Evaluation of the effect of oxamniquine, praziquantel and a combination of both drugs on the intramolluscan phase of Schistosoma mansoni.

The activity of oxamniquine (OXA), praziquantel (PZQ), and a combination of both drugs was evaluated at the intramolluscan phase of Schistosoma mansoni. Biomphalaria glabrata snails infected with S. mansoni were treated with 500 mg/kg OXA, 1000 mg/kg PZQ or with 250 mg/kg OXA and 500 mg/kg PZQ, in association, at the pre-patent and patent phases of infection. The results showed that either treatments with OXA or PZQ, alone, at the pre-patent period, delayed the parasite's development, increasing the pre-patent period by approximately 10 days. At the same pre-patent period, treatment with a combination of OXA/PZQ delayed the parasite's development even more, extending the pre-patent period up to 56 days. At the patent period, treatment with OXA and PZQ, alone, interrupted cercarial shedding. When the snails were treated with 1000 mg/kg PZQ, the cercarial production was re-established 15 days after treatment, but in lower numbers than those obtained before treatment, whereas the snails treated with 500 mg/kg OXA were able to shed cercariae in similar quantities to those observed before treatment. The association 250 mg/kg OXA+500 mg/kg PZQ, at the patent period, not only discontinued cercarial shedding, but also led to the "cure" of the snails, showing a synergistic effect of this combination of drugs. These results suggest that this model will be useful for selection of resistant parasites, as well as for screening new antischistosomal drugs.