Scrub typhus in Nan province (Thailand): Seventeen years of data to understand the impact of land cover change.

BACKGROUND

Scrub typhus, caused by Orientia tsutsugamushi and transmitted by chigger mites (Leptotrombidium), is a major health problem in northern Thailand, particularly in Nan province. Land cover change, by altering the ecosystem, could affect the ecology of the vector and consequently the risk of scrub typhus transmission.

METHODOLOGY/PRINCIPAL FINDINGS: This study investigated the impact of land cover changes on scrub typhus transmission in 2.5 km buffer zones around each village of Nan Province between 2003 and 2019. Using the open land cover data of the European Spatial Agency Climate Change Initiative (ESA CCI), we quantified land cover composition and land cover changes and integrated public health data on scrub typhus cases, as well as information on elevation, population, and slope. Generalized Additive Models were applied to assess the effects of land cover changes on annual scrub typhus cases. Scrub typhus cases increased significantly during the study period, peaking in 2012 and 2016, mainly in mountainous areas rather than in the Nan River valley. Land cover associated with cases included shrubland, mosaic land, broadleaf forest, and needleleaf forest. Cases increased with shrubland and mosaic land, displayed an inverted U-shaped relationship with broadleaf forest, and decreased with needleleaf forest. Key land cover change factors included shrubland transitions, population, and geographic interactions. Reforestation (from shrubland to broadleaf forest) showed an inverted U-shaped relationship with cases, whereas stable broadleaf forest and loss of shrubland to grassland became non-significant. Male population increased cases.

CONCLUSIONS/SIGNIFICANCE: This study highlights the importance of land cover changes in understanding disease transmission and suggests that landscapes disturbance may create optimal conditions for O. tsutsugamushi transmission. This is a novel regional-scale exploration of land cover impacts on scrub typhus in Thailand.