No effect on penetration or maturation was evident when Schistosoma mansoni cercariae were irradiated with 1000 r of gamma radiation from Co-60. Cercariae irradiated with 2500–3000 r developed into schistosomes in which egg production was greatly decreased as compared with normal worms. When irradiated with more than 4000 r, the worms did not survive 8 weeks in mice, but even irradiation with 10,000 r failed to affect cercarial penetration.
Repeated exposures of mice to cercariae irradiated with 6000–8000 r produced greater resistance than did a single exposure to irradiated schistosomes. Resistance was manifest either as a reduction in the number of worms maturing from a subsequent exposure to nonirradiated cercariae or as prolonged survival after exposure to otherwise lethal numbers of nonirradiated cercariae, or both.
Exposure of rats to irradiated cercariae enhanced their natural resistance as measured by reductions in worm burdens 4 and 9 weeks after exposure to a challenging infection. Four exposures of 5000 irradiated cercariae each produced greater resistance than did three exposures to 1000 irradiated cercariae. Infections with irradiated schistosomes induced greater resistance in rats than did infections with nonirradiated worms.
Present address: Department of Medical Zoology, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, D. C. 20012.