The course of Schistosoma mansoni infection in thymectomized rats.

Inbred rats were thymectomized, irradiated, and reconstituted with T cell-free bone marrow cells. Thymectomized-reconstituted (B rats) and control rats were infected with Schistosoma mansoni cercariae and the number of worms recovered was determined at various times after infection. The extent of immunosuppression was assessed by two criteria: 1) response to an injection of sheep erythrocytes (plaque assay, hemagglutination, hemolysis); 2) response to schistosome antigens (passive hemagglutination). Humoral responses to worm antigens were completely suppressed in almost all instances and anti-sheep erythrocyte responses showed a more variable but always very definite depression in B rats. The number of worms in B rats was about 4 times higher than in control animals at 5 weeks and about 3 times higher at 6 weeks. In a different experiment, rats were perfused at 4, 6, and 9 weeks after infection and the number of worms was found to be consistently higher in B rats, by a factor of about 2 at 4 weeks to a factor of about 4 or 6 at subsequent times. Although B rats had more worms than controls even at 9 weeks, a slow drop in their worm burden was noticeable with time in both experiments. Moreover, the size of worms in B rats was smaller than in controls and even 9-week-old worms failed to develop to normal size and appearance and could not be shown to produce fertile eggs. These experiments show a definite involvement of the immune system in the "self-cure" phenomenon, but may at the same time suggest that other non-immune mechanisms are involved in determining the pattern of S. mansoni infection in the rat.