多數火山島嶼由於面積小且物種不易到達，在缺乏捕食者和競爭者的情況下，外來種包括囓齒類動物一旦被引入，族群通常會變大，且能入侵利用各種棲地，而囓齒類動物是許多疾病的重要宿主。蘭嶼這一火山島為恙蟲病高風險區域，當地的硬蜱也帶有多種斑點熱群立克次體。囓齒類動物為恙蟎與硬蜱的主要宿主，而蘭嶼的囓齒類動物以外來種亞洲家鼠(Rattus tanezumi)為主，相較於在臺灣本島的亞洲家鼠僅出現在住家周遭，過去研究發現蘭嶼的亞洲家鼠也會出現在草地與森林等多種棲地中，因此本研究欲得知，恙蟎與硬蜱是否會伴隨著亞洲家鼠的擴張而進入草地和森林環境，因而增加當地民眾感染疾病的風險。本研究選擇蘭嶼的三塊部落住家、三塊草地、以及四塊森林，捕捉小型哺乳動物並調查體外寄生的恙蟎與硬蜱。2017年9月至2018年6月的調查只捕捉到亞洲家鼠，在三種棲地類型均會出現，但部落的家鼠捕捉數量(2.9±0.8隻；平均±標準誤差)顯著低於草地(5.9±1.2隻；p < 0.05)與森林(9.3±1.3隻；p < 0.001)，草地的捕捉數量也顯著低於森林(p < 0.05)，且12月的家鼠捕捉數量(10.3±1.9)顯著高於其他月份(均為p < 0.05)。所有亞洲家鼠都帶有恙蟎，共採集到三種恙蟎，但絕大多數(>95%)為地里纖恙蟎(Leptotrombidium deliense)；平均恙蟎寄生數量(恙蟎總數量/家鼠總數量)在棲地與季節間有顯著交互作用(p < 0.001)，在部落(285.8±84.9)、草地(545.5±100.9)與森林(450.7±77.1)有不同的季節變化，9月時草地有最高的恙蟎總數量(5579.3±1519.1)，森林恙蟎總數量則是在6月時出現高峰(4958.0±1357.4)。家鼠的硬蜱寄生率62.6%，共採集到三屬硬蜱，以粒形硬蜱(Ixodes granulatus)為主(>70%)；部落平均硬蜱寄生數量(硬蜱總數量/家鼠總數量；0.9±0.4)顯著低於草地(14.8±8.1；p < 0.001)，森林(6.9±1.9)與其他兩棲地則沒有差異(均為p > 0.05)，且12月顯著低於其他月份(p < 0.001)，此外草地與森林的硬蜱總數量在9月與12月顯著高於部落(均為p < 0.001)，並在3月時數量顯著上升(均為p < 0.05)。綜合上述結果，外來種亞洲家鼠以及恙蟎和硬蜱不但會在草地與森林中出現，且數量通常較部落住家環境多，顯示外來種囓齒類動物的入侵，不但會嚴重影響火山島嶼的生態系，也可能會增加島嶼居民感染鼠類相關疾病的風險。

Lack of predators and competitors in small and hard-to-reach volcanic oceanic islands can facilitate ecological release of introduced species, including rodents that serve as hosts or reservoirs of a variety of zoonotic diseases. The volcanic Lanyu Island has a high prevalence of scrub typhus, which is transmitted by chigger mites; in addition, spotted fever group rickettsiae have been detected in hard ticks. Rodents are the primary hosts of chiggers and hard ticks. The rodent community in Lanyu is dominated by the exotic Asian house rat (Rattus tanezumi), which can be observed in different types of habitats, including grasslands and natural forests. This is in contrast to Taiwan main island where this species is limited to human residence areas. This study investigated the role of the Asian house rat in hosting chiggers and ticks and whether the grasslands and natural forests in Lanyu also shelter chiggers and ticks. From September 2017 to June 2018, we set up rodent traps and collected ectoparasites from captured rodents in each of the three human residence sites, three grassland sites, and four natural forest sites. Results showed that the Asian house rat was the only rodent trapped across all three habitat types. Abundance of R. tanezumi was significantly lower in human residence sites (2.9±0.8 individuals; mean±SE) than in grasslands (5.9±1.2; p < 0.05) and in forests (9.3±1.3; p < 0.001); their abundance in grasslands also lower than in forest (p < 0.05). Meanwhile, rat abundance was significantly higher in December (10.3±1.9) than the other month (all p < 0.05). All rats were infested with chiggers, which includes three species, although the chigger fauna is dominated (>95%) by Leptotrombidium deliense. There was a significant interactive effect of habitat and season on mean chigger load (total number of chiggers/number of rats; p < 0.001); seasonal change in mean chigger load varied among human residence (285.8±84.9), grasslands (545.5±100.9) and forests (450.7±77.1). The grasslands had the highest total number of chiggers in September(5579.3±1519.1), while total number in forests peaked in June(4958.0±1357.4). Prevalence of tick infestation was 64.6%. Three tick genus were identified, with Ixodes granulatus being the most commonly observed (>70%). Mean ticks load (total number of ticks/number of rats) was significantly lower in human residence (0.9±0.4) than in grasslands (14.8±8.1; p < 0.001); while forests (6.9±1.9) did no differ from the other two habitats (both p > 0.05), and December had a significantly lower tick load than the other months (p < 0.001). In September and December, the total number of ticks in grasslands and in forests were significantly higher than in human residence (p < 0.001), and increased significantly in March (all p < 0.05). This study found that the exotic Asian house rats, and the chiggers and ticks that they carry, are also observed in grasslands and forests of Lanyu, and usually attained higher abundance than in human residence sites. These results suggest that the introduction of exotic rodents to volcanic islands will not only have devastating effect on ecosystems but can also increase human exposure to rodent-borne diseases.

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