Schistosoma mansoni: age-dependent susceptibility to immune elimination of schistosomula artificially introduced into preinfected mice

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Abstract
Mice chronically infected with S. mansoni exhibited a significant resistance to a 2nd infection with the same parasite, as demonstrated by their challenge worm burdens measured by portal perfusion. A decreased worm recovery was also exhibited by chronically infected mice when the challenge was administered i.v. using 3 h schistosomula obtained by the isolated skin technique or using 5, 6, 7 and 9 day old schistosomula obtained from the lungs of infected donor mice. Variable results were obtained with 10 and 11 day old forms, while schistosomula which were 12 days old or older did not undergo significant rejection when introduced into the mesenteric veins of preinfected mice. Attempts to analyze these phenomena using the lung assay were complicated by the observation that the day of maximum recovery from the lungs was dependent upon the age of injected worms.