Emerging local chikungunya virus transmission in a major urban area in Southern China: characteristics of clinical manifestations, viral evolution and climatic influences

The 2025 chikungunya fever outbreak in Foshan, China rapidly spread from a previously non-endemic area, raising significant public health concerns. This event underscores the need to understand the factors driving viral transmission, host responses and the influence of local environmental changes. The primary objectives of this study were to: (i) characterize the clinical manifestations of chikungunya patients at the initial stage of the outbreak to facilitate concise diagnosis and treatment; (ii) determine the viral genetic factors contributing to the outbreak's rapid spread; and (iii) investigate the influence of local environmental and climatic conditions on vector mosquito reproduction. We quickly collected and analyzed clinical data from 134 patients hospitalized for the purpose of quarantine at the beginning of the outbreak. While fever, arthralgia and rash are the typical symptom triad of chikungunya fever, we found that they did not always present simultaneously at onset. Arthralgia was the most common presenting symptom. Phylogenetic analysis revealed that the viral strains were highly homologous to those from the Réunion outbreak, suggesting an imported origin. Furthermore, we identified the presence of E1-A226V, E2-I211T and E2-L210Q mutations, which have been previously associated with enhanced transmission by <i>Aedes albopictus</i>. Local climatic conditions during the outbreak period were also found to be favorable for mosquito reproduction. In conclusion, we propose that the Foshan outbreak resulted from a combination of virus importation, a largely immunologically naïve population and a climate conducive to mosquito proliferation. Additionally, our findings suggest that clinicians should maintain vigilance for atypical symptoms to prevent misdiagnosis and missed cases.