Schistosoma mansoni infection in rhesus monkeys: comparison of the course of heavy and light infections.

The course of Schistosoma mansoni infection in rhesus monkeys exposed to 100 or 600 cercariae was compared. During the first 12 weeks, the numbers of eggs per worm pair passed in the faeces and retained in the tissues were similar in the two groups. Between weeks 12 and 27, two-thirds of the worms died in monkeys exposed to 600 cercariae, many surviving worms shifted from colonic to small intestinal venules, and the number of eggs per worm pair passed in the faeces decreased. None of these changes occurred in monkeys exposed to 100 cercariae and in these animals the number of eggs passed in the faeces consistently increased after the twelfth week. The findings indicate that study of a single level of infection is not adequate to define the host response, and that the relation between worm burden and the number of eggs in the faeces is considerably influenced by host response.Monkeys were also studied 6 and 7 weeks after exposure to 600 cercariae. After the seventh week of infection, inflammatory infiltrates in the colon and portal tracts became much less marked, and the granulomas surrounding mature eggs decreased in size.