Schistosoma japonicum in rabbits: differences in the host-parasite relationship over a seven-year period.

Rabbits were infected with Schistosoma japonicum of Japanese origin between 1973 and 1979. The distribution of lesions and eggs in the small intestine and colon changed significantly twice during that period, and the number of eggs per worm pair passed in the feces changed at least once. The shift in egg distribution and that in egg passage occurred at different times and appeared not to be causally related. The number of eggs per worm pair retained in the tissues of chronically infected rabbits was consistently greater in rabbits with colonic bilharziomas. We postulate that differences in the schistosome population were responsible for the alterations in worm behavior. These differences may have been caused by the introduction of new isolates from the field, or by selection during laboratory passage.