Immunization of mice with ultraviolet-attenuated cercariae of Schistosoma mansoni transiently reduces the fecundity of challenge worms.

In the present study cohorts of ICR and BALB/c mice were immunized with u.v.-irradiated cercariae of S. mansoni and challenged 5 weeks later, in parallel with unimmunized control mice, with approximately 100 cercariae. Total worm burdens at 5, 6, 7 and 8 weeks after challenge were significantly reduced by 27-65% in immunized mice. The total number of eggs and the number of eggs/female worm trapped in liver and small intestine were reduced significantly at 6 and 7 weeks post challenge in immunized, as compared to unimmunized mice. Decrease in tissue egg load could be achieved in BALB/c mice passively transferred with spleen cells from u.v.-attenuated cercaria-immunized mice. The proportion of female worms laying eggs in vitro was diminished only in worms recovered from highly resistant mice. The reduction in worm oviposition in immunized mice was no longer apparent at 8 weeks. The data taken together indicate that highly effective immunization of outbred and inbred mice with attenuated cercariae leads to significant, but transient, impairment in challenge worm egg production.