Laboratory comparison of the relative success of Biomphalaria glabrata stocks which are susceptible and insusceptible to infection with Schistosoma mansoni

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Abstract
A method of interrupting the life-cycle of the human blood fluke Schistosoma by increasing the proportion of genetically insusceptible intermediate host snails in natural populations was first proposed nearly 25 yr ago. The method assumes that insusceptible snails will be at a selective advantage over susceptible snails when the schistosome parasite is present, and therefore, natural selection will act to increase the proportion of alleles for insusceptibility. While these snails are at a selective advantage they are not predominant in natural populations that transmit disease. One explanation of this paradox is that insusceptibility may be associated with a disadvantageous character or a physiological defect. This study tests this hypothesis by measuring the relative reproductive success of susceptible and insusceptible snails under controlled conditions. Insusceptible (unsuitable) snails are negatively affected in the presence of either susceptible snails or schistosome parasites. In the presence of both susceptible snails and schistosome parasites, insusceptible snails are selectively disadvantaged compared to susceptible snails. These results obtained under laboratory-controlled conditions, suggest a plausible answer as to why insusceptible snails are not predominant in natural populations that transmit disease.

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