A comparative study of mechanisms underlying digenean-snail specificity: In vitro interactions between hemocytes and digenean larvae

A panel of 4 digenetic trematode species (Echinostoma paraensei, E. trivolv is, Schistosoma mansoni, and Schistosomatium douthitti) and 5 snail species (Biomphalaria glabrata, Helisoma trivolvis, Lymnaea stagnalis, Stagnicola elodes, and Helix aspersa) was examined to determine if known patterns of h ost specificity could be explained by the tendency of digenean larvae to be bound by snail hemocytes, or by the ability of larvae to influence the spr eading behavior of hemocytes. In short-term (1 hr) in vitro adherence assay s, there was no overall pattern to suggest that sporocysts were more likely to be bound by hemocytes from incompatible than compatible snails. Compare d with the other parasites, sporocysts of E. paraensei were less likely to be bound by hemocytes from any of the snail species tested. All rediae exam ined, including those of another species Echinoparyphium sp., were also rem arkably refractory to binding by hemocytes from any of the snails. Of all t he larvae examined, only sporocysts and young daughter rediae of E. paraens ei caused hemocytes to round up in their presence. This was true for hemocy tes from the compatible species B. glabrata and the incompatible lymnaeid s pecies S. elodes and L. stagnalis. The patterns of host specificity shown b y this particular panel of parasites and snails were not predicted by eithe r the extent of hemocyte adherence to digenean larvae or by the ability of larvae to affect hemocyte spreading behavior. The results of this study sug gest that a role for hemocytes, although likely, may require different assa ys, possibly of a more prolonged nature, for its detection. Also, different parasite species (notably E. paraensei) and intramolluscan stages have dis tinctive interactions with host hemocytes, suggesting that the determinants of specificity vary with the host-parasite combination, and with the paras ite life cycle stage.