Enhancement of Acquired Resistance against Schistosoma mansoni in Albino Mice by Intraperitoneal Immunizing Exposures

Immunization of albino mice against Schistosoma mansoni by multiple, low-grade cercarial exposures resulted in earlier and higher levels of resistance when exposures were administered intraperitoneally (ip) than when the percutaneous (pc) route was used. Mice which received a course of ten weekly ip injections of ten cercariae each had significantly fewer worms than their controls. This did not occur when the exposures were made percutaneously. Both courses elicited strong resistance against challenge, although the level of this resistance was higher following ip immunization. With pc immunization resistance apparently was directly related to the maturation of immunizing infections and accumulation of eggs. This was not the case with ip immunization since significant resistance against challenge was evident only 7 days after the last of five weekly ip exposures. In ip immunization the basis for resistance may be the prolonged immunologic stimulation of lungs and liver resulting from the delayed escape of young worms from the peritoneal cavity, and the tendency of worms that remain in the peritoneal cavity to persist as schistosomular stages for extended periods.