Modelling the global constraints of temperature on transmission of Plasmodium falciparum and P. vivax

Abstract

Background: Temperature is a key determinant of environmental suitability for transmission of human malaria, modulating endemicity in some regions and preventing transmission in others. The spatial modelling of malaria endemicity has become increasingly sophisticated and is now central to the global scale planning, implementation, and monitoring of disease control and regional efforts towards elimination, but existing efforts to model the constraints of temperature on the malaria landscape at these scales have been simplistic. Here, we define an analytical framework to model these constraints appropriately at fine spatial and temporal resolutions, providing a detailed dynamic description that can enhance large scale malaria cartography as a decision-support tool in public health.

Results: We defined a dynamic biological model that incorporated the principal mechanisms of temperature dependency in the malaria transmission cycle and used it with fine spatial and temporal resolution temperature data to evaluate time-series of temperature suitability for transmission of Plasmodium falciparum and P. vivax throughout an average year, quantified using an index proportional to the basic reproductive number. Time-series were calculated for all 1 km resolution land pixels globally and were summarised to create high-resolution maps for each species delineating those regions where temperature precludes transmission throughout the year. Within suitable zones we mapped for each pixel the number of days in which transmission is possible and an integrated measure of the intensity of suitability across the year. The detailed evaluation of temporal suitability dynamics provided by the model is visualised in a series of accompanying animations.

Conclusions: These modelled products, made available freely in the public domain, can support the refined delineation of populations at risk; enhance endemicity mapping by offering a detailed, dynamic, and biologically driven alternative to the ubiquitous empirical incorporation of raw temperature data in geospatial models; and provide a rich spatial and temporal platform for future biological modelling studies.

Figure 1

Temperature suitability index time-series for three example pixels. Each plot shows the Z(T) modelled value evaluated weekly across an average year for P. falciparum (green line) and P. vivax (red line) for individual pixels located in (A) Central Sumatra; (B) Western Kenya; (C) Central Afghanistan.

Figure 2

Mapped outputs of the temperature suitability model for P. falciparum. (A) Modelled limits of temperature suitability for transmission of P. falciparum. Areas shaded grey are those in which the annual temperature regime would be unable to support infectious vectors at any point during an average year. (B) The number of days in an average year in which the annual temperature regime could support potentially infectious vectors. (C) The normalized Z(T) index of temperature suitability that incorporates not just the duration but also the degree of suitability across an average year. See text for a full explanation of all three metrics.

Figure 3

Mapped outputs of the temperature suitability model for P. vivax. Panels correspond directly to those described for P. falciparum in Figure 2.