Cellular response to Echinostoma caproni infection in Biomphalaria glabrata strains selected for susceptibility/resistance

The Biomphalaria glabrata/Schistosoma mansoni system represented the only model available so far, for investigating snail resistance to parasites. A new host parasite model has been recently provided by selection of B. glabrata strains that are genetically resistant and susceptible to Echinostoma caproni. As a first approach in investigating resistance mechanisms in this model, we compared the hemocytic response and its effect on E. caproni development in a susceptible and a resistant strains. Histological analysis revealed that resistance does not prevent penetration or migration through the snail tissues. However, all mother sporocysts (MS) settled in the ventricle and the aorta of resistant snails were encapsulated and killed by 4 days post-exposure. MS abnormally settled in the pericardial cavity were not encapsulated and could survive for 5 days. Regardless of their level of encapsulation, all live MS observed during the first days of infection in resistant snails showed abnormal development and degeneration, raising the question of the possible role of humoral factors in the resistance to E. caproni. Finally, we observed an increased number of adherent hemocytes in the resistant as compared with the susceptible snails after parasite exposure. This different effect on circulating hemocyte subpopulations may reflect the failure of E. caproni to interfere with the hemocytic response of resistant snails.