Resistance following drug attenuation (Ro 11-3128 or oxamniquine) of early Schistosoma mansoni infections in mice.

A single dose of Ro 11-3128 was found to be 98-100% effective against Schistosoma mansoni infections at intervals of 3 h to 15 days following infection, and apparently killed the schistosomula stages soon after administration, thus allowing an assessment of the immunizing potential of progressive larval stages. Following infection with 500 unirradiated cercariae, optimum resistance was manifest by groups drug-treated at 48-96 h (60-75%). Significantly lower levels of resistance occurred with early (3 h) or later (6-15 day) treatments. Superimposition of an infection treated at 15 days on a prior infection which had been treated at 2 days did not reduce the level of resistance caused by the latter, indicating that the infection plus delayed treatment had not induced suppression. Thus the peak resistance manifest during the 48-96 h period may reflect optimum expression of protective antigens. Comparison of irradiated (20 krad.) with unirradiated infections showed that, when drug-terminated 24, 48 or 96 h post-infection, irradiated cercariae induced significantly less resistance than unirradiated cercariae, perhaps indicating a delayed appearance of protective antigens following radiation treatment.