The impact of climate change on the distribution patterns of Schistosoma transmitting snails in South Africa

Abstract

Schistosomiasis is a vector-borne disease transmitted by trematode worms. Two schistosome species, Schistosoma mansoni and Schistosoma haematobium, cause human schistosomiasis in South Africa (SA). This neglected tropical disease is transmitted by three gastropod intermediate host species, Bulinus africanus, Bulinus globosus and Biomphalaria pfeifferi. Temperature and rainfall can affect Schistosoma species' distribution by altering their prevalence and geographical distribution. Examining projected changes in rainfall, temperature shifts, and other climatic factors is imperative to understand how climate change may affect schistosomiasis transmission dynamics. The impacts of climate change on schistosomiasis distribution in SA remain unclear. This thesis aims to investigate the current and future impacts of climate change on the spatial distribution patterns of schistosomiasis intermediate host snails in SA. It combined ecological niche modelling with bioclimatic data from downscaled climate projections to predict future changes. Three ecological models and three General Circulation Models (GCMs) under two emission scenarios (RCP4.5 and RCP8.5) for two future periods (20402070) and (2070-2100) were used. The climate projection data for future periods (2040-2070) and (2070-2100) were derived from the Coupled Model Intercomparison Project (CMIP5) provided by the Copernicus Climate Change Service. A highly correlated variable (>0.85) was excluded from the analysis to reduce multicollinearity. The results showed that the historical distribution of Schistosoma-transmitting snails in SA has been influenced by temperature and rainfall, with the Random Forest (RF) model proving most effective in mapping the habitats of these intermediate hosts. The ensemble model, which combines various Ecological Niche Models (ENMs), demonstrated a greater performance in identifying suitable environments for these snails. Bulinus africanus, B. globosus, and B. pfeifferi have historically thrived in the eastern and coastal regions of South Africa, with B. pfeifferi being the most widespread, favouring warmer temperatures and average rainfall. Future projections for the distribution of Schistosoma-transmitting snails under climate change scenarios RCP 4.5 and 8.5 suggest a complex landscape of habitat suitability. The ensemble model forecasts a decrease in highly suitable habitats for B. pfeifferi in the eastern areas of SA, with potential expansion in the north of Kwa-Zulu Natal. Bulinus globosus may find fewer optimal areas but could adapt to warmer and hotter habitats in the interior and east of SA. Bulinus africanus is likely to experience a reduction in suitable regions, with some coastal areas potentially becoming more favourable due to warmer temperatures. The predicted impacts of climate change on Schistosoma-transmitting snails in SA indicate a shift in habitat suitability for these intermediate hosts. Bulinus africanus may see a reduction in habitats across several provinces but could expand into the southwest coast and central Free State. Biomphalaria pfeifferi is expected to encounter decreased suitability in the eastern regions of SA but may find new habitats in the Free State, Northern Cape, and western areas. Bulinus globosus is likely to face habitat declines but could adapt to suitable climates in the Free State and KwaZulu-Natal regions. These shifts suggest that warming climates may create new habitats for these snail species at higher elevations and cooler regions. This study established a foundational framework for subsequent research at provincial and municipal levels. This can be a foundation to develop strategies to prevent transmission and range expansion of schistosomiasis into previously unaffected areas.