A mechanistic approach for accurate simulation of village scale malaria transmission

Abstract

Background: Malaria transmission models commonly incorporate spatial environmental and climate variability for making regional predictions of disease risk. However, a mismatch of these models' typical spatial resolutions and the characteristic scale of malaria vector population dynamics may confound disease risk predictions in areas of high spatial hydrological variability such as the Sahel region of Africa.

Methods: Field observations spanning two years from two Niger villages are compared. The two villages are separated by only 30 km but exhibit a ten-fold difference in anopheles mosquito density. These two villages would be covered by a single grid cell in many malaria models, yet their entomological activity differs greatly. Environmental conditions and associated entomological activity are simulated at high spatial- and temporal resolution using a mechanistic approach that couples a distributed hydrology scheme and an entomological model. Model results are compared to regular field observations of Anopheles gambiae sensu lato mosquito populations and local hydrology. The model resolves the formation and persistence of individual pools that facilitate mosquito breeding and predicts spatio-temporal mosquito population variability at high resolution using an agent-based modeling approach.

Results: Observations of soil moisture, pool size, and pool persistence are reproduced by the model. The resulting breeding of mosquitoes in the simulated pools yields time-integrated seasonal mosquito population dynamics that closely follow observations from captured mosquito abundance. Interannual difference in mosquito abundance is simulated, and the inter-village difference in mosquito population is reproduced for two years of observations. These modeling results emulate the known focal nature of malaria in Niger Sahel villages.

Conclusion: Hydrological variability must be represented at high spatial and temporal resolution to achieve accurate predictive ability of malaria risk at the village scale, which can then be integrated appropriately to regional spatial scales and seasonal temporal scales. These results have important implications for models seeking to link the impacts of climate change and climate variability to malaria transmission. The highly focal nature of malaria in the Sahel makes detailed representation necessary to evaluate village-level risks associated with hydrology-related vector population variability.

Figure 1

Locations of the study villages Banizoumbou and Zindarou, Niger. The left panel shows the location of the square degree zone (2-3°E, 13-14°N) depicted in the right panel, which depicts topographic relief. The major fluvial feature in the right of the zone is the relict river valley known as the "Dallol Bosso".

Figure 2

An example of surface expression of groundwater in Zindarou, at a topographic low point.

Figure 3

Model domain, discretization, and topography for the 2 km × 2 km area surrounding Zindarou. The model domain is superimposed on an Ikonos image of the Zindarou environs. Grid cells are 50 m square. Zindarou village is located at the center of this model domain.

Figure 4

Soil moisture simulated and observed values at the Zindarou South TDR recording site, for the 2005 rain season. The influence of the groundwater table rise is evident in the 100 cm probe, as the volumetric water content reaches saturation for about six weeks. The modeled soil moisture reproduces the trend, but has a step shape because of the vertical soil discretization.

Figure 5

Comparison of observed and simulated groundwater levels in Zindarou. Near the wet season groundwater peaks, the observed groundwater level contains many measurements, whereas during the dry season measurements are sparse.

Figure 6

Simulated surface area of water (m²) over the entire Zindarou model domain. There are many areas with year-round surface water, including wells, ditches and swamps.

Figure 7

Total captured Anopheles gambiae s.l. mosquitoes captured in Banizoumbou and Zindarou in the period June 2005 - December 2006.

Figure 8

Cumulative simulated and observed mosquitoes in Banizoumbou and Zindarou. Close fits of simulation results to observations show that the model reproduces the inter-village differences in mosquito abundance as well as the interannual variability in abundance between the villages.