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Studies on Schistosomiasis

XVIII. Some Factors Affecting Resistance to Schistosoma mansoni Infections in Albino Mice^*

Experiments were conducted with albino mice to evaluate several variables in the production of acquired resistance, or "immunity", to Schistosoma mansoni infection. These variables included: (1) the number of "immunizing" cercarial exposures; (2) the time from initial "immunizing" exposure and final immunizing exposure until the challenge exposure by the same parasite; and (3) the size of the challenge exposure.

The results indicate that: (1) a limited number of cercarial exposures are no more effective in producing "immunity" to a challenge infection than is a single exposure to the same total number of parasites; (2) a period of approximately 60 days from the initial exposure to a challenge exposure is necessary for a demonstrable "immunity" to the challenge infection; and (3) different sizes of challenge exposures, after a uniform "immunizing" infection, do not prevent the demonstration of acquired resistance. These results appear consistent with the hypothesis that maturity of the initial infections with egg deposition is necessary before "immunity" to a challenge exposure with S. mansoni can be demonstrated.

^* This investigation was supported by Grant E-1893 from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Public Health Service.