Mechanistic modelling of the links between environment, mosquitoes and malaria transmission in the current and future climates of West Africa

Abstract

Malaria transmission in West Africa is closely tied to climate, as rain fed water pools provide breeding habitat for the anopheles mosquito vector, and temperature affects the mosquito's ability to spread disease. This thesis presents a framework of highly detailed, spatially explicit mechanistic modelling to explore the relationships between the environment and malaria in the current and future climate of West Africa. A mechanistic model of human immunity was incorporated into an existing agent-based model of malaria transmission, allowing us to move beyond entomological measures such as mosquito density and vectorial capacity to analyzing the prevalence of the malaria parasite within human populations. The result is a novel modelling tool that mechanistically simulates all of the key processes linking environment to malaria transmission. Simulations were conducted across climate zones in West Africa, linking temperature and rainfall to entomological and epidemiological variables with a focus on nonlinearities due to threshold effects and interannual variability. Comparisons to observations from the region confirmed that the model provides a reasonable representation of the entomological and epidemiological conditions in this region. While current generation climate models agree that mean temperatures in West Africa will likely increase by 2 to 4° C in the future by the end of the 21st century, they disagree on the magnitude and the direction of the change in rainfall. We analyzed the performance of CMIP5 climate models in simulating West African rainfall and temperature before selecting the most credible predictions of future climate. We used these predictions to simulate the expected change in malaria transmission in sensitive regions of West Africa. We found that the western subregion of West Africa is likely to become drier in the coming decades. The warmer temperatures will shorten mosquito life spans, and the drying will limit mosquito reproduction. As a result, we expect malaria transmission in this region to decrease. However, the eastern half of the region is expected to become wetter. In some areas, the positive effects of increased rainfall on mosquito reproduction may surpass the negative effects of high temperatures on mosquito longevity, leading to a small net increase in environmental suitability for malaria transmission.