In this work we have studied the modifications in the biological properties of Trypanosoma cruzi when the parasite is maintained for a long time in axenic culture. The studies were done with a clone from an avirulent strain (Dm30L) and a non-cloned virulent strain (EP) of T. cruzi. Both parasites were maintained, for at least three years, by successive triatomine/mouse alternate passage (control condition), or by serial passage in axenic medium (culture condition), or only in the mouse (mouse condition). The comparison between parasites of culture and control condition showed that metacyclogenesis capacity was reduced in the former and that the resulting metacyclics displayed an attenuated virulence. In order to compare the virulence of metacyclics from the urine of the insect vector, Rhodnius prolixus were infected by artificial feeding with parasites of the control or culture condition. After three triatomine/triatomine passages, there was observed an almost identical biological behavior for these parasites, hence indicating that the maintenance of T. cruzi for a long time in axenic culture affects the differentiation capacity and the virulence of the parasite. Additionally, it was demonstrated that it is possible to maintain T. cruzi exclusively through passages in the invertebrate host.

Biomphalaria glabrata and Schistosoma mansoni relationship was studied by light microscopy (LM) and freezefracture replica technique (FFR). We observed very thin cytoplasmic extensions of hemocytes in the LMMorphological and ultrastructural methods previously employed for the characterization of vertebrate immune systems (macrophages, lymphocytes, and polymorphonuclears) under normal conditions and parasitic infections were used for identifying mollusc hemocytes (Sminia & Barendsen 1980, Ottaviani & Franchini 1988, Abdul-Salam & Michelson 1980, Barracco et al. 1993. It has been lately suggested that some bacteria toxins are able to induce the re-arranging of Rho superfamily proteins and cytoskeleton alterations (stress fibers) in epithelial cells (Brest et al. 2003). These findings indicate a new perspective for studying the mechanisms mediating the B. glabrata hemocyte and S. mansoni larval interaction.The aim of this investigation was to study ultrastructural aspects of the interaction between B. glabrata hemocyte and S. mansoni occurring at the cellular membrane level. For this purpose, a useful methodology for the ultrastructural characterization of membrane systems, the freeze-fracture replica technique (FFR) was used. MATERIALS AND METHODSMiracidia -S. mansoni eggs were purified from the liver and gut sections of hamsters (Mesocricetus auratus) 7 weeks after infection. Briefly, dissected organs were enzymatically digested (0.25% trypsin and 0.05% collagenase in 0.15 M PBS, pH 7.2) then eggs were purified by shifting through metallic meshes, decreasing in size from 420 to 44 µm. Eggs were collected and hatched under sterile conditions, and the resulting miracidia were sedimented at 4 o C as described by Césari and Alarcón de Noya (1987). All the animals used in this study were sacrificed in accordance to Venezuelan laws for animal care.Cytoadherence -Hemocytes were obtained from B. glabrata between 7 and 10 mm diameter. Molluscs were externally rinsed with 25% isopropylic alcohol and then