SCHISTOSOMA-MANSONI - BIOMPHALARIA-GLABRATA - DYNAMICS OF THE SPOROCYST POPULATION IN RELATION TO THE MIRACIDIAL DOSE AND THE HOST SIZE

The rate of invasion of snails and percentage of the digestive gland occupied by daughter sporocysts decrease when the snails are larger and the miracidial dose is smaller. Growth of the sporocyst population is influenced not only by the miracidial dose, but also by the changes in the growth rate of the host. The larger the host and (or) the smaller the miracidial dose, the more significant is the volume occupied by the sporocyst population. Biomass- and (or) density-dependent regulatory mechanisms of different patterns intervene to limit the volume of parasites. Rapid colonisation and intensive exploitation of the host correspond to an immature plurimiracidial system, whereas progressive colonisation and less exploitation correspond to a mature monomiracidial system. The results are discussed in terms of possible demographic strategies that can be used to optimize parasite fitness in various environments.