Reducing Plasmodium falciparum Malaria Transmission in Africa: A Model-Based Evaluation of Intervention Strategies

Abstract

BackgroundOver the past decade malaria intervention coverage has been scaled up across Africa. However, it remains unclear what overall reduction in transmission is achievable using currently available tools.Methods and FindingsWe developed an individual-based simulation model for Plasmodium falciparum transmission in an African context incorporating the three major vector species (Anopheles gambiae s.s., An. arabiensis, and An. funestus) with parameters obtained by fitting to parasite prevalence data from 34 transmission settings across Africa. We incorporated the effect of the switch to artemisinin-combination therapy (ACT) and increasing coverage of long-lasting insecticide treated nets (LLINs) from the year 2000 onwards. We then explored the impact on transmission of continued roll-out of LLINs, additional rounds of indoor residual spraying (IRS), mass screening and treatment (MSAT), and a future RTS,S/AS01 vaccine in six representative settings with varying transmission intensity (as summarized by the annual entomological inoculation rate, EIR: 1 setting with low, 3 with moderate, and 2 with high EIRs), vector–species combinations, and patterns of seasonality. In all settings we considered a realistic target of 80% coverage of interventions. In the low-transmission setting (EIR∼3 ibppy [infectious bites per person per year]), LLINs have the potential to reduce malaria transmission to low levels (<1% parasite prevalence in all age-groups) provided usage levels are high and sustained. In two of the moderate-transmission settings (EIR∼43 and 81 ibppy), additional rounds of IRS with DDT coupled with MSAT could drive parasite prevalence below a 1% threshold. However, in the third (EIR = 46) with An. arabiensis prevailing, these interventions are insufficient to reach this threshold. In both high-transmission settings (EIR∼586 and 675 ibppy), either unrealistically high coverage levels (>90%) or novel tools and/or substantial social improvements will be required, although considerable reductions in prevalence can be achieved with existing tools and realistic coverage levels.ConclusionsInterventions using current tools can result in major reductions in P. falciparum malaria transmission and the associated disease burden in Africa. Reduction to the 1% parasite prevalence threshold is possible in low- to moderate-transmission settings when vectors are primarily endophilic (indoor-resting), provided a comprehensive and sustained intervention program is achieved through roll-out of interventions. In high-transmission settings and those in which vectors are mainly exophilic (outdoor-resting), additional new tools that target exophagic (outdoor-biting), exophilic, and partly zoophagic mosquitoes will be required. Please see later in the article for the Editors' Summary

The silent threat: asymptomatic parasitemia and malaria transmission

“…Similar observations have been noted in areas formerly considered as high-transmission areas, such as Zambia [7] and Uganda [8]. A mathematical simulation of Plasmodium falciparum transmission in Africa suggested that only in areas with the lowest baseline level of transmission (<three infective bites per person per year) could malaria be eliminated through a combination of ITNs, IRS and case management with an artemisinin-based combination therapy (ACT) [9].…”

“…Review to be supplemented by additional strategies for malaria to be eliminated in most of sub-Saharan Africa, and the recognition that there is not yet a feasible technical strategy to eliminate P. vivax from Latin America and Asia, there is increased attention being paid to the human parasite reservoir [9,11]. The human parasite reservoir consists of all malaria infections in people in a given area, including symptomatic and asymptomatic infections, and both the sexual and asexual stages of the parasite (the special case of dormant liver stages of P. vivax and Plasmodium ovale will not be addressed in this review).…”

Sensitivity analysis of infectious disease models: methods, advances and their application

“…In resource poor settings, models, coupled with SA, may be used as a first step towards understanding control options. Whether by means of modelling behavioural interventions, biomedical interventions or through evaluation of social conditions and their impact on the disease system, infectious disease models have been used by decision-making bodies to explore and evaluate various policy options [48]. Where data are meager, SA can be used to identify key parameters upon which to focus limited data collection resources.…”

Efficacy of indoor residual spraying with dichlorodiphenyltrichloroethane against malaria in Gambian communities with high usage of long-lasting insecticidal mosquito nets: a cluster-randomised controlled trial

“…Some models indicate that LLIN and IRS combined would interrupt transmission in areas of moderate transmission 7,8 . Yet others suggest that the impacts could be antagonistic against the major African vectors: Anopheles gambiae s.s. 9 and A arabiensis 10 .…”