Scrub typhus and tick-borne spotted fever group (SFG) rickettsioses are transmitted by chiggers (larval trombiculid mites) and hard ticks, respectively. We assessed exposure to these disease vectors by extensively sampling both chiggers and ticks and their small mammal hosts in eastern Taiwan during 2007 and 2008. The striped field mouse Apodemus agrarius Pallas (Rodentia: Muridae) was the most common of the small mammals (36.1% of 1393 captures) and presented the highest rate of infestation with both chiggers (47.8% of 110 760) and ticks (78.1% of 1431). Leptotrombidium imphalum Vercammen-Grandjean & Langston (Trombidiformes: Trombiculidae) and immature Rhipicephalus haemaphysaloides Supino (Ixodida: Ixodidae) were the most abundant chiggers (84.5%) and ticks (>99%) identified, respectively. Immunofluorescent antibody assay revealed high seropositive rates of rodents against Orientia tsutsugamushi Hyashi (Rickettsiales: Rickettsiaceae), the aetiological agent of scrub typhus (70.0% of 437 rodents), and tick-borne SFG rickettsiae (91.9% of 418 rodents). The current study represents a first step towards elucidating the potential hosts and vectors in the enzootic transmission of O. tsutsugamushi and tick-borne SFG rickettsiae in Taiwan. Further studies should focus on characterizing pathogens in L. imphalum and R. haemaphysaloides, as well as the proclivity of both vectors to humans. Uncovering the main hosts of adult ticks is also critical for the prevention of SFG rickettsial infections.
Summary1. The role and influence of exotic species in indigenous vector-borne diseases are important but remain understudied. Ascertaining whether disease vectors prefer exotic vs. native hosts has important implications for human health. Moreover, evaluating whether exotic hosts are intrinsically less susceptible to vectors, or whether vector loads can vary with environment, is instructive for possible disease dynamics in the face of range expansion of exotic species. Rattus exulans has recently been recorded for the first time in eastern Taiwan, where scrub typhus is prevalent. We assessed the role of R. exulans as a host for chiggers (larval trombiculid mites), and the degree to which vector loads of R. exulans exhibited spatial variation. 2. We deployed live traps in two villages in Taiwan that differed in human population density and in the incidence of scrub typhus. We recovered and tallied chiggers from small mammals, and identified over one-fifth of chiggers from each host to species. Chiggers were assayed for Orientia tsutsugamushi (OT) infection with nested-PCR. 3. R. exulans was the most common small mammal species captured (31AE4% total captures), and supported about one-fifth of total chiggers recovered. Leptotrombidium imphalum dominated the chigger assemblage of most native species (>90%), but Gahrliepia spp. was commonly found in R. exulans (39AE1%). We detected OT in the genus Leptotrombidium (39%) but not in the Gahrliepia. Prevalence and loads of chiggers in R. exulans were about 5· and 17· higher, respectively, in the less densely populated village; a similar trend also occurred with native R. losea. 4. Synthesis and applications. Currently, R. exulans appears to play a relatively minor role in supporting chiggers. However, the fact that both prevalence and loads of chiggers in R. exulans vary greatly with environment, along with the abundance of most exotic species and the ecological flexibility of R. exulans, implies a potential health risk as this species expands to areas with more chiggers. Our study suggests that a clearer understanding of interactions among native and exotic hosts and native parasite species can facilitate prediction of the impact of exotic hosts on the dynamics of vector-borne diseases.
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