Cellular response to Schistosoma mansoni infection in Biomphalaria alexandrina strains selected for susceptibility and resistance.

Exposure of juvenile and adult Biomphalaria alexandrina to Schistosoma mansoni miracidia resulted, typically, in three susceptibility patterns: a) non-infected snails; b) normal infections, and c) retarded infections. Under laboratory conditions, a vigorous resistant-type cellular response to invading miracidia was seen in the histological sections of non-susceptible snails. Accordingly, they were classified as resistant snails. Data pertaining to the influence of host size on suceptibility to S. mansoni indicates that adult snails (i.e. 10-20 mm shell diameter) were significantly less likely to harbour sporocysts than juvenile ones (i.e. 5-10 mm shell diameter). Cellular reaction to the infection varied with sporocysts location and length of infection. At 2 days post-exposure (DPE), most sporocysts were viable. Approximately 8-12% of the sporocysts had elongated shaped transverse constriction and were categorized "normal", while those showing no elongation were categorized "retarded". All remaining sporocysts at 4 DPE were categorized "dead", while at 30 DPE, most sporocysts were "amorphous" with eosinophilic masses. Although encapsulation of sporocysts never occurred in susceptible snails, haemocyte aggregations could sometimes be observed in the proximity of well developed sporocysts. In resistant snails, this cellular response continued to increase and resulted in the encapsulation of the sporocysts. In susceptible snails, the nucleus of secretory cells of the albumen cytoplasm were different in susceptible and resistant snails.