The potential effect of exotic Pacific rats Rattus exulans on vectors of scrub typhus.
Abstract
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. 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. R. exulans was the most common small mammal species captured (31.4% 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 (39.1%). 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. 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.