Invasive plants facilitated by socioeconomic change shelter vectors of scrub typhus and spotted fever

Abstract

BackgroundEcological determinants of most emerging vector-borne diseases are little studied, particularly for neglected tropical disease; meanwhile, although socioeconomic change can have significant downstream effect on human risks to vector-borne diseases via a change in land cover, particularly facilitating the invasion of exotic plants, related studies remain very scarce. Scrub typhus and spotted fever are neglected diseases emerging around the globe and are transmitted by chigger mites and ticks, respectively, with small mammals as the primary hosts of both vectors.\n\nMethodology/Principal findingsWe investigated how invasion of Leucaena leucocephala plant after extensive abandonment of farmlands driven by industrialization in Penghu Islands of Taiwan affected abundance of chiggers and ticks by trapping small mammals in three types of habitats (invasion site, agricultural field, human residence) every two months for a year. Invasion sites sheltered more chiggers and ticks than the other two habitats; moreover, both vectors maintained higher abundance in early winter and populations of chiggers were more stable across seasons in invasion sites, suggesting that the invasive sites could be a temporary refuge for both vectors and might help mitigate the negative influence of unfavorable climate. Infective rates of etiologic agents in chiggers and ticks were also higher in invasion sites. Top soil temperature and relative humility were similar across the three habitats, but invasion sites harbored more Rattus losea rat, on which infested chiggers and ticks were more well fed than those from the most commonly trapped species (Suncus murinus shrew), implicating that abundance of superior hosts instead of microclimate, might determine the abundance of both vectors.\n\nConclusions/SignificanceThis study highlights an important but largely neglected issue that socioeconomic change can have unexpected consequence for human health mediated particularly through invasive plants, which could become a hotspot for emerging infectious diseases but usually are very hard to be eradicated. In the future, a more holistic perspective that integrates socioeconomy, land use, exotic species, and human health should be considered to fully understand potential emergence of vector-borne diseases.\n\nAuthor summaryUnderstanding how environmental factors, such as land use change, affect risks to vector-borne diseases helps control and prevent human diseases, but ecological preference of vectors of most neglected diseases remain little investigated. In this study, we found that vectors of scrub typhus (chigger mites) and spotted fever (hard ticks), two emerging neglected diseases, were much more abundant in sites invaded by exotic plants than the other major land cover types in a small island of Taiwan; moreover, populations of chigger mite in invasion sites were more stable across seasons, suggesting that plant invasion sites could be a refuge for disease vectors under unfavorable climate. Higher abundance of chigger mites and ticks was related to higher abundance of a superior rodent host instead of a difference in soil micro-climate. More significantly, these invasive plants are facilitated by extensive abandonment of farmlands driven by industrialization and rural to urban migration, thus demonstrating an important but largely neglected issue that socioeconomic change, when mediated through a change in land cover, can have unexpected downstream effect on emerging neglected tropical diseases.