Title : The influence of evolving land covers on Scrub typhus spread in Nan Province, Thailand (2003-2019)
Abstract:
Scrub typhus is a vector-borne disease caused by Orientia tsutsugamushi and transmitted by chigger mites of the genus Leptotrombidium. Endemic to the tsutsugamushi triangle, including Japan, Southeast Asia, and Australia, scrub typhus affects up to 77% of people in some regions of Thailand, particularly in the northern provinces like Nan. This disease predominantly impacts rural populations living in areas with significant spatial heterogeneity, with farmers and the elderly being especially at risk.
This study investigates the relationship between land cover changes and scrub typhus incidence in Nan Province, Thailand, from 2003 to 2019. The objective is to identify the risk or protective effects of evolving land covers within a 2500 m buffer zone around each village. By analyzing land cover surfaces using ESA CCI data, combined with public health surveillance records, elevation, population density, and mean slope data, we aimed to understand how these variables influence scrub typhus transmission.
The number of scrub typhus cases in Nan Province increased from 2003 to 2019, peaking in 2012 and 2016. The disease was more prevalent in the mountainous and piedmont areas to the north, northwest, and east of the Nan River valley, with lower rates in the valley itself. Key land covers significantly associated with scrub typhus cases include shrubland, mosaic lands, and forests. Specifically, scrub typhus cases increased with the extent of shrubland and mosaic land within village buffer zones. A reversed U-shape relationship was observed with broadleaf forests, indicating that both very low and very high forest coverage were associated with increased disease risk, while needleleaf forests had a negative correlation. Rainfed cropland was also linked to an increase in cases, after reaching a plateau.
Further analysis of land cover changes revealed that stable forests were not significantly correlated with an increase or scrub typhus cases whereas the reforestation process through the transition from shrubland to forest was. The presence of stable rainfed cropland was at risk when in small proportion in the village surroundings but became protective when in bigger proportion. The process of urbanisation through the loss of rainfed cropland to urban areas also had a protective effect for people.
This study underscores the importance of land cover as a key factor in scrub typhus distribution among human populations. The interface between human habitats and wild or disturbed areas, especially those frequented by farmers, is critical in understanding and managing disease risk. While previous research has focused on the influence of climatic factors on scrub typhus distribution, our study is one of the first to explore the impact of land cover changes at this scale in Thailand, offering valuable insights for disease prevention and control.
