Climatic Drivers of Spatiotemporal Heterogeneity in Dengue Vector Seasonality and Transmission Across a North–South Gradient in Laos: A Time-Series Analysis (2024–2025)

Abstract

Background: Laos experiences a pronounced north–south gradient in dengue vector seasonality and transmission, yet the climatic drivers underlying this heterogeneity remain unquantified. Objective: To quantify the associations between climatic factors (temperature, precipitation, relative humidity) and Aedes larval indices (Breteau Index, BI) and dengue case incidence across three latitudinally distinct sites in Laos, and to estimate lag effects for evidence-based differentiated vector control. Methods: Monthly BI and dengue case data (January 2024–December 2025) were obtained from longitudinal entomological and epidemiological surveillance in Luang Prabang (north, 19°53′N), Vientiane (central, 17°58′N), and Attapeu (south, 14°48′N). Gridded monthly climate data (mean temperature, total precipitation, mean relative humidity) at 0.1° resolution were extracted from ERA5-Land. Cross-correlation functions (CCF) were used to identify optimal lag months between climate variables, BI, and cases. Distributed lag non-linear models (DLNM) were fitted to estimate the non-linear and delayed effects of climate on BI and dengue incidence, adjusting for seasonal trends. Attributable fractions were calculated for key climatic windows. Results: A clear north–south climatic gradient was observed: Attapeu had the highest annual mean temperature (26.5°C), earliest rainy season onset (March), and highest cumulative rainfall (2,340 mm); Luang Prabang had the lowest temperature (22.8°C) and latest monsoon onset (June). Temperature explained 68–74% of BI variance across sites, with peak effects at 1-month lag. Precipitation showed stronger effects on BI in Luang Prabang (r=0.81 at 1-month lag) than Attapeu (r=0.52 at 0-month lag), reflecting baseline water availability differences. The optimal climatic windows for dengue cases—defined as the climatic conditions associated with the highest predicted dengue risk and vector proliferation in the DLNM framework—were: Attapeu, April–May (temperature 26–29°C, rainfall 180–250 mm/month); Vientiane, May–July (27–30°C, 220–300 mm/month); Luang Prabang, July–September (25–28°C, 250–350 mm/month). Relative humidity ≥75% was necessary but not sufficient for BI elevation. DLNM revealed that a 1°C temperature increase above site-specific thresholds was associated with a 12–18% increase in dengue risk at 1–2 months lag. Cumulative rainfall in the wettest 3-month period accounted for 52–63% of annual dengue cases (attributable fraction). The BI–case lag was consistently 1–2 months across all sites. Conclusion: Temperature and precipitation jointly drive the north–south gradient in dengue vector seasonality in Laos, with site-specific optimal climatic windows. These findings support a differentiated "south-early, central-mid, north-late" vector control calendar: source reduction initiation in Attapeu (March–April), Vientiane (April–May), and Luang Prabang (June–July). Integrating seasonal climate forecasts into dengue early warning systems can enhance proactive control in Laos.