Abstract
BackgroundMathematical models can be used to identify areas at risk of increased or new schistosomiasis transmission as a result of climate change. The results of these models can be very different when parameterised to different species of host snail, which have varying temperature preferences. Currently, the experimental data needed by these models are available for only a few species of snail. The choice of density-dependent functions can also affect model results, but the effects of increasing densities on Biomphalaria populations have only previously been investigated in artificial aquariums.MethodsLaboratory experiments were conducted to estimate Biomphalaria sudanica mortality, fecundity and growth rates at ten different constant water temperatures, ranging from 13-32°C. Snail cages were used to determine the effects of snail densities on B. sudanica and B. stanleyi mortality and fecundity rates in semi-natural conditions in Lake Albert.ResultsB. sudanica survival and fecundity were highest at 20°C and 22°C respectively. Growth in shell diameter was estimated to be highest at 23°C in small and medium sized snails, but the relationship between temperature and growth was not clear. The fecundity of both B. sudanica and B. stanleyi decreased by 72-75% with a four-fold increase in population density. Increasing densities four-fold also doubled B. stanleyi mortality rates, but had no effect on the survival of B. sudanica.ConclusionsThe optimum temperature for fecundity was lower for B. sudanica than for previously studied species of Biomphalaria. In contrast to other Biomphalaria species, B. sudanica have a distinct peak temperature for survival, as opposed to a plateau of highly suitable temperatures. For both B. stanleyi and B. sudanica, fecundity decreased with increasing population densities. This means that snail populations may experience large fluctuations in numbers, even in the absence of any external factors such as seasonal temperature changes. Survival also decreased with increasing density for B. stanleyi, in contrast to B. sudanica and other studied Biomphalaria species where only fecundity has been shown to decrease.
Highlights
Mathematical models can be used to identify areas at risk of increased or new schistosomiasis transmission as a result of climate change
Sufficient data are only available to accurately parameterise models of temperature and snail ecology to three species of Biomphalaria: B. pfeifferi, B. alexandrina, and B. glabrata [6], and only one of these species can be found in subSaharan Africa [7], where the majority of schistosomiasis occurs [8]
We describe laboratory and field experiments conducted with two species of Biomphalaria: B. sudanica and B. stanleyi
Summary
Mathematical models can be used to identify areas at risk of increased or new schistosomiasis transmission as a result of climate change. Mathematical models of water temperature and the lifecycles of schistosomes and their intermediate host snails can be used to gain valuable insight into the possible effects of environmental changes, including climate change, on the distribution and intensity of schistosomiasis [1,2,3,4]. It is crucial, to consider the species of intermediate host snail found in an area when parameterising a model, and when using the model to make predictions of the potential effects of environmental change on schistosome transmission [5,6]. This is the case for lakeside areas in eastern Africa, where prevalences and intensities of intestinal schistosomiasis are often very high [11], and where B. pfeifferi are not commonly found in large numbers [9,12,13,14]
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