The Influence of Infection Intensity of Schistosoma mansoni on Resistance Against Existing and Subsequent Infections in Macaca mulatta

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  • U. S. Army Tropical Research Medical Laboratory, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Department of Pathology, Peter Bent Brigham Hospital, and Harvard Medical School, Fort Brooke, San Juan, Puerto Rico 09851


The biology of light infections of Schistosoma mansoni and their immunogenic effects were studied in seven Macaca mulatta by means of routine egg counts, worm numbers and cellular reactions against schistosomula in the lungs. Two monkeys showed statistically significant increases in eggs when re-exposed at 250 days. One of these was strongly resistant to added infection at 500 days and the other relatively so. Three other animals re-exposed for the first time after 500 days were almost normally susceptible at that time. Thereafter, one of these failed to spontaneously terminate an infection with high egg counts, resulting from 20 light exposures, although its resistance to reinfection became strong.

Two other monkeys, receiving only a single initial exposure of 50 cercariae, produced eggs uniformly for much longer periods than is characteristic of heavy infections. One of these two animals, when re-exposed at 800 days, showed a statistically significant increase in egg output, while the other was strongly resistant at 1100 days. Following terminal challenge at 1400 days, worm counts and cellular reactions against schistosomula in the lungs indicated strong resistance had been induced. This indicates that it required a longer time to induce a comparable resistance in the lightly infected animals than in those with heavy initial infections.