澎湖恙蟎數量在不同棲地間的差異

Abstract

恙蟲病(scrub typhus)是由帶有恙蟲病立克次體(Orientia tsutsugamushi)的恙蟎叮咬所感染，小型哺乳類為恙蟎主要宿主。臺灣近年來以澎湖縣的恙蟲病病例數最多，澎湖由於人口外移與人口老化，許多農耕地逐漸荒廢，並為外來植物銀合歡(Leucaena leucocephala)所佔據。本研究在澎湖縣選擇四塊銀合歡林，三塊農耕地與三塊住宅樣區，每兩個月在每個樣區放置30至33個捕鼠籠，進行四天三夜的捕捉，以比較三種棲地間，哺乳類動物種類、數量以及哺乳類動物身上恙蟎的數量以及種類的差異。2016年12月至2017年10月的調查結果顯示，捕獲率(隻數/捕捉籠夜)以錢鼠(Suncus murinus)最高(0.198)，其次為家鼷鼠(Mus musculus；0.071)、小黃腹鼠(Rattus losea；0.029)與溝鼠(Rattus norvegicus；0.001)，但總共採集到的42,274隻恙蟎中，有76.3%寄生於小黃腹鼠上，其次為錢鼠(18.7%)、家鼷鼠(2.9%)與溝鼠(2.1%)。共鑑定恙蟎10,815隻，扣除掉因蟲體破損或缺失無法辨識的1,035隻，其餘的9,780隻恙蟎，皆為地里纖恙蟎(Leptotrombidium deliense)，其中採集自溝鼠和小黃腹鼠的地里纖恙蟎相對飽食程度(分別為16.1±1.1 104µm2，15.5±0.3 104µm2)顯著大於家鼷鼠(6.6±0.6 104µm2)和錢鼠(5.2±0.3 104µm2) (皆為p< .0001)。小黃腹鼠的捕獲率，在不同棲地類型與不同捕捉月份間皆有顯著差異(均為p < .001)，且棲地和月份之間有交互作用(p < .01)，多數月份，在銀合歡林的捕獲率顯著大於農地與住宅(皆為p < .05)；自小黃腹鼠身上採集到的恙蟎，有74.4%來自銀合歡林，23.5%來自農地，2.0%來自住宅；小黃腹鼠的總恙蟎寄生量在六個月份間的變異係數(coefficient of variation)，以銀合歡林最低(0.72)，其次為農地(0.92)，住宅則是最高(1.47)。結果顯示在澎湖，小黃腹鼠數量雖不多，但多數恙蟎寄生在小黃腹鼠身上，且相較於數量最多的錢鼠，前者恙蟎的飽食程度大約是後者的三倍，顯示小黃腹鼠是恙蟎最重要的宿主；三種棲地中，則以銀合歡林採集到的恙蟎數量最多，且季節間的變動程度最低，代表銀合歡林可能為恙蟎於氣候條件較不佳時的庇護所。本研究發現透過提供病媒宿主較佳的棲地環境，外來種植物的入侵會增加病媒的數量，可能增加人類感染疾病的風險。Scrub typhus is transmitted by chigger mites infective of Orientia tsutsugamushi (OT), and small mammals are the primary hosts of chiggers. Prevalence of scrub typhus is high in Penghu, where most agricultural fields are abandoned and invaded by the exotic Leucaena leucocephala. In this study, four L. leucocephala invasion sites , three cultivating agricultural fields, and three human dwelling areas were surveyed every two months (for a whole year) by deploying 30 to 33 rodent traps in each study site for three consecutive nights. Number and species of small mammals and their associated chiggers were compared among the three habitat types. Suncus murinus was more abundant (trapping rate = 0.198, individuals/trapping nights) than Mus musculus (0.071), Rattus losea (0.029) and Rattus norvegicus (0.001), but 76.3% of the 42,274 collected chiggers came from R. losea, in comparison with 18.7% from S. murinus, 2.9% from M. musculus and 2.1% from R. norvegicus. I have successfully identified 9,780 chiggers, and all are Leptotrombidium deliense. Relative engorgement degree of chiggers collected from R. norvegicus and R. losea (16.1±1.1 104µm2，15.5±0.3 104µm2，respectively) were significantly higher than those from M. musculus (6.6±0.6 104µm2) and S. murinus (5.2±0.3 104µm2) (all p< .0001). Trapping rate of R. losea differed among habitat types and months (both p < .001), and there was an interaction between habitat and month (p < .01): in most months, trapping rate was higher in L. leucocephala invasion sites than in cultivating fields and human dwelling areas. Among all chiggers collected from R. losea, 74.4% came from L. leucocephala invasion sites, 23.5% from cultivating fields, and 2.0% from human dwelling areas. Coefficient of variation for chiggers on R. losea, interms of monthly variation, was lower in L. leucocephala invasion sites (0.72) than in cultivating fields (0.92) and human dwelling areas (1.47). I have found that although R. losea is not the commonest species in Penghu, it is parasitized by the majority of chiggers; moreover, relative engorgement degree of chiggers collected from R. losea is around three times that of chiggers collected from the most abundant host S. murinus, demonstrating that R. losea is the primary host of chiggers in Penghu. Among the three habitat types, most chiggers were collected from L. leucocephala invasion sites; in addition, monthly variation in chigger abundance was the lowest in this habitat type, revealing that L. leucocephala invasion sites could be the shelter for chiggers under unfavorable climatic conditions. This study shows that via providing better habitat for vertebrate hosts, the invasion of exotic plants could increase the number of disease vectors, thus elevating human risks for vector-borne diseases.