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

BACKGROUND: The deployment of the Wolbachia wMel 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 wMel 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 of the wMel infection status. While wMel-uninfected population recovered within weeks, wMel-infected population remained at low levels likely due to the fitness costs imposed by wMel on egg viability.

OBJECTIVES AND METHODS: Evaluate the updated frequency of Wolbachia wMel strain in neighborhoods that received deployments. Fort that, we assessed the long-term persistence of wMel following its demographic collapse, by performing mosquito sampling across 12 neighbourhoods in Rio de Janeiro, 30 months after mosquito populations crashed.

FINDINGS: Our findings reveal that wMel introgression remains suboptimal, with a mean frequency of 9.87% across the sampled areas. Only two neighbourhoods exhibited wMel frequencies exceeding 15%, likely reflecting ongoing localised releases.

MAIN CONCLUSIONS: The reduced prevalence underscores the challenges of achieving self-sustaining wMel establishment in complex urban environments and highlight critical considerations for the implementation of Wolbachia-based dengue control programs in endemic regions.