Climate drivers and winter constraints of dengue epidemics: a 10-year epidemiological perspective study in the Lao People’s Democratic Republic

BACKGROUND: Dengue fever is hyperendemic in the Lao People's Democratic Republic (PDR), where transmission is driven by Aedes mosquitoes and influenced by large-scale climatic phenomena, including the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). As a landlocked nation, the Lao PDR experiences sharper winter temperature declines than coastal regions, which may impose a seasonal "bottleneck" on vector survival and dengue transmission. This study examined whether winter minimum temperatures act as a seasonal transmission bottleneck, alongside the Oceanic Niño Index (ONI) and the Dipole Mode Index (DMI), during 2014-2023. METHODS: Monthly dengue case counts reported to the National Center for Laboratory and Epidemiology, Ministry of Health, Lao PDR, from January 2014 to December 2023 were analyzed using region-specific quasi-Poisson distributed lag nonlinear models. Models incorporated 3-month-lagged ONI/DMI cross-basis functions, winter minimum temperature hinges, long-term trends, and seasonality, with population as an offset. Region-specific estimates were pooled using multivariate meta-analysis to generate best linear unbiased predictions (BLUPs). Optimal lag structures and temperature thresholds were selected by minimizing the quasi-Akaike information criterion and residual sum of squares. RESULTS: A total of 134,093 dengue cases were reported, with substantial regional heterogeneity. The Capital Region had the highest burden (40,672 cases; annual incidence 35.4 per 100,000), followed by the Southern Mountains and Tropical Rainforests region (20,176 cases; 23.3 per 100,000). Annual incidence in each region appeared constrained by region-specific winter minimum temperature thresholds. Pooled BLUPs analyses adjusted for covariates revealed monotonic cumulative relative risk increases with ONI [RR = 2.83 at ONI = 2.0; 95% confidence interval (CI): 1.46-5.49) and decreases with DMI (RR = 0.37 at DMI = 1.5; 95% CI: 0.24-0.59). CONCLUSIONS: Winter cold functions as a primary bottleneck for dengue transmission in the Lao PDR, with ENSO amplifying and IOD suppressing outbreak risk. These findings support the development of climate-integrated, region-specific early warning systems. Incorporating 3-month-lagged climate indices may enhance public health preparedness for future dengue outbreaks.