Effect of the duration of Schistosoma mansoni infections on the degree of protection against subsequent exposures

Prolongation of initial infection with S. mansoni in rodents results in decreased levels of acquired resistance despite continued presence of adult worms. In the albino rat a significant decrease in resistance occurs between 32 and 64 weeks after primary exposure, although resistance is still at a significant level at the latter time. In hamsters acquired resistance is lost sometime after 12 weeks and before 32 weeks. The mechanism of acquired resistance in the rat develops independently from the mechanism responsible for innate resistance in this host; is presumably unrelated to the latter; and has characteristics similar to the same mechanism in the highly susceptible mouse and hamster.