In vitro interactions between the defense systems of resistant and susceptible Biomphalaria alexandrina and sporocysts of Schistosoma mansoni.

Biomphalaria species that act as an intermediate host for Schistosoma mansoni have different degrees of susceptibility and different internal defense system responses against parasites. Of these species, Biomphalaria alexandrina represents the only intermediate host in Egypt. Given the limited data on the efficacy of the B. alexandrina internal defense system in comparison to that of other species, we sought to better understand its defense against S. mansoni. We performed in vitro hemocyte adherence assay using whole hemolymph and in vitro reaction using the hemocyte-free hemolymph of susceptible and resistant snails against transformed mother sporocysts. The results demonstrated that the interacting factors between the parasite and the hemolymph of the resistant and susceptible snails do not act in a similar manner. Destruction of the parasite was a restricted function of the hemocytes among resistant snails only. This study demonstrates the key role played by snail hemocytes as a first line of defense against the parasite. The incubation of the hemocyte-free hemolymph of both susceptible and resistant snails with the sporocysts did not lead to any changes in the sporocysts shape or integrity. This immunological variance demonstrated between susceptible and resistant snails could be useful to differentiate between susceptible and resistant snails in future field studies. In addition, the results may help further studies to explain the process of attraction, encapsulation and subsequent killing of S. mansoni in its intermediate host.

Genetic variation between Biomphalaria alexandrina snails susceptible and resistant to Schistosoma mansoni infection.

Much effort has been made to control schistosomiasis infection in Egypt. However, enduring effects from such strategies have not yet been achieved. In this study, we sought to determine the genetic variability related to the interaction between Biomphalaria alexandrina snails and Schistosoma mansoni. Using RAPD-PCR with eight (10 mers) random primers, we were able to determine the polymorphic markers that differed between snails susceptible and resistant to Schistosoma mansoni infection using five primers out of the eight. Our results suggest that the RAPD-PCR technique is an efficient means by which to compare genomes and to detect genetic variations between schistosomiasis intermediate hosts. The RAPD technique with the above-noted primers can identify genomic markers that are specifically related to the Biomphalaria alexandrina/Schistosoma mansoni relationship in the absence of specific nucleotide sequence information. This approach could be used in epidemiologic surveys to investigate genetic diversity among Biomphalaria alexandrina snails. The ability to determine resistant markers in Biomphalaria alexandrina snails could potentially lead to further studies that use refractory snails as agents to control the spread of schistosomiasis.

Biomphalaria species in Alexandria water channels.

Of the several species of Biomphalaria snails worldwide that serve as the intermediate host for Schistosoma mansoni, Biomphalaria alexandrina is a species that is indigenous to Egypt. Recently, there has been much debate concerning the presence of Biomphalaria glabrata and the hybrid of the species with Biomphalaria alexandrina. Due to this debate, the absence of a clear explanation for the presence of B. glabrata in Egyptian water channels and the probability that they may be reintroduced, we conducted this field study to identify Biomphalaria species present in Alexandria water channels. Laboratory-adapted susceptible snails to Schistosoma mansoni of the following species were used as a reference; Biomphalaria alexandrina, Biomphalaria glabrata and their hybrid. These snails were used to perpetuate the Schistosoma life cycle at the Theodor Bilharz Research Institute (TBRI), Cairo, Egypt. Morphological and molecular studies were conducted on these reference snails as well as on the first generation of Biomphalaria snails from two areas in the Alexandria governorate. The morphological study included both external shell morphology and internal anatomy of the renal ridge. The molecular study used a species-specific PCR technique. The results demonstrated that there was an absence of Biomphalaria glabrata and the hybrid from Alexandria water channels. Moreover, the susceptibility patterns of these reference snails were studied by measuring the different parasitological parameters. It was found that Biomphalaria glabrata and the hybrid were significantly more susceptible than Biomphalaria alexandrina to the Egyptian strain of Schistosoma mansoni. The results demonstrated that if Biomphalaria glabrata was reintroduced and adapted to the local environment in Egypt, it would have important epidemiologic impacts that would have a serious effect on the health of Egyptian people.