入侵紅火蟻雙順子病毒台灣分離株之特性及對入侵紅火蟻之影響

Abstract

入侵紅火蟻 (Solenopsis invicta) 於 2003 年底首度於台灣發現，根據現今調查紀錄，除火蟻病毒 (Solenopsis invicta virus-1, SINV-1；Solenopsis invicta virus-2, SINV-2) 外，尚未發現其他的天敵或病原體；以防治角度而言，若不考慮引進其他天敵物種，將此病毒綜合餌劑使用似乎是可發展的防治技術。於擬定整合性管理前，火蟻病毒如何於族群中及族群內傳播成為此防治計畫成敗的關鍵，但前人研究大都著重於分子特性方面，使得傳播模式尚未明朗，因此，本研究致力於描述 SINV-1 的傳染力、傳播力，及感染蟻巢所產生的病徵特性，並試圖進一步探討應用此病毒之可行性。鏡檢結果顯示，自 SINV-1 感染蟻巢中純化出來之病毒顆粒，無論形狀或大小與前人純化之病毒顆粒符合。隨後之序列分析也呈現與前人發表之 SINV-1 RdRp gene 序列有高度相似性，因此，可進一步確認此病毒為 SINV-1，但 capsid gene 呈現高度變異性與前人發表資料分歧甚大。SINV-1 在台灣火蟻族群內廣泛分佈，且野外感染率達 45％。經由人工感染方式，將已感染之個體磨碎混入糖水後，餵食健康群體八天後，可成功感染，顯示該病毒於群體內具有強烈的水平傳染力。健康蟻巢於置入染病四齡幼蟲 30 天後可經 PCR 偵測到 SINV-1反應訊號，證實四齡幼蟲為 SINV-1 水平傳染重要途徑之一。病理方面，以組織切片比較感染與健康火蟻四齡幼蟲，感染幼蟲之中腸細胞出現凋亡現象 (apoptosis)，且健康四齡幼蟲經過人工感染，於不同感染時間亦能發現細胞凋亡現象逐漸顯著，推測此細胞凋亡現象對於幼蟲死亡率有密切關係。未來若加入 SINV-1 作為整合管理策略，針對餌劑選用、病毒量產及傳播都需要考量，最終才能完成一套適合台灣條件的火蟻防治策略。

The red imported fire ant, Solenopsis invicta, was first reported in Taiwan in late 2003, and no natural enemy, especially pathogen group, could be detected except two viruses, Solenopsis invicta virus-1 (SINV-1) and Solenopsis invicta virus-2 (SINV-2). An alternative control strategy that integrates virus with ongoing bait treatment therefore seems feasible and may have potential to long term suppress the fire ant in Taiwan. One core issue for this strategy is that how the virus transmits within or between colonies as well as how the virus impacts its host, which remains fairly unclear given most of studies focus on its molecular characterization. Hence, the present study aims to determine the infectivity and transmissibility of SINV-1, and characterize the symptom of infected colonies. Virus particles have been purified from infected colonies (confirmed by RT-PCR) and are consistent in size and shape under electron microscopic examination with those previously published. Subsequent sequence analyses confirmed these virus particles as SINV-1 by highly similar RdRp gene sequence identity. However, highly divergent capsid gene sequence was found in Taiwan. SINV-1 appears to be widespread in field with infection rate ranging from 30~ 45％. Furthermore, the healthy colony could be artificially infected by feeding SINV-1-infected individuals (homogenized with food source) for eight days, suggesting SINV-1 is transmissible at inter-colony level. However, the horizontal transmission was evident, most likely through trophallaxis from infected larvae to workers, in 3 of 5 replicates after 30 days. Forth instar larvae showed apoptosis in epithelial cell. Artificially infected larvae displayed serial cytopathologic effects through infective duration gradually. Induction of apoptosis by SINV-1 suggests significant association of SINV-1 infection with brood death. In future, combining SINV-1 and chemical controls for the integrated fire ant management, such as baits selection, mass production of virus and viral transmission are estimated. Hence, combination of SINV-1 and chemical pesticides to accomplish an end product of fire ant control strategy in Taiwan.