Abstract
Thailand experienced an explosive country-wide outbreak of leptospirosis in the late 1990s, followed by high endemic transmission. The key barrier to effective control has been a lack of knowledge about the factors driving the timing and spatial distribution of this persistent transmission. We obtained data on weekly leptospirosis incidence in the 320 districts of northeastern Thailand between 2000 and 2014 from the Thai passive notifiable disease surveillance system (R506). We modeled incidence using a spatiotemporally explicit Poisson model and first examined the effects of current and lagged rainfall and temperature (Thai Meteorology Department). We then collected data on environmental covariates—land use (Thai Land Development Department), livestock and irrigation (FAO), NDVI, NDWI, and elevation (Google Earth Engine)—and evaluated their effects on spatial variation in incidence. Between 2000 and 2014, 53,719 cases of leptospirosis were reported in northeastern Thailand. The timing of peak incidence varied between early August and mid-October and did not coincide with periods of rice planting or harvesting. Instead, weekly incidence was strongly associated with rainfall and temperature in the current and two prior weeks. Districts with high flooding propensity (NWDI, OR = 95.24 per 0.01 index point), a high percentage of rice paddies (OR = 1.057 per %), and low cattle density (OR = 0.98 per head) had significantly higher leptospirosis incidence. We also encountered significant spatiotemporal residuals in our model that appear to represent focal outbreaks. Our study found that rainfall and temperature, not specific events in the agricultural cycle, were the main determinants of peak transmission. We also identified specific environmental features associated with persistent high transmission which may serve as targets for prevention. However, in addition to this endemic pattern, outbreaks contribute to the burden of leptospirosis. Understanding the sources of these epidemics will be important for leptospirosis control in this region.