Spatial analysis of scrub typhus infection and its association with environmental and socioeconomic factors in Taiwan

Abstract

We analyzed the spatial distribution of human cases of scrub typhus on the main island of Taiwan from 2003 to 2008 and implemented an island-wide survey of scrub typhus vectors (trombiculid chiggers) in 2007 and 2008. The standardized incidence rate 'SIR' incorporating inter-district variations in population, gender and age was correlated with environmental and socioeconomic variables. Higher incidence and SIR rates were clustered in the less developed, mountainous regions of central and eastern Taiwan. Higher SIRs were also associated with a higher proportion of dry-field farmers in the population, a higher normalized difference vegetation index (NDVI) and lower mean annual temperature, but was not associated with rainfall. Small mammal hosts in high-SIR districts harbored more chiggers and had higher rates of seropositivity against Orientia tsutsugamushi Hyashi, the etiologic agent of scrub typhus, compared to low-SIR districts. The concurrence of a higher proportion of dry-field farmers and higher NDVI has likely led to the clustering of scrub typhus in the mountainous regions of Taiwan. Further individual-level study of the risk factors associated with scrub typhus, and a better understanding of the effect of environmental factors on chigger abundance, should help to prevent scrub typhus in Taiwan.