Wolbachia introgression in Rio de Janeiro remains at sub-optimal levels 30 months after its crash: challenges in the sustainability of w Mel interventions for dengue control

The deployment of the Wolbachia w Mel strain is currently underway in multiple dengue-endemic municipalities across Brazil. The efficacy of this strategy in Rio de Janeiro remains uncertain, primarily due to the difficulty in sustaining high w Mel prevalence in regions previously subjected to large-scale releases. A key contributing factor was the routine rotation of insecticides within the framework of Integrated Vector Management (IVM), which led to the use of the larvicide Spinosad for Aedes control in urban areas. This compound was associated with a precipitous decline in Aedes aegypti mosquitoes, regardless the w Mel infection status. While w Mel-uninfected population recovered within weeks, w Mel-infected population remained at low levels likely due to the fitness costs imposed by w Mel on egg viability. To assess the long-term persistence of wMel following this demographic collapse, we conducted mosquito sampling across 12 neighborhoods in Rio de Janeiro, 30 months after mosquito populations crashed. Our findings reveal that w Mel introgression remains suboptimal, with a mean frequency of 9.87% across the sampled areas. Only two neighborhoods exhibited wMel frequencies exceeding 15%, likely reflecting ongoing localized releases. The reduced prevalence underscores the challenges of achieving self-sustaining w Mel establishment in complex urban environments and highlight critical considerations for the implementation of Wolbachia -based dengue control programs in endemic regions.