Less is more: modelling the impact of species-targeted versus broadcast larviciding in Anopheles funestus-dominated malaria transmission settings.

backgroundLarval source management (LSM) was once central to malaria control before insecticide-treated nets and indoor residual spraying dominated. Renewed interest in LSM raises questions about its effectiveness in rural Africa, where habitats are dispersed, and vector species contribute unequally, and whether species-targeted larviciding could offer greater gains than broadcast approaches.

methodsThis modelling study quantified the potential impact of larviciding in African settings where multiple vector species contribute unequally to malaria transmission. We modeled malaria transmission in southeastern Tanzania using the EMOD v2.20, an individual-based malaria transmission model incorporating seasonal dynamics, insecticide resistance, and semi-field biolarvicide efficacy. Outcomes were entomological inoculation rate, malaria incidence in under-fives, and operational larviciding costs.

findingsLarge-scale deployment of biolarvicides with > 1-week residual activity substantially reduced malaria transmission, with disproportionately greater gains when control efforts were preferentially focused on the dominant vector species, Anopheles funestus, compared to broadcast approaches treating both An. funestus and An. arabiensis habitats. In the absence of ITNs, a four-month fortnightly larviciding campaign targeting An. funestus at 80% coverage reduced EIR by 57.7% (CI 56.7-58.7%) and incidence by 39.7% (CI 38.1-41.3%), versus 70.1% (CI 67.5-72.8%) EIR and 55.7% (CI 52.1-59.3%) incidence reductions under broadcast strategies; targeting An. arabiensis alone yielded only 3.1% (CI 2.1-4.2%) EIR and 1.3% (CI 0-2.9) incidence reductions. Starting with 80% ITN coverage, funestus-targeted larviciding reduced EIR by 77.3% (CI 72.1-82.7%) and incidence by 69.8% (CI 62.9-76.8%), versus 90% (CI 87.3-92.6%) and 84.8% (CI 81-88.7%), respectively, with broadcast strategies, suggesting broadcast larviciding provided limited additional reductions beyond those achieved by the funestus-targeted approach. At 40% ITN coverage, reductions were 62% (CI 60.4-63.5%) of EIR and 45.6% (CI 43.3-47.9%) in incidence (funestus-targeted) versus 76.4% (CI 74.5-78.4%) and 62.6% (CI 60.2-65.1%), respectively (broadcast). The targeted campaigns preserved a 30-50% cost advantage while sustaining > 50% dry-season transmission reductions. Finally, high-coverage (e.g. 80%) funestus-targeted larviciding campaigns achieved greater impacts than lower-coverage (e.g. 40-60%) targeting both species.

conclusionsIn settings where multiple vector species contribute unequally to malaria transmission, preferentially targeting larviciding against the dominant vector species can deliver substantial epidemiological impact with lower operational costs than broadcast approaches targeting multiple vectors. In south-eastern Tanzania, where An. funestus drives most transmission, concentrating larviciding efforts on its characteristic aquatic habitats may offer a scalable, low-cost complement to established tools such as ITNs.