Examining the impact of larval source management and insecticide-treated nets using a spatial agent-based model of Anopheles gambiae and a landscape generator tool

Abstract

BackgroundAgent-based models (ABMs) have been used to estimate the effects of malaria-control interventions. Early studies have shown the efficacy of larval source management (LSM) and insecticide-treated nets (ITNs) as vector-control interventions, applied both in isolation and in combination. However, the robustness of results can be affected by several important modelling assumptions, including the type of boundary used for landscapes, and the number of replicated simulation runs reported in results. Selection of the ITN coverage definition may also affect the predictive findings. Hence, by replication, independent verification of prior findings of published models bears special importance.MethodsA spatially-explicit entomological ABM of Anopheles gambiae is used to simulate the resource-seeking process of mosquitoes in grid-based landscapes. To explore LSM and replicate results of an earlier LSM study, the original landscapes and scenarios are replicated by using a landscape generator tool, and 1,800 replicated simulations are run using absorbing and non-absorbing boundaries. To explore ITNs and evaluate the relative impacts of the different ITN coverage schemes, the settings of an earlier ITN study are replicated, the coverage schemes are defined and simulated, and 9,000 replicated simulations for three ITN parameters (coverage, repellence and mortality) are run. To evaluate LSM and ITNs in combination, landscapes with varying densities of houses and human populations are generated, and 12,000 simulations are run.ResultsGeneral agreement with an earlier LSM study is observed when an absorbing boundary is used. However, using a non-absorbing boundary produces significantly different results, which may be attributed to the unrealistic killing effect of an absorbing boundary. Abundance cannot be completely suppressed by removing aquatic habitats within 300 m of houses. Also, with density-dependent oviposition, removal of insufficient number of aquatic habitats may prove counter-productive. The importance of performing large number of simulation runs is also demonstrated. For ITNs, the choice of coverage scheme has important implications, and too high repellence yields detrimental effects. When LSM and ITNs are applied in combination, ITNs’ mortality can play more important roles with higher densities of houses. With partial mortality, increasing ITN coverage is more effective than increasing LSM coverage, and integrating both interventions yields more synergy as the densities of houses increase.ConclusionsUsing a non-absorbing boundary and reporting average results from sufficiently large number of simulation runs are strongly recommended for malaria ABMs. Several guidelines (code and data sharing, relevant documentation, and standardized models) for future modellers are also recommended.

Landscape Epidemiology Modeling Using an Agent-Based Model and a Geographic Information System

“…On the other hand, the version described and used in this paper reflects the most recent updates in an attempt to enrich the models with features that reflect the population dynamics of An. gambiae in a more comprehensive way, as described in [15,16]. Since the most recent ABM is tested using the rigorous V&V and replication techniques, the results presented in this paper entail much higher confidence from both the epidemiological and the simulation perspectives.…”

“…Killing effectiveness refers to an increased mortality (increased probability of death of a mosquito), toxicity, or killing efficiency due to the insecticidal killing effects of the ITNs; the insecticide kills the mosquitoes that come into contact with the ITNs. Repellence refers to the insecticidal excito-repellent properties of the ITNs which repel the blood meal seeking mosquitoes; it adds a chemical barrier to the physical one, further reducing human-mosquito contact and increasing the protective efficacy of the ITNs (see [15,16] …”

“…As mentioned before, each aquatic habitat is set with a predefined carrying capacity (CC), which regulates the aquatic mosquito population that the habitat can sustain, and reflects the habitat heterogeneity (e.g., in terms of productivity) to some degree (see [15] for details). A total of 941 houses, each having a mean of five occupants, are also identified.…”

“…The spatial ABM, described in detail in [14,15], simulates the life cycle of the mosquito vector An. gambiae by tracking attributes relevant to the vector population dynamics for each individual mosquito.…”

“…The second column refers to the exploratory features from the previous versions [12][13][14]. The third column refers to the most recent features from [15,16], which are used for this paper. Resource-seeking includes both host-seeking and oviposition.…”