Commensal rodent habitat expansion enhances arthropod disease vectors on a tropical volcanic island

Release from predators and competitors on volcanic islands can lead to increased body size and population density, as well as expanded habitat usage of introduced animals relative to their mainland counterparts. Such alterations might help the spread of diseases on islands when these exotic animals also carry pathogenic agents, but is rarely investigated. The commensal Asian house rat (Rattus tanezumi) is confined to human residential surroundings in Taiwan mainland but can be observed in the primary forests of the nearby Orchid Island, a tropical volcanic island. Orchid Island is also a hotspot of scrub typhus, a lethal febrile disease transmitted by larval trombiculid mites (chiggers) infective of the rickettsia Orientia tsutsugamushi (OT). We predicted an increase in chigger abundance when rodents, the primary host of chiggers, invade forests from human settlements as soils are largely devoid of in the latter habitat but is necessary for the survival of nymphal and adult mites. A trimonthly rodent survey in ten sites of three habitats (human resident, grassland, and forest) found only R. tanezumi and showed more R. tanezumi and chiggers in forest than in human residential sites. There was a positive association between rodent and chigger abundance, as well between rodent body weight and load of chiggers. Lastly, >95% of chiggers were Leptotrombidium deliense and their OT infection rates were similar among the habitats. Our study demonstrated potentially elevated risks of scrub typhus when the commensal rat is allowed to invade natural habitats on islands. In addition, while the success of invasive species can be ascribed to their parasites being left behind, island invaders might instead attain more parasites when the parasite requires only a single host (e.g. trombiculid mite), is a host generalist (L. deliense), and is transferred from unsuitable to suitable habitats (human settlement on mainland to forest on island).