Weather-driven dynamics of an intermediate host: mechanistic and statistical population modelling of Oncomelania hupensis.

Abstract

Population models of intermediate host species' abundance are vital tools for assessing the impacts of control activities and changing environmental conditions on host-parasite dynamics. Public health practitioners require reliable predictions to inform disease control decisions that target the intermediate host of human schistosomes. A synthesis of mechanistic and statistical modelling approaches was undertaken to develop a weather-driven mathematical population dynamics model for Oncomelania hupensis, the intermediate host for Schistosoma japonicum. The model was parameterized with data from a novel, longitudinal mark-recapture technique incorporating enclosures in order to distinguish birth from immigration and death from emigration. Snail density, recruitment and death rates were estimated monthly, and environmental variables recorded continuously at three sites in a mountainous region of south-western China. Generalized estimating equation models were used to relate birth and death parameters to lagged and unlagged environmental variables, clustered by site. Competing mechanistic models, combining these submodels, were assessed based on their prediction error against observed population dynamics. In the best-fit overall model, O. hupensis seasonal cycling was largely governed by temperature and episodic precipitation events >15 mm, with characteristic early spring and late summer population peaks. Density-dependent effects were not detected. The implications for snail control and, in particular, the influence of human hydrological disturbances on snail populations were examined. The model quantified the response of O. hupensis to small changes in climate variables, providing vital data for assessing the potential impact of climate change on parasite intermediate host populations. Synthesis and applications. The results of this study can form the basis of management measures aimed at controlling the Asian intermediate host of a widespread human parasite, with relevance to nearly 30 million affected people. As a component of larger disease transmission models, a description of snail population dynamics is valuable both for explaining the patterns of occurrence of disease in natural populations, and for evaluating the relative impact of different environmental scenarios on host-parasite interactions. Having integrated the first direct measurements of O. hupensis population parameters in a validated, weather-driven population model, managers may now quantitatively evaluate the response of O. hupensis to changing environmental conditions, including environmental modifications designed to achieve population reductions. Results suggest that, in particular, the sensitivity of O. hupensis to heavy flow events can be exploited as a potential control technique.

Key words

• climate change
• disease control
• disease vectors
• helminthoses
• helminths
• host parasite relationships
• hosts
• infection control
• infections
• intermediate hosts
• mathematical models
• parasites
• parasitoses
• population density
• population dynamics
• schistosomiasis
• snail-borne diseases
• temperature
• trematode infections
• vector control
• vectors
• weather