One hundred and ten Swiss albino mice in three groups were experimentally infected with Schistosoma mansoni cercariae. In the first group the spleen weight was recorded from the 3rd to the 79th day after one exposure. The animals in the second group were reinfected weekly beginning 60 days after the primary infection, and the average spleen weight was recorded. The third group was reinfected daily, beginning 60 days after initial exposure.
The average spleen weight in the three groups, compared with that of a control group, increased (splenomegaly), markedly so in the daily reinfected animals. Histologically there was lymphatic and reticular hyperplasia which began on the 3rd day after infection (toxemic stage). In animals which survived from the 40th day on, the lesions were more evident, probably as a result of greater phagocytic activity due to the destruction of adult parasites and eggs in the tissues of the host. In animals reinfected weekly the splenic lesions were similar to those in the first group, but the hyperplasia was more intense. In the group of mice reinfected daily, the intensity of the lymphatic and reticular hyperplasia was even greater.