Autoradiographic study of the attrition of migrating schistosomula in the skin of mice. II. Mice challenged with normal S. mansoni cercariae after immunization with highly irradiated cercariae.

The migratory pattern and attrition of 75Se-labelled S. mansoni challenge cercariae in CF1 mice immunized 0, 1, 3, or 5 times with 12kR- or 48kR-irradiated cercariae were investigated by compressed organ autoradiography. Mice were percutaneously inoculated with 500-1000 cercariae for each immunization and with a known count of approximately 90 cercariae at a different site for challenge. The skin, lungs, liver, other organs, and carcass were processed for autoradiography. Hepatic and mesenteric perfusions for worm collection were also sampled from day 19 through 34. In naive mice, 25% of challenge cercariae died in the skin. Skin attrition of challenge cercariae in mice immunized with 12kR-irradiated cercariae was 43% with 1 immunization, 54% with 3, and 58% with 5, whereas in mice immunized with 48kR-irradiated cercariae, skin attrition was 48% with 1, 63% with 3, and 76% with 5 immunizations. In all immunized mice, a high percentage of migrating schistosomula died in the skin within 24 hours of a challenge infection. Utilizing autoradiography, it was found that the silver foci created by single labelled worms were difficult to differentiate from those of paired worms. This study documents the induction of acquired resistance against a challenge infection in the skin by vaccination with highly irradiated cercariae, with the degree of attenuation of immunizing cercariae and the number of immunizations being closely related to the attrition rate in the skin. Finally, we discuss the contradictory views concerning the site of attrition of challenge cercariae.