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.