Ecology of scrub typhus in Taiwan: The impact of environment, rodent community, and socio-economy.

摘要

Ecological knowledge can be valuable for the prevention of human infectious disease, which is exemplified in this study with scrub typhus, a lethal human disease vectored by larval trombiculid mites (i.e., chiggers) harboring the rickettsia Orientia tsutsugamushi (OT) and with rodents as main hosts.;In Chapter 1 I show that more incidences and higher standardized incidence rate (SIR) clustered in less developed, mountainous central and eastern Taiwan. Higher SIR was related to higher proportions of dry_field farmers, higher normalized difference vegetation index, and lower annual mean temperature, but could not be explained by rainfall.;In Chapter 2 I document that in Hua-lien of eastern Taiwan, Apodemus agrarius presented the highest rate of infestation with both chiggers and ticks (the latter vectoring tick-borne spotted fever group (SFG) rickettsioses). Leptotrombidium imphalum and immature Rhipicephalus haemaphysaloides were the most abundant chiggers and ticks identified. Immunofluorescent antibody assay revealed high seropositive rates of rodents against OT, and tick-borne SFG rickettsiae. In Chapter 3, however, it was shown that naturally occurring L. imphalum recovered from Rattus losea and Bandicota indica were 1.4x and 1.3x more engorged than those from A.agrarius. Moreover, I did not observe significant variation in chigger engorgement both within and among host species across replicated plots, contradicting predictions of the Ideal Free Distribution.;In Chapter 4 I report that the abandonment of flooded rice paddies after Taiwan joined the World Trade Organization resulted in increased numbers of ticks and chiggers, whereas periodic plowing, a policy intending to decrease agricultural pests, may unexpectedly mitigate vector burdens. This chapter underscores the fact that regional or global economic policies may have unexpected and important local impacts.;Finally, in Chapter 5 I demonstrate that prevalence and loads of chiggers, as well as infective rates of OT in R. exulans, a recently introduced and expanding rodent species in eastern Taiwan, was moderate in comparison with other native species; however, both prevalence and loads of chiggers in R. exulans vary greatly with environment, implying a potential health risk as this species expands to areas with more chiggers.;These studies demonstrate the prevalence of scrub typhus as an outcome of complex interactions among environment, rodent community, and socio-economy, and call for a more holistic approach for the prevention of this disease.