Experimental studies of age-prevalence curves for Schistosoma mansoni infections in populations of Biomphalaria glabrata.

We report the results of experimental studies of the generation of age-prevalence curves for Schistosoma mansoni infections in laboratory populations of Biomphalaria glabrata. Within snail populations of varying sizes and age structures, the net force of infection is shown to be linearly dependent on the rate at which miracidia are introduced into the aquatic habitat of the host. For individual snails, the per capita force of infection is shown to be related to snail age and size, and the death rate of shedding snails is demonstrated to be dependent on the period of time during which a snail has been releasing cercariae. Both factors are important determinants of the proportion of infected snails within populations of hosts and may generate convex age-prevalence curves. Comparisons of snail abundance in populations either exposed, or not exposed, to infection suggest that S. mansoni can act to significantly depress the population growth of its intermediate host. Mathematical models are developed, incorporating the age structure of snail populations, to aid in the interpretation of experimental results.