Determinants of Compatibility in Mollusc-Trematode Parasitism

The low prevalence of schistosome-infected snails in hyperendemichabitats, and the demonstrated ability of snails, in general,to recognize and eliminate a myriad of foreign substances and/orinfectious agents, lead to the postulate that host resistanceto larval trematodes must be considered the "rule," while susceptibility(compatibility) represents an exceptional occurrence. In thisreview, we discuss a variety of possible mechanisms by whichcompatibility between trematodes and their molluscan host mightbe attained. Included among these are parasite mimicry of snailhost molecules, prevention of opsonization, interference withhemocyte behavior or differential stimulation of hemocyte metabolicprocesses. Evidence that compatibility is the result of theability of larvae to resist toxic host molecules or to acquireprotective host components on their surface membranes is lacking.Clearly, there are multiple variables of both host and parasiteorigin which ultimately determine compatibility. Careful identificationand dissection of these variables will be required before weachieve a complete understanding of how compatible snail-trematodeassociations are established and maintained.