COS 266-6 - Spatiotemporal variability in transmission risk of human schistosomes in a seasonally desiccating landscape

Abstract: Variability in environmental conditions across multiple spatial scales interact to create highly heterogenous patterns of disease outcomes across space and time. Different populations of hosts and parasites experience distinct seasonal fluctuations in rainfall and temperature depending on local-scale biotic and abiotic factors. Urogenital schistosomiasis is a neglected tropical disease caused by parasitic trematodes (Schistosoma haematobium). Their intermediate hosts are aquatic snails (Bulinus spp.) that are highly adapted to landscapes with extreme annual rainfall seasonality. The ponds that Bulinus snails occupy are at risk of complete desiccation each year, often forcing snails to aestivate (akin to dry season hibernation) for up to seven months of the year. While Bulinus snails have a remarkable capacity for rebounding following aestivation in these ephemeral habitats, parasite survival is greatly diminished. Thus, does ephemerality act as a dampener to schistosome transmission risk? We investigate the potential for seasonal pond desiccation to deter parasite transmission using a year-round evaluation of snail-parasite dynamics in 109 variably ephemeral ponds in the Lake Victoria watershed of Tanzania. First, we found that ponds have two peaks of transmission risk through a year’s cycle regardless of tendency for ephemerality, though of lower magnitude in the ponds that desiccate fully when compared to those that do not. Both prevalence and infection intensity peak following the onset of rain (likely associated with the emergence of infected aestivators) and as ponds start to dry up (likely associated with new infections). Second, we evaluated total yearly infection prevalence across a gradient of ephemerality, finding ponds with intermediate ephemerality (% pond size reduction) to have the highest infection rates. While East Africa is expected to experience increased frequency and intensity of droughts with climate change, local-scale impacts are likely to be spatially heterogenous. Our study found that peak schistosome transmission risk is seen at intermediate pond ephemerality, thus the impacts of increasing landscape desiccation could result in increases or decreases in transmission risk depending on a pond’s natural tendency for drying. Future studies should identify the factors that promote these hotspots of transmission, such as variable aestivation survival rates or human water use behavior. Additionally, as we anticipate changes in the environmental conditions and disease risk within an increasingly desiccated landscape, it is imperative to understand how humans will adapt to increase water availability and maintain their livelihoods.