The susceptibility of two Venezuelan (YT and SM) and one Brazilian (BH) strain of Schistosoma mansoni to single oral doses of praziquantel (Pz; 250 or 500 mg/kg), oxamniquine (Ox; 40, 60, or 100 mg/kg) or to low-dose combinations of both drugs (33 mg/kg Pz and 25 mg/kg Ox; 66 mg/kg Pz and 12.5 mg/kg Ox; 250 mg/kg Pz and 40 mg/kg Ox) was experimentally evaluated in mice. At lower doses of either drug, adult worms of the SM isolate were less susceptible than those of the BH and YT isolates. However, no difference in liver or intestinal egg counts (IECs) could be detected among the isolates after this treatment. At such doses, Pz was better than Ox at reducing IECs. In spite of lowered IECs, eggs continued to accumulate in the liver after Ox treatment. At higher individual doses or following treatment with low-dose combinations of both drugs, no difference in susceptibility could be detected among the parasite isolates. Under such conditions, oviposition was drastically reduced in all three isolates. We confirm that Ox preferentially kills male parasites and present for the first time evidence for the preferential killing of female worms by Pz. We propose that the synergistic effect obtained in the present study and in other investigations using low-dose combinations of both drugs may be due to the preferential cytotoxicity of each drug against a different parasite sex.

Serum or immunoglobulin fractions of serum from CBA/Ca mice vaccinated three or four times with radiation-attenuated cercariae of Schistosoma mansoni have been investigated for their capacity to confer protection upon naive mice. The data confirm that around 35% protection can be transferred with polyvaccine mouse serum administered in 0.5-ml aliquots 1 h before challenge (intravenously) and 24 h post-challenge (intraperitoneally). We show in addition, however, that polyvaccine serum is also protective when injected into the skin site of challenge as a single 0.05-ml aliquot. In contrast, lymphocytes obtained from the donors of protective serum conferred only 13% protection upon recipient mice. The passive cutaneous anaphylaxis assay showed that IgG1 is incremented by polyvaccination, while passive transfer experiments revealed that of the different isotypes fractionated from whole protective serum, only IgG1 has the capacity to protect naive recipients against challenge. The resistance transferred by IgG1 represents more than 60% of that obtained with whole serum and can be achieved using either the intravenous/intraperitoneal or the subcutaneous administration regimen. Recipients of serum given via the subcutaneous route exhibit cutaneous inflammatory focal reactions which comprise 20% eosinophils and 80% mononuclear cells; these foci entrap challenge larvae. The importance of IgG1 subclass expression to the success of serum-transferred resistance is discussed.

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.

Naive CBA/Ca mice and CBA/Ca mice vaccinated 4 weeks previously with radiation-attenuated cercariae of Schistosoma mansoni were challenged with normal cercariae and then treated with 500 mg/kg body weight of Praziquantel (Pzq). The drug was administered intramuscularly or intradermally on day 1, but intramuscularly only on day 6. The results show clearly that in naive mice skin-stage larvae were susceptible to Pzq provided the drug was given intradermally on day 1. Lung worms were susceptible to Pzq given intramuscularly on day 6. Drug efficacy in naive mice is thus dependent upon treatment being given at the correct time and via the optimal route. The efficiency of Pzq treatment was enhanced in vaccinated mice, but was again affected by the treatment regime. Analysis of the data revealed a highly significant synergistic effect between drug treatment and vaccination when Pzq was given intramuscularly on day 6. Synergy was detectable but only marginally significant when the drug was administered intradermally on day 1, and could not be demonstrated when Pzq was given via the intramuscular route on day 1. These findings are discussed in the light of known sites and mechanisms of vaccine resistance in mice, as well as in relation to the mode of action of Pzq against schistosome parasites.

Naive CBA/Ca mice and CBA/Ca mice vaccinated 4 weeks previously with radiation-attenuated cercariae of Schistosoma mansoni were subjected to 550 rad of whole body (gamma) irradiation and then challenged 3 days later with normal cercariae. The perfusion recovery data showed that this procedure reduced the primary worm burden in naive mice by 22% and the challenge worm burden in vaccinated mice by 82%. Irradiation also ablated the peripheral blood leucocytes of both mouse groups by 90-100% at the time of challenge. Histological data revealed that such treatment caused a dramatic change in number, size and leucocyte composition of cutaneous inflammatory skin reactions that characterize challenged vaccinated mice and are known to entrap invading larvae; cutaneous eosinophils were preferentially abolished by this treatment. Polyvaccine mouse serum that conferred protection passively upon naive recipient mice, failed to protect naive/irradiated mice when administered by the same protocol. Distraction of macrophages by treatment of mice with silica did not affect the establishment of a primary worm burden and reduced the protection exhibited by vaccinated mice by only 16%. These data indicate that radio-sensitive cells are important to both innate and specific acquired resistance in this mouse model and that macrophages contribute only marginally to the expression of vaccine immunity.