Genomic and epidemiological evidence linking a pre-season dengue case to local overwintering transmission in Guangdong, China

Background In March 2025, Guangdong Province reported China’s earliest locally acquired dengue case, occurring two months before the typical transmission season. This unprecedented event raised critical questions about potential overwintering transmission mechanisms in subtropical regions. The investigation sought to determine whether this case represented a resurgence from the 2024 outbreak or a new importation event. Methods We conducted comprehensive epidemiological, entomological, and laboratory investigations to trace the origin of infection. Standardized case interviews were conducted to record the patient’s 14-day exposure history, including travel patterns. A targeted investigation within a 100-meter radius of the case exposure sites was conducted. Active surveillance and entomological assessments—including Breteau Index calculations and ovitrap deployment—were implemented within 200-meter radius of exposure sites. Serum samples were tested for DENV using real-time RT-PCR, NS1 antigen, and IgM/IgG antibodies. The envelope (E) gene was sequenced using Sanger sequencing, followed by phylogenetic analysis against global variants with the Neighbor-Joining method in MEGA software. Principal Findings Genomic analysis of the envelope (E) gene revealed 99.93% identity between the case’s dengue virus serotype 1 strain and a locally acquired 2024 case, differing by only a single-nucleotide variant. The high-risk area showed a relatively high dengue prevalence within the previous outbreak year. Meteorological data confirmed that 22 of 26 days during the exposure window met temperature thresholds (20–25°C) conducive to mosquito activity. Conclusions This investigation provides genomic and epidemiological evidence suggesting overwintering dengue transmission in China. The findings are consistent with local persistence from the 2024 outbreak, with vertical transmission in overwintering mosquitoes as a leading hypothesis.. These results challenge traditional assumptions about seasonal transmission patterns and underscore the need for year-round vector surveillance in subtropical regions experiencing climate warming.