RESUMO
Distribution pattern of Schistosoma mansoni miracidia level of host susceptility/resistance and the basic cellular responses brought into play during the parasite development. Several snail stocks showed a wide spectrum of host reaction to the parasite. From the present study, a vigorous "resistant-type" cellular response to invading miracidia is seen in the histological sections of non-susceptible snails. In this respect, they were classified in our study as "resistant snails". B. alexandrina experimentally infected with S. mansoni exhibit a wide range of histopathological and immunological changes. The rate of phagocytosis as well as the actual number of hemocytes were determined in different groups of snails during the infection cycle. The significant fluctuation (increase and/or decrease) in circulating hemocyte number was only correlated with a shift in hemocyte populations in the first two days after exposure to the infective stage. The in vitro phagocytic activity of resistant snail hemocytes was found to be higher than that observed in susceptible snails. The observed phagocytic activity indicates that susceptible snail hemocytes were capable of recognizing parasite antigen in vitro, despite of that, they were not able to clear the infection in vivo. Thus suggesting the presence of endogenous factors preventing the immune system of susceptible snails from destroying the developed parasite larvae. Therefore, the mechanism underlying the susceptibility of the snails should be investigated by studying the host-parasite interactions.
