A study of small-scale local transmission of dengue fever in an area without previous epidemic based on geographic information system and molecular tracing: a case study of an outbreak in Rongchang district, Chongqing, 2025

Objective To systematically characterize the spatiotemporal distribution, risk factors, and viral genetic characteristics and origin of a local dengue outbreak with limited size in Rongchang district, Chongqing municipality, in 2025, and provide evidence for the transmission tracing and precision control of dengue fever in areas without previous dengue fever epidemic.

Methods The clinical data of laboratory-confirmed dengue fever cases in the local outbreak in Rongchang during July - October 2025 were collected. Kernel density estimation and epidemic centroid migration analyses were performed by using the geographic information system (GIS). The correlations between daily case count and meteorological variables with a 14-day lag were analyzed. Based on the temporal distribution of the outbreak, 16 serum samples were selected from the cases for whole-genome sequencing of dengue virus, followed by phylogenetic analysis.

Results A total of 274 dengue fever cases were reported in the outbreak in Rongchang, all the cases were caused bys dengue virus 1 (DENV-1). The incidence curve showed a bimodal incidence peaks. Kernel density analysis indicated that the outbreak usually occurred in the old urban area, gradually spreading to the new urban area, and the epidemic centroid exhibited an overall northeastward shift. Daily case count was positively correlated with lagged daily mean relative humidity and rainfall, but negatively correlated with daily mean temperature. Phylogenetic analysis demonstrated that all the 16 strains of DENV-1 belonged to genotype I and shared high sequence homology.

Conclusion Based on the GIS and molecular tracing, this study revealed that the dengue fever outbreak in Rongchang in 2025 might be caused by the viruses from other areas or with same source. Subsequent spread was associated with the combined effects of urban area heterogeneity and meteorological factors. These findings can be used as reference for the early warning and control of dengue fever outbreaks in areas without previous dengue fever epidemic.