Modelling the effects of temperature changes on Schistosoma mansoni transmission

Abstract

Schistosomiasis is a chronic parasitic disease, estimated to affect 237 million people worldwide. It is caused by infection with Schistosoma helminths, which spend part of their lifecycles in aquatic snails. The mortality, development and fecundity rates of the parasites and their intermediate host snails are very sensitive to water temperature. The distribution and prevalence of schistosome parasites are therefore likely to be affected by climate change, however the potential effects of this have been largely neglected. Only two mathematical models of temperature and schistosome transmission in Africa have previously been developed, and neither explicitly simulated all temperature-dependent stages of the parasite and snail lifecycles.

The aim of this thesis is to advance understanding of the potential effects of climate change on S. mansoni transmission, using an agent-based modelling approach. A mathematical model of water temperature, snail population dynamics and S. mansoni transmission was developed. The model was parameterised using data from Biomphalaria pfeifferi, the most widespread intermediate host species in Africa, and the dynamics of the model were explored. Infection risk was shown to be highest (above 90% of the maximum) at a constant temperature of 15-19°C. Simulating diurnal variation in temperature and/or higher cercaria and miracidium removal rates increased the optimum temperature for transmission to 16-26°C. The effect of simulating different species of intermediate host snail was also investigated. Simulating Bi. alexandrina and Bi. glabrata increased the temperature at which infection risk was highest to 19-21°C and 20-26°C respectively.

The model was run using climate projections for eastern Africa. Comparisons of model output at baseline with empirical data showed that suitable temperatures are necessary but not sufficient for both schistosome transmission, and for high prevalences of schistosomiasis. All else being equal, infection risk may increase by up to 20% over most of the area over the next 20 and 50 years. Increases may be higher in Rwanda, Burundi, south-west Kenya and eastern Zambia, and schistosomiasis may become newly endemic in parts of these areas. The results for 20 years’ time are robust to changes in simulated snail habitats. There is greater uncertainty about the effects of different habitats on changes in risk in 50 years’ time.