可有效治療由KHV感染鯉引發(fā)的病癥,持續(xù)給藥4~6周能顯著降低KHV的感染發(fā)病強(qiáng)度。以色列烈日大學(xué)Vanderplasschen教授認(rèn)為防治KHVD最有效的方法是接種疫苗,但KHV疫苗需滿足以下3個方面:(1)研發(fā)的KHV疫苗必須適用于大規(guī)模接種;(2)疫苗的成本效益比率應(yīng)盡可能低,生產(chǎn)和管理成本過高是制約漁用疫苗推廣應(yīng)用的重要因素;(3)疫苗的安全性需得到保證,且能產(chǎn)生較好的免疫效果,即相對免疫保護(hù)率接近100%(Boutier et al.,2019)。
3 其他病毒性疾病
3. 1 鯉病毒性浮腫病
鯉病毒性浮腫?。╒EC)又稱錦鯉嗜睡?。↘oi sleepy disease,KSD),其病原體為鯉浮腫病毒(Carp edema virus,CEV)。CEV是一種雙鏈DNA病毒,屬于痘病毒科(Poxviridae)。1976年CEV在日本首次被發(fā)現(xiàn),隨后在世界各地迅速傳播(Haenen et al.,2014;Jung-Schroers et al.,2015;Swaminathan et al.,2016;Matras et al.,2017)。CEV主要感染鯉和錦鯉,是一種急性感染源,可造成87.5%的感染率及80.0%~100.0%的死亡率。感染CEV的魚體行動緩慢,呼吸功能減弱,常浮頭于水面,或側(cè)臥于池塘底部,呈昏睡狀,最終因缺氧而死。病魚體表潰爛,皮下組織水腫,鰓絲無血色、末端潰爛(Miyazaki et al.,2005)。
VEC常暴發(fā)于春秋兩季,暴發(fā)水溫一般維持在15~25 ℃。但也有研究顯示,當(dāng)水溫在6~22 ℃時,CEV可感染鯉或錦鯉,并造成流行傳播(Oyamatsu et al.,1997;Amita et al.,2002);而水溫高于28 ℃時,CEV難以感染錦鯉,不會造成VEC大規(guī)模暴發(fā)流行。目前,尚未發(fā)現(xiàn)對CEV敏感的細(xì)胞系,故難以獲得大量純化的病毒粒子,而制約CEV基因組測序及其疫苗的研發(fā)(Swaminathan et al.,2016)。
3. 2 鯉痘瘡病毒病
鯉痘瘡病毒?。–arp pox disease,CPD)是由CyHV-1感染致病,因此CyHV-1又被稱為鯉痘瘡病毒(Carp pox virus)或乳頭瘤病毒(Papilloma virus)。CyHV-1是一種DNA病毒,與KHV同屬于皰疹病毒科(Alloherpesviridae)鯉皰疹病毒屬(Cyprinivirus)。CyHV-1的病毒核心為二十面體,外面有囊膜包被,病毒粒子核心和囊膜的直徑分別為113和190 nm (Sano et al.,2004)。
CyHV-1于1985年從日本患病錦鯉表皮組織中分離獲得,能感染EPC和FHM,造成這2種細(xì)胞系出現(xiàn)CPE,被感染的細(xì)胞發(fā)生空泡化,且在核內(nèi)形成包涵體(Sano et al.,1985)。CyHV-1感染鯉的臨床癥狀表現(xiàn)為:感染初期魚體表出現(xiàn)白色斑點,并分泌大量白色黏液,隨著病情發(fā)展,體表的白色斑點范圍逐漸擴(kuò)大,且逐漸凸起,形成增生物。增生物為上皮細(xì)胞與結(jié)締組織增生形成的乳頭狀凸起,其表面光滑呈石蠟樣或玻璃樣。增生物的主要成分為膠原纖維,能自然脫落,但在脫落部位又會重新出現(xiàn)增生物。CyHV-1常于流行春冬季,暴發(fā)流行時的水溫一般在10~16 ℃。當(dāng)水溫高于22 ℃時,患病魚體能自然痊愈。CyHV-1對成魚的危害較小,其死亡率低于10%,但感染2周齡仔魚的死亡率可高達(dá)60%~90%。相對于其他病毒,CyHV-1的致病率和致死率均較低,且危害較小,因此針對CyHV-1的研究較少,通常認(rèn)為該病毒是通過接觸傳染,但也有研究證實水生寄生蟲是其傳播媒介(Sano et al.,1985)。
4 展望
當(dāng)前,針對鯉病毒性疾病的防控主要采取消滅傳染源、切斷傳播途徑及保護(hù)易感魚群。消滅傳染源是通過對池塘和塘泥進(jìn)行徹底消毒,發(fā)病池塘水體需全面消殺;切斷傳播途徑主要通過加強(qiáng)苗種檢疫,對疫區(qū)親魚、苗種和成魚嚴(yán)格管控;保護(hù)易感魚群主要通過對敏感魚類注射疫苗或提高魚體免疫力。由于缺乏高效的防控方法,因此加強(qiáng)檢疫和免疫防控仍是防控鯉病毒性疾病的主要手段。疫苗的導(dǎo)入方式直接決定其免疫效果。疫苗通過腹腔注射方式導(dǎo)入魚體,可誘導(dǎo)魚體產(chǎn)生抗病毒免疫反應(yīng),但這種導(dǎo)入方式對魚體應(yīng)激較大,且成本較高,難以實現(xiàn)大規(guī)模推廣應(yīng)用(Adelmann et al.,2008)。魚類的黏膜組織在抵御病毒侵染過程中發(fā)揮重要作用(Costes et al.,2009;Gomez et al.,2013),但通過腹腔注射方式接種疫苗難以激活黏膜免疫系統(tǒng)??诜呙缈沙晒せ铘~體黏膜免疫系統(tǒng),且這種疫苗導(dǎo)入策略已在多個魚種上成功誘導(dǎo)抗病毒免疫應(yīng)答反應(yīng)(Chen,2000;Liu et al.,2012),但口服疫苗需抵抗魚體消化系統(tǒng)的侵蝕才能到達(dá)后腸而能被黏膜免疫系統(tǒng)識別和吸收,因此,研發(fā)新型口服疫苗載體及傳送系統(tǒng)是今后魚類病毒疫苗的重點研究內(nèi)容之一。
隨著集約化、工廠化養(yǎng)殖模式的推廣普及,尾水排放等環(huán)境污染問題日益加劇,同時為病毒性疾病的大暴發(fā)流行創(chuàng)造了條件。環(huán)境友好型生態(tài)養(yǎng)殖模式是水產(chǎn)養(yǎng)殖發(fā)展的必然趨勢,生態(tài)養(yǎng)殖需具備優(yōu)良的養(yǎng)殖品種及優(yōu)質(zhì)的養(yǎng)殖環(huán)境和飼料,同時要構(gòu)建完善的養(yǎng)殖生態(tài)系統(tǒng)。近年來,中藥等植物源藥物在水產(chǎn)養(yǎng)殖中的應(yīng)用研究逐漸深入。趙倩等(2013)以嗜水氣單胞菌對鯉進(jìn)行攻毒,結(jié)果發(fā)現(xiàn)中藥喂食組鯉的死亡率顯著低于基礎(chǔ)飼料組,且魚體溶菌酶活力顯著提高。謝炎福等(2015)研究表明,在黃河鯉基礎(chǔ)飼料中添加中藥制劑能顯著上調(diào)魚體超氧化物歧化酶(SOD)活性,同時降低丙二醛(MDA)含量。中藥制劑還具有凈化水質(zhì)及抑制水體中病原微生物增殖的作用(湯菊芬等,2016;孟彬等,2018)。目前,利用中藥對鯉病毒性疾病進(jìn)行防治的研究較少,中藥在鯉抵御病毒入侵過程中發(fā)揮的作用機(jī)理尚有待進(jìn)一步探究。
KHV可感染多種鯉科近緣魚類,但為何只發(fā)展成為病毒攜帶者而不發(fā)???養(yǎng)殖水溫是否是決定病毒感染的關(guān)鍵因素?病毒可通過哪些途徑逃避免疫監(jiān)視而進(jìn)入魚體?在不同感染階段病毒的傳播方式是否存在差異?可見,鯉病毒性疾病仍存在諸多問題亟待深入研究,以揭示病毒與魚體間的相互作用機(jī)理及病毒免疫逃逸的作用機(jī)制。近年來,RNA干擾(RNAi)技術(shù)在抗病毒藥物研發(fā)領(lǐng)域已取得長足進(jìn)展,并獲準(zhǔn)進(jìn)行下一步臨床試驗(Gotesman et al.,2015)。因此,今后要加強(qiáng)對病毒致病機(jī)理和傳染途徑的研究,并基于RNAi技術(shù)研制能有效防治病毒性疾病的藥物,以確保我國鯉養(yǎng)殖產(chǎn)業(yè)的健康可持續(xù)發(fā)展。
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(責(zé)任編輯 蘭宗寶)