錦鯉疱疹病毒傳播途徑之探討

Abstract

錦鯉疱疹病毒 (koi herpes virus；KHV)，目前被分類為鯉科疱疹病毒的第三類 (Cyprinid herpesvirus 3, CyHV-3)。是一種會引起錦鯉 (Cyprinus carpio koi) 及鯉魚 (Cyprinus carpio carpio) 急性死亡的病毒。KHV自1998年發生於以色列，之後許多國家的錦鯉及鯉魚養殖場也陸續爆發本病。KHV能夠快速的在各國間傳播可能是藉由錦鯉魚的買賣，而台灣也在2002年12月確定成為疫區。由於本病傳播速度快、死亡率高且目前還沒有任何治療方法，所以如何預防及阻斷本病的傳播成為當前最重要的課題。本實驗室針對一般錦鯉養殖池內會出現的生物進行同居感染試驗，以期找出KHV在環境中的傳播途徑。同居試驗生物包括金魚、朱文錦、虎皮蛙、大肚魚、黑殼蝦及石螺，檢測方法採用胸腺嘧啶激酶 (thymidine kinase, TK) 片段基因的聚合酶鏈鎖反應 (polymerase chain reaction, PCR)。結果顯示金魚、朱文錦、虎皮蛙及大肚魚皆可在與發病錦鯉共同飼養三天後於體表檢測出陽性結果。進一步將這些生物與健康錦鯉共同飼養後，自第三天起便可在錦鯉鰓部檢測出陽性反應，並使部分健康錦鯉魚發病死亡。但是在原位雜交後的結果顯示僅有金魚可以在其體內觀察到陽性訊號，而陽性訊號主要出現在腎臟、脾臟及腸道之巨噬細胞內。所以由本實驗可知，金魚可以感染KHV而不發病，為其保毒者 (reservoir)，而朱文錦、虎皮蛙及大肚魚則僅能機械性攜帶本病毒。

Koi herpes virus (KHV), tentatively categorized under the family Herpesviridae as the novel designation Cyprinid herpesvirus 3 (CyHV-3), is an emerging virus which causes fatal disease in koi (Cyprinus carpio koi) and common carps (Cyprinus carpio carpio). In 1998 many carp and koi farms were found to be afflicted by KHV with a high mortality in Israel. Since then, the same disease has been identified in many carp farms in different countries. The rapid spread of the disease is suggested due to the intensive worldwide trade of koi, and Taiwan has been identified to be an infected zone since December 2002. Because of the high mortality, rapid spread, and deficiency of effective therapies, how to prevent and disrupt its transmission pathway becomes the most important thing now. In order to find the KHV transmission vectors in the field, we developed some cohabitation experiments to mimic the natural transmission of KHV in koi farms. The commonly-found aquatic animals in koi farms involved in the cohabitation experiments include the goldfish, common goldfish, Chinese bull frog, mosquito fish, shrimp and snail. Our results revealed that the goldfish, common goldfish, Chinese bull frog and mosquito fish showed KHV positive by the PCR method after cohabiting three days with the KHV infected koi. Those KHV-positive creatures further cohabited with the normal koi, which were examined and proved to be KHV positive three days latter. However, the positive signal could only be found in goldfish visceral organs including kidney, spleen and intestine by in situ hybridization. Taken together, we concluded that the goldfish is the potential reservoir of KHV, and the common goldfish, Chinese bull frog and mosquito fish could just play a mechanical vector carrying the virus on their body surface.