Current Concepts of the Ecology of Chigger-Borne Rickettsiosis (Scrub Typhus)*

Abstract

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Bayesian spatial modelling and the significance of agricultural land use to scrub typhus infection in Taiwan

“…The legacy of frontier cultivation in Taiwan may, therefore, be increased risk of scrub typhus infection. The association of frontier colonization with outbreaks of scrub typhus has also been documented in several other countries (Traub and Wisseman, 1974), although our study is the first one to quantitatively investigate such an association, suggesting that, similar to other vector-borne diseases, land use policy that encourages agricultural practices in the frontiers should also contemplate the potential for elevated burdens of scrub typhus infection and other vector-borne diseases (Walsh et al, 1993;Singer and de Castro 2001).…”

“…Scrub typhus frequently occurs in habitats characteristic of transitional vegetation, such as forest edges, shrub-covered margins along streams or lands left untended after forest clearance/agricultural practices (Traub and Wisseman, 1974). Abandonment of agricultural cultivation creates ideal habitats for chiggers because crop remnants and natural vegetative regrowth provide the food and shelter necessary for proliferation of rodent hosts; secondary vegetation also helps provide moist terrain essential for chigger survival (Traub and Wisseman, 1974;Muul et al, 1977;Kuo et al, 2012).…”

“…Abandonment of agricultural cultivation creates ideal habitats for chiggers because crop remnants and natural vegetative regrowth provide the food and shelter necessary for proliferation of rodent hosts; secondary vegetation also helps provide moist terrain essential for chigger survival (Traub and Wisseman, 1974;Muul et al, 1977;Kuo et al, 2012). In this study, based on univariate Poisson regression, scrub typhus incidence in Taiwan was found to increase with the proportion of land covered with crop-vegetation mosaics and forest, while it decreased with the proportion of land covered with cropland although it was not correlated with areas of forest-grass-shrub mosaics.…”

“…Thus, their distribution is constrained by the availability of suitable hosts (i.e. rodents) along with environmental conditions that can affect their survival (Traub and Wisseman, 1974). Chigger abundance is limited by temperature (particularly in temperate areas) and precipitation (particularly in tropical areas) (Gentry et al, 1977).…”

“…Chigger abundance is limited by temperature (particularly in temperate areas) and precipitation (particularly in tropical areas) (Gentry et al, 1977). Land cover is also an important factor in determining the occurrence of the disease, with higher incidence occurring in areas of secondary or transitional vegetation, presumably in relation to the preferred habitat of the rodent species on which chiggers generally feed (Traub and Wisseman, 1974). In addition, socioeconomic factors may influence transmission: higher incidence tends to occur in rural areas, with farmers at higher risk of infection, and it has been shown that increasing urbanisation and school enrolment can decrease incidence of scrub typhus (Olson and Bourgeois, 1979;Ogawa et al, 2002;Lee et al, 2006;Vallee et al, 2010;Kuo et al, 2011).…”