Agricultural development dramatically alters the environment, and may affect abundance of disease vectors, thereby altering susceptibility of humans to vector-borne diseases. Many intensive and extensive farming practices are driven by global market demands, in some cases leading to increases in vector-borne diseases. On the other hand, globalization can lead to abandonment of farms in countries with high agricultural costs. We document that such farm abandonment may have important implications for human health. After Taiwan joined the World Trade Organization, extensive areas of rice cultivation were deserted due to intense price competition. Periodic plowing is subsidized to suppress agricultural pests, but this may terminate due to severe economic burdens. Hard ticks and chiggers (larval trombiculid mites), which can transmit spotted fever (Rickettsia spp.) and scrub typhus (Orientia tsutsugamushi) to humans, are subject to environmental change and are likely to emerge after rice paddies are abandoned. In 2007 and 2008, we compared ectoparasite loads (ticks and chiggers) and disease prevalence on rodents, the main hosts for both vectors, in 17 periodically plowed plots and 16 abandoned (fallow) plots in eastern Taiwan, where scrub typhus is endemic and spotted fever circulates. Flooded rice paddies were not suitable for rodent trapping, and were assumed to sustain very few, if any, ticks and chiggers.
Results/Conclusions
Apodemus agrarius are common rodents in the study site, and serve as main hosts for ticks and chiggers. Although relative population size of A. agrarius was similar in plowed and fallow plots, tick and chigger loads were 6X and 3X higher in fallow than plowed plots. Moreover, ticks and chiggers were dominated by species that also infest humans. Seroprevalence of spotted fever and scrub typhus was very high in both types of plots (>95% and > 80%, respectively), but the latter was significantly higher in fallow fields, implicating elevated disease risks for local people. Other less common rodent species (Rattus losea and Bandicota indica) may be competent hosts for both diseases, but relative population size, disease prevalence, and ectoparasite loads of both host species were either similar in both treatments or higher in fallow plots. Ticks and chiggers proliferate after rice paddies are abandoned, but periodic plowing may mitigate vector burdens. This study suggests that an unexpected consequence of globalization in Taiwan may be elevated disease risk, although this can be modified by agricultural policies, calling for further research on vector-borne diseases and their control from broader perspectives.
