Susceptibility of Biomphalaria glabrata to infection with Echinostoma paraensei: correlation with the effect of parasite secretory-excretory products on host hemocyte spreading.

Susceptibility of the snail Biomphalaria glabrata to infection with the digenetic trematode Echinostoma paraensei was correlated with the ability of secretory-excretory products (SEP) derived from sporocysts of this parasite to interfere with the spreading behavior of host hemocytes in an in vitro assay. In 15 separate experiments, the prevalence of infection achieved by miracidia was positively correlated (C.C. = 0.779, P < 0.001) with the ability of in vitro-transformed sporocysts derived from the same batches of miracidia to produce SEP that inhibited host hemocyte spreading. Under assay conditions reflecting differences in hemolymph volumes between juvenile snails (susceptible to infection) and adult snails (relatively refractory to infection), SEP had a significantly greater effect on hemocytes from juvenile snails. No significant differences in response to SEP were noted when equivalent numbers of adult and juvenile hemocytes were used in the assay. Snails of the M line strain of B. glabrata are significantly more susceptible to infection with E. paraensei than 13-16-R1 strain snails. Exposure to SEP significantly increased the number of unspread hemocytes for both strains. However, significantly more 13-16-R1 than M line hemocytes remained spread following SEP treatment. Echinostoma paraensei sporocyst SEP effects on host hemocyte spreading mirror observed patterns in both age- and strain-related susceptibility of B. glabrata to this parasite. The results suggest that the number of hemocytes available to a particular snail influences its vulnerability to infection with E. paraensei.