How raising snails allows to better understand the dynamics of a parasite?by Thomas Boyer and Thibault Dindart
Published by the July 19, 2021 on 9:38 AM
The analysis of epidemiological dynamics depends on host and parasite interactions. But these interactions fluctuate because hosts and parasites are heterogeneous entities that exist in dynamic environments. Resource availability is a powerful environmental constraint of intra-host infection dynamics (temporal patterns of growth, reproduction, parasite production and survival).
In this study, researchers developed, parameterized and validated an explicit resource infection dynamics model by incorporating a parasitism module in the energy balance theory. The mechanisms explained are the multivariate dynamic responses of the human parasite Schistosoma mansoni and its intermediate host snail to resource variation and host density. This parasite is widespread in Africa and inter-tropical America. Worldwide, more than 200 million people are infected with it, 9 million suffering from its symptoms, which cause more than 200,000 deaths every year. It tends to have erratic localizations (liver, spleen) and the accumulation in these organs of lost eggs makes the severity of the infection.
The most common symptoms are diarrhea and even dysentery. Complications can appear such as rectal prolapse, fistulas, occlusion, appendicitis.
To do this, they have parameterized the model using an experiment that manipulates food resources and follows the growth, reproduction, parasite production and survival of snail hosts. The model is then validated by simulating the dynamics of infection for host individuals undergoing different levels of intraspecific competition and comparing these predictions with the results of another experiment that manipulated host and resource density, and hence the intensity of resource competition
This bioenergy perspective suggests that variation in resource availability and competition could explain the infective dynamics of this parasite. To begin with, total cercaria production could be low when snail densities are low (because there are few infected snails) or when snail densities are high (if competition limits per capita parasite production). This potential relationship between snail density and the risk of human exposure could be the reason for the success or failure of current and proposed methods of schistosome control, which depend on reducing the density of snail vectors by molluscicides or predators. If resource competition in natural snail populations is strong enough, then snail reduction programmes could backfire, as reducing intermediate host densities could free the remaining hosts from resource competition, thereby increasing parasite production rates per host.
To conclude this bio-energetic model seems to indicate that the fight against the parasite Schistosoma mansoni begins with the regulation of snail populations (first host of the cycle). In order to better regulate this infectious dynamics, measures must be taken collectively between the countries concerned and must be directed towards a total reduction of snails or a limitation by intra-species competition. This model would establish priority levels of parasite infection for certain areas and would be followed by measures to control the parasite. A first measure would be the installation of pipelines to prevent the release of infected faeces into watercourses.
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