The migratory pattern and attrition of

75

Se-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 cer- cariae 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 catrcass were processed for autoradiography.Hepatic and mesenteric perfusions for worm collection were also sampled from day 19 through 34.In naive tnice

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 was48% with 1

63% with 3

and 76% with 5 immunni- zations.In all immunized mice

a high percentage of migrating sehistosomula 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 attemtation of immunizing cercariae and the number of immunizations being closely related to the attritiotn. rate in the skin.Finally

we discuss the contradictory views concerning the site of attritiotn of challenge cercariae.