A large-scale mosquito larviciding in Tanga Region, Tanzania, reduced mosquito densities to varying degrees across malaria transmission risk strata.

In 2019, the Government of Tanzania endorsed a nationwide scale-up of mosquito larviciding. Prior to full implementation, a pilot project was conducted in the Tanga Region from June 2022 to April 2024. The intervention targeted three councils representing high, moderate, and low malaria epidemiological risk strata. Six rounds of larvicide application were conducted, each lasting eight weeks and scheduled according to local rainfall patterns. All mosquito breeding habitats identified by trained community members were treated using Bacillus thuringiensis israelensis and Bacillus sphaericus. Each intervention council was paired with a control council, and longitudinal entomological monitoring was conducted in 120 villages (60 intervention, 60 control). Larviciding was generally associated with lower densities of late-stage Anopheles larvae across strata, with Incidence Rate Ratios (IRRs) of 0.20 (95% CI: 0.12-0.35) in high-risk, 0.51 (95% CI: 0.26-1.01) in moderate-risk, and 0.40 (95% CI: 0.24-0.68) in low-risk areas. Adult Anopheles gambiae sensu lato densities were also reduced in moderate- and low-risk strata, while no significant reductions were observed in Anopheles funestus populations. These findings suggest that larviciding can reduce mosquito densities in varied ecological settings, though effectiveness may vary by species and transmission context. Optimizing implementation strategies may enhance the entomological impact.