Abstract
BACKGROUND: Since July 2025, an outbreak of mosquito-borne chikungunya fever has occurred in Foshan City, Guangdong Province, China. This was the second large-scale local outbreak in China after the one that occurred in Dongguan City, Guangdong Province, in 2010. As of 23 August, more than 10,000 human cases had been reported. This study aims to investigate mosquito infection and viral genomic characteristics during the Foshan outbreak. METHODS: Adult Aedes albopictus were collected using BioGents Sentinel trap in three hotspot towns (Beijiao, Chencun and Lecong). Mosquitoes were morphologically identified and pooled by species, sex and environment type. Each pool was homogenized, and the homogenate was clarified by centrifugation; and the supernatant was used for viral RNA isolation. The isolated RNA was screened using CHIKV RT-qPCR. Positive pools underwent Sanger sequencing and whole-genome sequencing. The CHIKV lineage and mutational profiles were inferred using maximum likelihood phylogenetic analysis and comparison with human- and mosquito-derived genomes. RESULTS: Over 11 days of trapping, 2803 mosquitoes were captured. 1569 (55.97%) female Ae. albopictus were divided into 77 pools and 9.09% (7/77) of these pools were CHIKV-positive. The minimum infection rate (MIR, per 1000 females) for local Ae. albopictus was 4.46, while the MIR for residences in Lecong Town was the highest at 9.17 per 1000 females. The MIR for parklands was slightly higher than for residences (4.60 vs. 4.30 per 1000 females). Five complete Ae. albopictus-derived CHIKV genome clustered within the East/Central/South African-Indian Ocean lineage genotype, and harbored novel E1 and E2 mutations consistent with those detected in the 2025 Reunion Island human strain. Amino-acid mutations E1-A226V/E2-L210Q were detected, enhancing adaptability to Ae. albopictus and increasing the transmission capacity. CONCLUSIONS: This study represents the first mosquito-derived CHIKV whole-genome sequence obtained from the 2025 Foshan outbreak. Ae. albopictus was confirmed as the primary vector, and the presence of adaptive mutations indicated an enhanced transmission potential. Despite the outbreak emerging earlier in the season and affecting a dense urban population, it was effectively controlled through timely and intensive vector interventions. These findings highlighted the critical role of mosquito surveillance in early outbreak preparedness and effective vector management.