First Evidence of Co-circulation and Co-transmission of Dengue Virus (DENV-1 and DENV-2) and Chikungunya Virus (ECSA Lineage) by a Single Aedes aegypti Population in Southern Iran (Bandar Abbas and Chabahar, 2024–2025), with Novel Pyrethroid Resistance Mechanisms Identified in the Same Vector Population

Abstract Background In 2024–2025, the port cities of Bandar Abbas and Chabahar (southern Iran) experienced the country’s first documented simultaneous local transmission of dengue virus (DENV-1, DENV-2) and chikungunya virus (CHIKV ECSA lineage) by Aedes aegypti . Methods From June 2024 to March 2025, active human case detection, intensive entomological surveillance, BG-Sentinel trapping, and larval surveys were conducted. Mosquitoes and acute-phase human plasma were screened by multiplex real-time RT-PCR. Near-complete viral genomes and the voltage-gated sodium channel gene were sequenced from the same individual virus-positive mosquitoes. Phylogenetic, phylogeographic, and resistance-genotyping analyses were performed alongside WHO bioassays. Results Of 18,742 females Ae. aegypti tested, 512 (2.7%) were virus-positive, including 57 (11.1%) co-infected with DENV and CHIKV. Minimum infection rates reached 42.6/1,000 in peak months. All circulating strains originated from Pakistan (2024 introductions). Extreme pyrethroid resistance (mortality < 8% at diagnostic dose) was near-fixed, with triple kdr mutations (S989P+V1016G+F1534C) and CYP9J28 amplification significantly associated with virus-positive status (aOR 8.7, 95% CI 5.4–14.1). Among 1,684 febrile patients, 1,237 (73.5%) were laboratory-confirmed, including 19.6% dual infections. Conclusion Southern Iran has become a new focus of intense, insecticide-resistant, simultaneous DENV–CHIKV transmission, driven by recent introductions from Pakistan and pre-existing high-level pyrethroid resistance. Urgent deployment of non-pyrethroid tools and regional genomic surveillance is required.