太平洋白蝦感染桃拉症病毒後血漿之分析

Abstract

臺灣地區養殖太平洋白蝦(Litopenaeus vannamei)於1999年3月發生大量死亡現象，病蝦具有軟殼以及紅色色素擴散的情形，倖存的蝦子其殼上出現許多任意分佈、不規則形狀、凹陷的黑化潰爛斑點。殼下的表皮組織以及結締組織發現有細胞核濃縮與破裂的情形，細胞質中有明顯的圓形包涵體(inclusion)，呈現“胡椒粉”的樣子。針對病蝦採用桃拉症病毒專一性核酸探針作原位雜交診斷顯示87％的檢驗病蝦呈現桃拉症病毒陽性反應，經純化的病毒顆粒負染再於穿透式電子顯微鏡下觀察為正二十面體(icosaherdal)，直徑為28-31nm。這些結果指出這一次臺灣地區白蝦大量死亡的主因為桃拉症病毒，桃拉症病毒可能經由慢性感染或帶原的蝦母由西半球傳播到東南亞的臺灣。 其次探討桃拉症病毒感染太平洋白蝦對血漿(hemoplasma)的影響，並對感染組與對照組作以下的血淋巴液分析，包括細菌凝集素效價(bacterial agglutinating titer)、抗菌活性分析(antibacterial activity)、血漿凝血蛋白含量(plasma clottable protein concentration)、血漿中血青素含量(plasma hemocyanin content)以及血漿中鎂鈣離子濃度(Plasma Mg++/Ca++ level)。急性桃拉症造成太平洋白蝦死亡，人工感染後到了第六天存活率為0％。血淋巴分析結果顯示隨著桃拉症病毒的感染，血漿中血青素和銅含量、血漿中鈣離子濃度與凝血蛋白量都下降，但血漿中鎂離子濃度於感染第二天顯著上升，至於抗菌活性分析，對照組與感染第0天均無法在血漿中測得活性，然而於感染第2和4天卻可於血漿中測得對抗哈維氏弧菌Vibrio harveyi的活性。本研究結果顯示急性桃拉症感染會改變蝦血漿中成份及抗菌活性，推測或許血淋巴的變化可以反映出蝦子健康的狀況，並提供作為疾病的診斷的依據。

In Taiwan, high mortality of cultured Pacific white shrimp, Litopenaeus vannamei,juveniles occurred within 3 days of disease onset in March, 1999. Moribund juveniles had a soft shell and a pale reddish coloration and survivors were found with multifocal, melanized, cuticle lesions on the cephalothorax. Prominent necrosis in the cuticular epithelium and subcutis had pyknotic nuclei and cytoplasmic spherical inclusion that gave lesions a peppered or buckshot-riddled appearance. In situ hybridization analyses using Taura syndrome virus (TSV)-specific cDNA probes showed that 87% of the juveniles examined were TSV positive. Negative staining of the purified viral preparations revealed the presence of icosahedral particles, 28-31nm in diameter. These results indicate that TSV is responsible for the acute mortality of white shrimp in Taiwan. TSV may have been introduced to Southeast Asia with chronically infected brooders or asymptomatic carriers imported from the Western Hemisphere. Study was further conducted to examine the changes in hemoplasma of Pacific white shrimp L. vannamei artificially infected with TSV. Hemoplasma obtained from infected and control shrimps were analyzed for bacterial agglutinating titer, antibacterial activity, plasma clottable protein concentration, plasma hemocyanin content, plasma Mg++/Ca++ level. Acute TSV infection caused high mortality in Pacific white shrimp and the survival rate was 0 % on the day sixth post infection. Hematological results showed plasma hemocyanin content, plasma Ca++ level and clottable protein content significantly decreased as disease progressed by TSV infection. A significant increase in plasma Mg++ level was found toward second day of TSV group. There is no antibacterial activity in the plasma of both control group and the day 0 of TSV group. However, on the day second and fourth after TSV-infection, we can detect the antibacterial activity against V. harveyi in the plasma. These results indicate that some hemoplasma parameters changed after TSV infection and they probably could be useful as the indicators of healthy status.