Schistosoma mansoni: distribution patterns of miracidia among Biomphalaria glabrata snail as related to host susceptibility and sporocyst regulatory processes.

Parasite prevalences, miracidia developmental capacity, mother sporocyst mean intensities, sporocyst distribution patterns, and cercarial production levels were determined after individual exposure of Biomphalaria glabrata snails to increased doses of Schistosoma mansoni miracidia for two geographical strains (Brazilian, BRE, and Guadeloupean, GUA) of host and parasite. For a high level (100%) of host-parasite susceptibility and in the absence of mother sporocyst regulatory processes for the BRE combination, parasites were randomly dispersed among snail hosts with a frequency distribution conforming to a positive binomial. In contrast, for a moderate level (65%) of host-parasite susceptibility and in the presence of mother sporocyst regulatory processes for the GUA combination, parasites were overdispersed among snail hosts with a frequency distribution conforming to the negative binomial. Levels of cercarial production were found to be strain dependent, to be determined during early development of mother sporocysts, and to be correlated with the number of developed mother sporocysts. Results were analyzed in the general context of the infrapopulation dynamics of the intramolluscan stages of trematode and are discussed in terms of their consequences on the distribution of the genetic diversity of adult schistosomes among the definitive host population.