Schistosoma mansoni: decay of resistance induced by gamma irradiated cercariae in the mouse.

The level of resistance induced by a single percutaneous infection of cercariae which had been previously exposed to 20 kr of gamma irradiation was compared in two strains of mice. Challenge cercariae were given at several time intervals after the preliminary immunization either to the same skin site as that used for immunization or to a previously unexposed skin site. No significant difference in resistance to infection was observed when a different skin site was used for the challenge. A higher degree of resistance to challenge was observed in CBA/Ca mice than in BALB/c mice. Immunity in both strains declined to about two thirds of the original level after six months and could not be detected when challenge cercariae were given 70 weeks after the single immunizing dose of irradiated cercariae.