First Whole-Genome Chikungunya Virus Sequence Detected in Mosquitoes during the 2025 Foshan Outbreak: Evidence of Field Vector Infection and Transmission Potential in China

Background: Since July 2025, an outbreak of mosquito-borne chikungunya fever occurred in Foshan City, Guangdong Province, China. This was the second outbreak in China following the one that occurred in Dongguan City, Guangdong Province, in 2010. Moreover, the intensity of this outbreak was significantly greater than that of the previous one. Updates to 23 August, more than 10,000 human cases had been reported. Here, we present the first full genome sequence of the chikungunya virus (CHIKV) derived from field-trapped mosquitoes during the outbreak. Methods: Adult Aedes albopictus were BG-trap captured from residences and parklands in three hotspot towns with high density of confirmed human cases. Mosquitoes were morphologically identified and pooled by species, sex and environment types. RNA was extracted, screened by CHIKV RT-qPCR, then positive pools underwent sanger and whole-genome sequencing for complete sequences. Lineage and mutational profiles were inferred by maximum likelihood phylogenetic and comparison against human and mosquito genomes. The distribution of amino acid site mutations in different protein coding regions was also analyzed. Result: Through 11 days of collection using 10 BG-traps, 2,803 mosquitoes were captured. 1569 (55.97%) female Ae. albopictus were divided into 77 pools and 9.09% (7/77) of the pools tested positive for CHIKV. The local Ae. albopictus minimum infection rate (MIR, per 1000 females) was 4.46, while the MIR for residences in Lecong Town was the highest at 9.17. The MIR for parklands was slight higher than for residences (4.60 vs. 4.30). All the 5 Ae. albopictus-derived complete CHIKV genome clustered within ECSA-Indian Ocean lineage genotype, closely related to human-derived genomes on 2025 Reunion Island. Amino-acid mutations E1-A226V/E2-L210Q were detected in the strains, which enhanced adaptability to Ae. albopictus and increased the transmission capacity. Novel mutation observed on E1 and E2 were totally consist to the patient-derived CHIKV in 2025 Reunion Island. Conclusions: It was the first mosquito-derived CHIKV whole-genome during the 2025 Foshan outbreak, filling a critical gap between human case and entomological surveillance. Ae. albopictus was confirmed as the primary vector during the outbreak. The current outbreak CHIKV strain with particular amino-acid mutations had adapted to Ae. albopictus transmission. Compared to previous Chikungunya outbreaks over the past decade, the Foshan outbreak occurred earlier (early July), in a larger urban area (with a population of over 9.5 million), and with abundant breeding sites for the vector mosquito Ae. albopictus. However, the outbreak was quickly brought under control, with daily case numbers consistently decreasing, which is closely linked to the strong vector control measures implemented by the Chinese government in the early stages of the outbreak. Moreover, this event once again underscores the necessity of early monitoring of vector mosquitoes and the importance of implementing highly effective vector intervention measures as soon as possible after an outbreak occurs.