三斑家蚊及白線斑蚊對牛流行熱病毒之病媒勝任性之評估

Abstract

牛流行熱 (Bovine ephemeral fever) 是由彈狀病毒科 (Rhabdoviridae) 中暫時熱病毒屬 (Ephemerovirus) 的牛流行熱病毒 (Bovine ephemeral fever virus) 所引起。此病會造成牛隻的產乳量、換肉率下降，治療費用高等後果，而造成嚴重的經濟損失。雖然從野外捕獲的庫蠓 (Culicoide spp.) 有分離出牛流行熱病毒之記錄，但庫蠓的地理分布與牛流行熱流行區域並不一致，所以推測牛流行熱病毒有庫蠓以外的其他傳播媒介。本研究的目的在探討蚊子對牛流行熱病毒之感受性與傳播潛力。利用人工血餐以及胸腔注射兩種方式，使蚊子獲得病毒後，再以免疫組織螢光染色搭配雷射共軛焦分光顯微鏡，檢測病毒於蚊子體內感染之情形。此外，測試牛流行熱病毒是否能藉由受感染的蚊子再次取食而隨唾液分泌，並成功感染試驗用之幼倉鼠細胞 (Baby hamster kidney cell)。結果表示此病毒能夠感染三斑家蚊 (Culex tritaeniorhynchus) 與白線斑蚊 (Aedes albopictus) 許多組織與器官，其中以後中腸、唾腺組織與腦部為主要的感染器官。以胸腔注射病毒方式跨越中腸屏障則使三斑家蚊與白線斑蚊之感染率上升、感染進程加速與感染強度增加。不論哪一種方式獲得病毒皆顯示三斑家蚊之感受性較白線斑蚊來得高。由感染牛流行熱病毒的蚊子唾液接種幼倉鼠細胞，僅三斑家蚊之唾液具有對細胞之感染能力，表示三斑家蚊可能為此病毒的媒介昆蟲。而白線斑蚊於兩種方式感染病毒後皆無法以唾液感染幼倉鼠細胞，顯示白線斑蚊不具有傳播此病毒的潛力。綜合前人田間調查的生態資料與以上實驗結果，牛流行熱發生與本研究所證實之潛在病媒昆蟲—三斑家蚊，在地理分布、病媒豐度、偏好性及血清學研究上是相符合的。藉由確認三斑家蚊為此病毒可能之傳播媒介，有助於針對易孳生蚊子之環境進行預防及殺蟲藥劑之應用。

Bovine ephemeral fever is caused by Bovine ephemeral fever virus (BEFV) that belongs to the genus Ephemerovirus of the family Rhabdoviridae. This disease causes significant economic loses through loss in milk production, loss of condition in beef herd and high cost of medication. Although BEFV has been isolated from field collected biting midges (Culicoides spp.), but the disparity of bovine ephemeral fever epidemic area and distribution of biting midges suggests that vectors other than Culicoides may be involved in the transmission of BEFV. The objective of this research was to investigate the susceptibility and transmission potential of mosquitoes to BEFV. Artificial bloodmeal or intra-thoracic injection was used to let mosquitoes acquire the virus. The infection of BEFV in mosquitoes was examined by immunofluorescence assay and confocal laser scanning microscopy. To examine the BEFV transmission potential of virus-infected mosquitoes, the saliva of mosquito was used to infect baby hamster cells. Results showed that BEFV infected many organs and tissues of Culex tritaeniorhynchus Giles and Aedes albopictus Skuse after oral infection through artificial bloodmeal, and the major infection was detected in posterior midgut, salivary glands and brain. Intra-thoracic injection helped BEFV to escape from the midgut barriers of Cx. tritaeniorhynchus and Ae. albopictus, and it increased virus infection rate, promote infection progress and enhance infection intensity. The susceptibility of Cx. tritaeniorhynchus to BEFV was always higher than that of Ae. albopictus no matter using oral infection or virus injection. Only the saliva collected from BEFV-infected Cx. tritaeniorhynchus had cytopathogenic effect in baby hamster cells that implies Cx. tritaeniorhynchus may be the vector of BEFV. Neither through oral infection nor intra-thoracic injection of BEFV, the saliva collected from virus-infected Ae. albopictus did not infect baby hamster cells that implies Ae. albopictus is a non-vector of BEFV. According to the ecological data from field surveys and our results, geography distribution, population dynamic, blood-feeding preference and serological preference well support the hypothesis that Cx. tritaeniorhynchus may transmit BEFV in Taiwan. This study identified Cx. tritaeniorhynchus as a potential vector of BEFV, and the information would be helpful to prevent the disease by environmental management and insecticide application.